Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Jahn–Teller effect wikipedia , lookup
Ring-closing metathesis wikipedia , lookup
Metal carbonyl wikipedia , lookup
Hydroformylation wikipedia , lookup
Evolution of metal ions in biological systems wikipedia , lookup
Spin crossover wikipedia , lookup
Metalloprotein wikipedia , lookup
INTRODUCTION The most common approach for determining reaction mechanisms of chemical reactions of coordination complex species is the interpretation of results from kinetics investigations. The experimental aspects of kinetics measurements, for the determination of rate laws and their interpretation and the use of the Eyring equation to derive the enthalpy and entropy of activation have been described thoroughly in by Atkins [1], The principal method of monitoring reactions of coordination compounds is UV/visible spectrophotometery as many reactions of transition metal complexes are accompanied by changes in the electronic spectra. In coordination compounds metal atom or ions are surrounded by donor groups (anions on neutral molecules) that are called ligands. The type of groups that may surround a metal atom or ion are greatly varied, but they may be broadly considered to be of two types i.e. ligands that bind to metal atoms or ions through carbon atoms and ligands that do not. The former are involved in organometallic compounds. The branch of inorganic chemistry concerned with the remaining combined behaviour of central metal ions and their ligands is called coordination chemistry. The main justification for classifying many substances as coordination compounds is that their chemistry can conveniently be described in terms of a central cation Mn+, about which a great variety of ligands L, L L and so on may be placed in an essentially unlimited number of combinations. The overall charge on the resulting complex [MLx LY Lz…] is determined by the charge on M, and the sum of the charges on the ligands. The ability of a co-ordination compound to engage in reactions that result in displacing one or more ligands in its coordination sphere (by other -1- ligands in solution, for instance) is called its lability. Those coordination compounds for which such substitution reaction are rapid, called labile, whereas those for which such substitution reactions proceed slowly (or not at all) are called inert. It is to be noted that these terms should not be confused with thermodynamic stability and unstability [2].For example [Co(NH3)6]3+ ion, which will persist for months in an acidic medium because of its kinetic inertness (slow reactivity) despite of the fact that it is thermodynamically unstable. As shown by the large equilibrium constant (K~1025) for reaction [Co(NH3)6]3+ + 6H3O+ [Co(H2O)6]3+ + 6NH4+; in contrast, the overall formation constant (β=1022) for the reaction Ni2+ + 4CN– [Ni(CN)4]2– indicates that the thermodynamic stability of [Ni(CN)4]2– is high. A practical definition of the terms labile and inert can be given. Inert complexes are those whose substitution reaction have half life longer than a minute. Such reactions are slow enough to be studied by the classical techniques where the reagents are mixed and changes in absorbance, pH, gas evolution and so on. Labile complexes are those that have half-lives for a reaction under a minute. Special techniques are required for monitoring such reactions, as they may appear to be finished within the time of mixing. In the first transition series, virtually all octahedral complexes say those of Cr(III) and Co(III) and sometimes Fe(II), are normally labile, i.e., ordinary complexes came to equilibrium with additional ligands (including water) so rapidly that the reactions appear instantaneous by ordinary techniques of kinetic measurement. Complexes of Co(III) and Cr(III) ordinarily undergoes substitution reaction with half lives of hours, days, or even weeks at 25oC. -2- I. Ligand Substitution Reactions In reference to coordination chemistry, the coordinated ligand to a central transition metal ion may be exchanged by a free ligand or the exchange of coordination central metal ion itself by a free metal ion in solution. The detailed knowledge obtained from the comprehensive kinetics of substitution reactions can be of immense importance in deducing most appropriate conditions under which new metal complexes can be synthesized. Therefore, such studies can definitely improve the older methods of preparation of such complexes in general and refining the available analytical procedures depending on coordination chemistry. Several excellent reviews [3-5] and books [6,7] have been published on the kinetics and mechanism of inorganic reactions in order to visualize the nature of a variety of intermediates and transition states produced during the course of such reaction. I.1 General Mechanism of Ligand Substitution Reactions In these reactions, a coordinated ligand is substituted by an uncoordinated ligand. The simplest and most common of these is the swapping of coordinated water with solvent water (known as water exchange) but any ligand can in principle exchange with another. There are three classes of ligand substitution reaction mechanism: -3- In the dissociative mechanism, the M-X bond is broken before the entering group Y attaches. The coordination number of M decreased by one in the transition state. This mechanism is denoted as D corresponding to the notation SN1 used for substitution at a carbon centre developed by Hughes and Ingold [8]. In the associative mechanism, the entering group attaches to M before any weakening of the M-X bond occurs. The coordination number of M is increased by one in the transition state. The mechanism is denoted as A. The interchange mechanism lies in between these extremes as it involves the synchronous weakening of the M-X bond and attachment of Y. It is denoted as I and corresponds to the notation SN2 used in organic chemistry [7]. If bond weakening makes a larger contribution to the energy of the transition state then the interchange mechanism is labelled Id. When bond attachment makes a larger contribution, the mechanism is labelled, Ia. When the contributions are equal, the label I is used. This reformulation was made by Langford and Gray [9]. -4- I.2 Mechanism of substitution in Octahedral Complexes I.2.1 Dissociative Mechanisms In dissociative mechanisms, the rate determining step involves bond breaking and the energy required for this determines the activation energy. (a) D Mechanism In the D mechanism, there are only two elementary steps. In step (I), the complex LnMX gains enough energy to break completely the M-X bond. The 5-coordinate intermediate L5M may exist long enough to be detectable. In step (2), the intermediate L5M reacts with Y. It must be noted that Y is often solvent if this can coordinate as it is in large excess. (b) Id Mechanism In the Id mechanism, as the M−X bond starts to break, M begins to from a new bond with Y. The entering group Y must be present in the region around the complex L5MX when the M−X bond begins to break. Thus before the substitution occurs, Y must enter the outer sphere of L5MX, Ligands X and Y then swap over in the rate determining step, before X completely leaves the outer sphere of L5MY, It should be noted that, unlike in the D mechanism, no 5-coordiante intermediate is predicted. -5- I.2.2 Associative Mechanism (a) A Mechanism In the A mechanism, step (I) involves the formation of a 7-coordiante intermediate, which may exist long enough to be detectable. This is the rate determining step and the activation energy is determined by the bond making to the entering group and the ensuing steric crowding around M. In step (2), the products are then formed by breaking the M-X bond in the intermediate: k L5MXY L5MY + X 2 (b) Ia Mechanism The Ia mechanism is very similar to that shown above for the I d mechanism. The difference between the two lies in the nature of the 7coordiante transition state in the rate determining step: In the Id case, the M-X bond is very sensitive to the approach of Y and begins to weaken when Y is relatively far away. As bond breaking is more important than bond making, the activation energy is determined to a large degree by the strength of the M-X bond. In the limit, the M-X, bond breaks when Y is absent and a genuinely 5-coordiante intermediate is formed and the mechanism becomes D. In the Ia case, the M-X is unaffected until Y gets closer to M. As bond making is more important than bond breaking, this determines the activation energy. In the limit, the M-Y bond is formed before any weakening of the M-X bond. A genuinely coordinate intermediate is formed and the mechanism becomes A. -6- Where L represents a non labile ligand, X is the leaving ligand and Y the incoming ligand. I.3 Water Exchange in Aqua Ions Since many reactions in which complexes are formed occur in aqueous solution. One of the most fundamental reactions in which the water ligands in the aqua ion [M(H2O)n]m+ are displaced from the first coordination shell by other ligands included. Here is the simple case in which the new ligand is another water molecule that is the water exchange reaction. It is convenient to divide the ions into four classes [10], depending on these rate constants for water exchange [2]. Class I. The exchange of water is extremely fast, First-order exchange rate constants are on the order of 108 s-1, which approaches the maximum possible rate constant (calculated to be 109 to 1011 s-1 for a diffusion controlled reaction). The complexes are bound by essentially electrostatic forces and include the complexes of the alkali metals and larger alkaline earth metals. The metal ions are characterized by low charge and large size; Z2/r ratios range up to about 10×10-28 C2 m-1. Class II. The exchange of water is fast. First-order rate constants range form 105 to 108 s-1, Metal ions belonging to this group are the dipositive transition metals, Mg2+, and tripositive lanthanides. These ions form complexes in which the bonding is somewhat stronger than in those of Class I ions, but LFSEs are relatively small. The Z2/r values for ions in this category range from about 10 to 30 ×10-28 C2 m-1. Class III. The exchange of water is relatively slow compared with Classes I and II, although fast on an absolute scale, with first-order rate constants of 1 -7- to 104 s-1, The metal ions of this group are most of the tripositive transition metal ions stabilized to some extent by LFSE, and two very small ions, Be2+ and Al3+. The Z2/r ratio are greater than about 30×10-28 C2 m-1. Class IV. The exchange of water is slow, these are the only inert complexes. First-order rate constants range from 10-1 to 10-9 s-1. These ion are comparable in size to Class III ions and exhibit considerable LFSE: Cr3+ (d3), Ru3+ (low spin d5), Pt2+ (low spin d8). Best estimates for Co3+ , which oxidized water and is therefore unstable in aqueous solution, also place it in this class. I.4 Formation Reaction The formation of metal complexes takes place in media where usually water acts as a solvent. The rate of these reactions vary from very slow to very fast. The generally accepted mechanism for complex formation was originally proposed by Eigen and Tamm [11-13]. For complexes of unidentate ligands it involves the formation of an outer sphere complex between solvated metal ion and the incoming ligand followed by loss of a solvent molecule from this outer sphere complex to give the desired species. The mechanism of formation of complexes of multidentate ligands with the minor modification that the ring closure may constitute the rate determining step also comes in this category. I.5 Dissociative Reactions The dissociation of metal complexes can be considered as the reverse of the complex formation. These reactions are, generally much slower than the formation reactions. The mechanism proposed for the complex formation -8- also accounts for the dissociation rates of complexes bearing unidentate, bidentate or ambidentate ligands. In case of complexes of bidentate or ambidentate ligands, the rate constant depends upon opening of the chelate ring or sometimes rupture of penultimate metal-ligand bonds. Both these situations have been encountered frequently with outgoing bidentate or multidentate ligand groups. The presence of an acid generally enhances the dissociation rate because of protonation of the released ligand stablises the intermediate relative to the fully coordinated form [16-20]. I.6 Types of Ligand Substition Reaction Ligands substitution reactions of coordination compounds have been studied as intensively as many class of inorganic reactions. The kinetics of these processes have been investigated extensively for octahedral and to a lesser extent for square planer complexes. A very wide span of rate is found ranging from the extremely slow exchange of CN with [NiCYDT]2- (no evidence for the formation of [Ni(CN)4]2-, t1/2= 1440 hours) [21] to the almost diffusion controlled exchange of H2O between [Cu(H2O)6]2+ and water (t1/2= 10-8 sec) [22]. The following four types of substitution reaction have been reported in coordination chemistry. I.6.1 Unidentate Ligand Substitution Reactions (excess ligand). This section is concerned with the kinetic and mechanisms of substitution in complexes where unidentate ligands substitution with unidentate or multidentate ligands taken place. I.6.1.1 Unidentate by Unidentate Ligands The substitution of one unidentate ligand by another is the simplest case to consider and has been extensively used for investigating the mechanisms of substitution in many octahedral complexes as well as some -9- square-planer complexes. The general reactions involve the substitution of a monodentate ligand present in the inner-coordination sphere in the solvent media. There are two basic mechanism for unidentate ligand substitution in aqueous solution. For example the nickel-hexamine system, the first mechanism would be a dissociative type. r .d . s . [Ni (NH3)5]2+ + NH3 [Ni (NH3)6]2+ (1) NH [Ni (NH3)6]2+ [Ni (NH3)6]2+ (2) 3 The second would be a bimolecular mechanism involving water molecules: . d .s . [Ni (NH3)6]2+ + H2O r [Ni (NH3)5]H2O]2+ + NH3 (3) [Ni (NH3)5]2+ + NH3 [Ni (NH3)6]2+ + H2O (4) It is difficult to distinguish between such mechanism in aqueous solutions. However, the lack of NH3 attack on [Ni (NH3)6]2+ and the fact that a change of 30% in H2O concentration produced no observable effect support a dissociative mechanism [23]. The reactivity of low-spin Ru(II) complexes with respect to substitution by Unidentate ligand has received little attention in comparison with the large number of data available for other octahedral, low-spin complexes such as those of Co(III) [24] Rh(III) [24] and Ru(II) [25,26]. However, the kinetics and mechanism of ligand substitution in pentacyano (ligand)ruthenate(II) [Ru(CN)5L](3-n)- complexes have been the subject of considerable interest, in recent past, for several reasons [27-34]. These low spin Ru(II) species represent models for active sites in biological system and the reactions with imidazole (ImidH) have been investigated in this regard [30]. Another feature of these complexes is their use in redox reactions with metalloproteins [25]. - 10 - The kinetics of substitution reaction of a series of complexes of the type [Fe(CN)5L](3-n)- have been studies by Toma et al. [35,36] where L was an aromatic nitrogen heterocycle and the substituting ligand was nitroso-Rsalt. The rate of substitution varied with the nature of L and a saturation kinetics typical of rate-determining loss of L from the complex followed by rapid addition of the incoming ligand was reported. This is the first study [6] about monodentate ligand substitution reactions of simple low-spin Fe(II) complexes which generally proceed by D or SN1 (lim) mechanism. In contrast to the work on the pentacyanoferrate(II) complexes described above, very little work has been reported on the analogous Ru(II) system until recently. A detailed kinetic study of ligand substitution reactions of substituted [Ru(CN5)L](3-n) (where Ln+ is dimethyl sulphoxide or nitrogen heterocycle and entering ligand Ym+ is primarily dimethyl sulphoxide or N-methylpyrazinium cation) has been made previously [30]. The substitution kinetics in [Ru(CN)5en]3- complexes by pyrazine have also been studied in the recent past by Olabe et al. [31]. The reactions of [Ru(CN)5L)(3-n)- in presence of an excess of Ym+ resulted in a first order formation of [Ru(CN)5Y](3-m)– or loss or [Ru(CN)5L)(3-n)-. The deviation of the first order behaviour after 4 or 5 half lives (-24 hrs) were observed in some reactions and are attributed to possible side processes such as cyanide substitution and dimer formation [37]. The dissociative [D or SN1 (lim)] mechanism of ligand substitution is proposed for exchange of the L and Y ligands in [Ru(CN)5L)(3-n)- complexes according to eq. (5). - 11 - (5) Where n and m are the charges on the ligands L and Y respectively. The limiting reaction rates, at sufficiently large concentrations of entering ligand Ym have been observed with all leaving ligands as described above. The exchange of L and Y has been found to obey a second order rate law as. d [Ru(CN)5L)(3-n)- = kY [Ru(CN)5H2O3-][Ln] dt (6) The rate constants and activation parameters of the dissociation reactions of [Ru(CN)5L)(3-n)- complexes have been compiled in Table 1.1. - 12 - Table 1.1 Rate and activation parameters for ligand sunstitution of L n+ by Ym+ in [Ru(CN)5L](3–n)– complexes (pH = 7.00, I = 0.10MNaCl) Ln+ Ym+ 105k–Ls–1 N-Mepyz+ Me2SO 6.31 Py 5.97 Im 6.74 pyz 5.91 Me2SO 4.17 Py 3.79 Im 4.32 bpy Me2SO Pyz pyrpyr+ H (kJ mol–1) 102.402.1 S (JK–1 mol–1) 20.98.3 Ref. 100.31.7 8.34.2 27 6.79 92.80.8 -12.54.2 27 Me2SO 1.77 93.67.1 -20.920.9 N– 1.57 27 27 Mepyz+ 107.01.3 29.216.7 27 92.82.9 –8.38.3 27 104.91.2 16.74.2 27 1021.7 –4.24.2 27 11.40 832 335 34 tri 51.20 781 181 34 PCI tri 10.10 922 161 34 PyCN tri 18.10 902 151 34 isox tri 20.60 861 192 mtr tri 52.20 801 212 tt tri 68.40 881 222 Py Me2SO 3.34 N-mepyz+ 3.39 N-mepyz+ 10.70 py 11.70 Me2SO 1.67 N-mepyz+ 1.33 M-mepyz+ 0.85 Py 0.81 Pyz 0.77 en Pyz bpe Im Isonic– Me2SO 34 34 34 - 13 - The rate constants and activations parameters reported for formation and dissociation reactions of the analogous [Fe(CN)5L](3–n)– complexes are also compiled in Table 1.2. Table 1.2 : Kinetic and activation parameters at 298 K for the formation and dissociation of various pentacyano(ligand)ferrate(II) complexes. 365 kd104, s–1 11.0 H (kJ mol–1) 103.7(67.3)* - 11.5 100.3) Isonicotinamide 296 7.3 108.7(66.0) 4-Picoline 354 - –(63.1) –(16.7) 38 Pyrazine 380 4.2 110.3 (64.4) 58.5 (20.9) 36 N-Methylpyrazinium 550 2.8 114.9 (70.2) 74.6 (41.8) 36 Dimethylsulphoxide 240 0.75 110.8 (64.4) 46.0 (16.7) 36 - 6.2 110.8) 66.9 36 Thiourea 286 390.0 69.4 (65.6) -37.6 (20.9) 39 Allylthiourea 196 451.0 68.1 (69.8) -41.8 (33.4) 39 Dimethylthiourea 238 813.0 75.2 (64.8) -12.5 (16.7) 39 Glycinate 28.0 26.7 97.0 (61.5) 29.2 (-12.5) 40 Imidazole 240 13.3 101.6 (63.5) 41.8 (12.5) 40 N–Histidine 320 5.3 105.3 (64.4) 46.0 (20.9) 40 N3–Histidine 320 1090 91.1 (64.4) 41.8 (20.9) 40 Cyanide 38 - - - 41 Thiocyanate 64 - - - 41 370 - - - 41 42 - - - 41 1 2 3 4 5 DMNA - 12.0 - - 41 Ligand Pyridine 4-Methylpyridine 4, 4- Bipyridine 3-Cyanopyridine Nitrile kf M–1s–1 Ref. S mol–1) 46.0(29.3)* 36,38 (JK–1 37.6 36 58.5 (25.1) 36,38 6 - 14 - Aniline - Ca 0.20 - - 42 Cyclohexylamine - 13.56 96.5 46.0 40 Ethanolamine - 7.72 97.8 38.0 42 Morpholine - 7.16 103.2 53.9 42 NH3 190 12.0 102.4 63.1 42 MeNH3 130 4.46 103.2 53.9 42 Me2NH 80 7.79 100.3 49.7 42 Me3N 60 12.2 91.4 28.8 42 BunNH2 250 7.47 105.7 66.9 42 EtNH2 180 7.54 104.1 38.0 42 PrnNH2 200 7.38 107.0 71.0 42 - 8.43 94.9 29.0 43 en 330 51.5 97.0 37.7 43 enH+ 620 104.0 99.9 50.1 43 Sulphite - 0.57 Ca119.5 Ca75.2 43 Nitrosobenzene - 0.016 Ca117.0 Ca41.8 43 N-Methylimidazole 418 32.4 81.5 - 44 Isonicotinohydrazide 325 7.3 107.8 59.8 45 pd - 54.0 100.5 58.7 46 pdH+ - 83.0 100.5 53.7 46 bd - 46.0 102.4 58.7 46 bdH+ - 69.0 104.5 62.7 46 ptd - 45.0 103.7 58.7 46 ptdH+ - 64.0 101.6 53.7 46 hxd - 41.0 100.5 45.8 46 hxdH+ - 53.0 96.5 37.8 46 1, 4–Tx - 5.71 112.0 71.0 47 1, 4–DT - 5.58 105.0 44.0 47 1, 3–DT - 3.39 108.0 50.0 47 Pipyridine * Numbers in parenthesis give the values of H and S for the formation reactions. - 15 - Shephered et al. [48,49]and Toma et al. [36,38] have recently employed these complexes and binuclear derivatives in spectroscopic studies for comparing the behaviour of low spin d6 moieties [35-36] viz. [Fe(CN)5L)](3-n) , [Ru(CN)5L]3- and [Ru(NH3)5]2+. Malin and co-workers [35-36] have observed that the intermediate [Fe(CN)5]3 is quite insensitive to the nature and charge on the attacking reagent. Coelho and co-workers have also made the similar observations [47] on the kinetic studies of [Fe(CN)5L)(3-n) (where L = sulphur heterocylic ligands). There has been several reports in literature on the complex formation reaction involving [Fe(CN)5H2O]3- complex ion beside the discussion on the tendency of this ion to dimerize at higher concentration and the nature of the dimeric species [50]. However, the replacement of coordinated water ligand in [Fe(CN)5H2O]3- ion by other ligands, such as aromatic nitrogen heterocycles which gives rise to an equally extensive series of substituted pentacyanoruthenate(II) complexes [27]. Hoddenbagh and Macartney have recently reported that results of kinetic study of the substitution reactions of [Ru(CN)5H2O]3- ion [51], which implicates an ion-pair dissociative mechanism with a water exchange rate of 10±5 s-1 one order of magnitude lower than found for the [Fe(CN)5H2O]3- ion. The rate data and activation parameters on the formation of some [Ru(CN)5H2O](3-n)- complexes by the reaction of few nitrogen heterocyclic ligands with [Ru(CN)5H2O]3- have also been reported in Table 1.3. - 16 - Table 1.3. Rate and equilibrium constants for ligand substitution in pantacyano(ligand)ruthenate(II) and pentacyano(ligand)ferrate(II) complexes. k m–1 s–1 H kcal mol–1 N-Methylpyrazinium 47.2 17.3 [Ru(CN)5L] S kcal (3–n)– 105 mol–1 105 k–Ls–1 kL m–1 7.1 6.31 7.5 Pyridylpyidinum 44.4 17.0 6.1 4.17 11.0 2050d 4,4-bipyridine 14.4 17.2 4.3 6.79 2.1 365d 0.62g 5.9 Pyrazine 10.5 17.2 4.0 1.77 5.9 380e 0.42g 9.0 Pyridine 05.4 17.9 4.8 3.34 1.6 365e 1.10g 3.3 Imidazole 05.1 17.0 1.6 10.70 0.4 240e 1.33e 1.8 Ligand(L) [Fe(CN)5L](3–n)– kL 103 M–1s–1 k–1 550e Ref. 10–5 Km–1 0.28g 20 2.6d 7.9 51 8.0 2.6 1.67 2.1 60d 13.1b - - 0.85 15.0 240f 01.8 1.2 Isonicotinate 03.5 Dimethyl sulfoxide 2,3-pyraznetic 17.5 16.7 - - - 0.40d 1.5 0.07f 32 - - carboxtate a Reference 79 (I=0.10 M), bReference 70 (I=0.10M), cReference 73 (I=0.50M), except for N-mepyz+ (where I=1.0M), dMacartney, D. H.: unpublished results (I=0.10M), (I=0.10M) e Reference (72) (I=1.00M), gReference (I=1.0M). The results on the substitution reaction on [Ru(CN)5H2O]3- ion [51] have demonstrated that these pentacyano(ligand)ruthenate(II) complexes are relatively inert species in solution. This decreased lability is expected because of relatively stronger Ru-L bonds. The larger radial extension of the 4d-orbital on ruthenium would allow for greater π back bonding to the cyanide ligands and to the nitrogen heterocycle in the sixth position. The increased back-donation would enhance the M(III) character of Ru(II) relative to that of Fe(II) and account for slower exchange in [Ru(CN)5H2O]3- - 17 - . The back bonding interactions, metal to ligand charge transfer (MLCT) transition and back-bonding stabilization energy (BBSE) for M [CN)5Ru]3-, [CN)5Fe]3-, [(CO)5W], [(NH3)5Ru]2+ or [(NH3)5Os]2+ and L aromatic nitrogen heterocycle can be easily understood through the molecular orbital description developed by Zwicked and Creutz based on symmetry and overlap arguments [52]. The metal to ligand charge transfer band is described as a transition from a 'd' level to the ligand B2 anti bonding π level. Symmetry orbitals are constructed for dzx (metal) and π2* (ligand) orbitals, producing a bonding orbital of predominantly metal character and an antibonding orbital of predominantly metal character and an antibonding orbital of predominantly ligand character. The MLCT transition occurs between these two orbitals. Here, the back-bonding stabilization energy (BBSE) is the energy difference between purely metal orbital and molecular ground state derived from mixing of metal orbital with the ligand π* orbital. Qualitatively, following two factors will influence the extent of this orbital mixing and the magnitude of BBSE. (i) The radial extension of the metal d-orbital (ii) The energy difference ( E) between the metal orbital and the ligand orbital. Greater interaction is expected for orbital which are close in energy to metal orbitals of high energy, favourable for the back bonding interaction. The ability for metal ligand orbital interactions is also enhanced if the metal d orbitals extend far into space, enabling effective overlap with π orbital. These considerations lead to the well known order of back-bonding ability of 5d>4d>3d. In considering the ability of the metal centers to back bond to L in the complexes as described in Table 1.3 attention must be paid to the remaining ligand environment, the “spectator” ligand viz. NH3, CO and CN-. - 18 - The NH3 will not have much effect on the energy of the t2g orbital of the metal centre and should not alter ability for back-bonding to π-acceptors such as pyrazine in these complexes. Conversely CN- and CO will be greatly diminished. A much longer difference in substitution lability is found when the water exchange rates of [Ru(OH2)6]2+ and [Fe(OH2)6]2+ ions are compared (Table I.4). This difference may be attributed due to change in the spin configuration from a low-spin Ru(II) to a high-spin Fe(II) system. The water exchange rate constant (kex) for [Ru(CN)5OH2]3- may also be compared with values for other octahedral ruthenium(II) complexes containing coordinated water molecules (Table I.3). The anionic complexes [Ru(CN)5OH2]3- and [Ru(edta)OH2]2- (edta4- = ethylenediamine-tetraacetate) are at least two orders of magnitude more labile than cationic complexes such as [Ru(OH2)6]2+ and [Ru(NH3)5OH2]2+. The labilization of the Ru−OH2 bond is presumably a results of a reduction in the effective positive charge on the metal the centre. This in turn lowers the energy barriers for water dissociation by the interaction between the metal and the ion pairs on the oxygen atom. When spectator ligands, such as CO or 2,2-bipyridine, are present in the cationic Ru(II) complexes, the removal of electron density via n-back donation to these ligands results in an increase in the effective charge on the metal. The lability of the coordinated water in these complexes is further reduced. - 19 - Table 1.4. Water substitution rate constants for iron(II) and ruthenium(II) complexes 25C Complex kex,s–1 Ref. Ru(H2O)62+ 1.410–2 53 Fe(H2O)62+ 4.4106 54 Ru(CN)5OH23– 10 51 Fe(CN)5OH23– 300 44 Ru(edta)OH22– 15 55 Ru(NH3)5OH22+ 0.10 56 Trans-Ru(NH3)4(Pyr)OH22+ 2.010–2 57 Cis-Ru(NH3)4(CO)OH22+ 4.010–6 57 Ru(terpy)(bpy)OH22+ 7.010–5 57 The water substitution rate constants for both [Fe(CN)5OH2]3- and [Ru(CN)5OH2]3- decrease substantially in acidic solution, as a coordinated cyanide ligand is protonated. Cyanide protonation would tend to draw some electron density away from the metal centre, strengthening the M-OH2 bond. The pka vaule for [(HCN)Ru(CN)4OH2]-2, 2.24±0.10 (μ=0.10M) and [(HCN)Ru(CN)4OH2]-2, 2.63±0.12 (μ=1.0M) [58] may be compared with values reported for hexacyanocomplexes: (HCN)Ru(CN)53-, 2.53 (μ=0.10M) [59], (HNC)Fe(CN)53- , 3.16±0.03 (μ=0.10) [60]. The basicity of the cyanide ligands is reduced by the replacement of one cyanide by a water molecule. This trend continues when two cyanides are replaced by 2,2- bipyridine ligand to from [(HCN)M(CN)3bpy]–, for M=Fe, pka=1.69±0.02 (20.0 oC μ=0.089M) [61] for M=Ru, pka=0.12±0.0.06 (21oC, ionic strength not held constant [62]. Taking into consideration the differences in ionic strength, the pka values for the respective Ru(II) and Fe(II) complexes are quite similar. A - 20 - slightly lower pka for the Ru(II) species may reflect somewhat stronger metal-ligand interactions consistent with the more inert Ru-OH2 bond. The ratio of formation and dissociation rate constants (kL/k-L) for few pentacyano (ligand) ruthenate(II) complexes have been evaluated and found to be similar to the corresponding Fe(II) counterpart (Table 1.3).; The trends in kL and k–L are also found similar for two metal systems, with dimethyl sulphoxide the most inert and the imidazole the most labile with respect to substitution. The ligand-exchange rate constants have also been reported for some other octahedral Ru(II) complexes, of the type [Ru(NH3)5L]2+[56-63] and [Ru(edta) L]2- [58] . The rates of ligand substitution in these complexes are of similar magnitude and are found to be dependent on the nature of 'L', consistent with a dissociatively activated process. The reactivity of coordinated NO+ in [Ru(CN)5NO]2- with OH – , SH – and SO32 – are reported to be similar to those found for the corresponding nitroprusside anion [64]. The intensive absorption bands (325, 430 and 420 nm respectively) are observed initially, which slowly decay to generate a final absorption band at Ca. 285 nm. This is a common feature for all the reactions. On the other hand, the reactions with N-coordinating nucleophiles such a hydrazine and concentrated ammonia are rapid processes that lead to the evolution of gaseous products, together with the formation of the same final band at 285nm. The overall picture implies the general eq. (7). [Ru(CN)5NO]2- + L {[Ru(CN) NOL]n-} X285 + products (7) Here X285 is the final reaction product and identified as [Ru(CN)5H2O]3-. The [Fe(CN)5H2O]3- has been identified as final reaction product for the reaction of {Fe(CN)5NO]2- with excess hydroxylamine in - 21 - alkaline medium [65]. The mechanism involving the five coordinated intermediate [Ru(CN)5]3- is likely the same for most of these reactions as found in the corresponding iron(II) counterpart. On the other hand, cyanochemistry of Ru(III) has been least studied until recently. The solutions of [Ru(CN)6]3- were prepared by reacting [Ru(CN)6]4- with strong oxidizing agents [66]. (e.g. acidic Ce(IV). The aqueous solution of [Ru(CN6)]3- is highly unstable. However, reaction of [Ru(CN6)]3- has been observed at pH=6. This reaction is found to be catalysed by HgCl2. No other substitution reaction of [Ru(CN)6]3- is reported so far. Recently the [Ru(CN)6]3- ion is synthesized in aqueous solution and isolated as [Ph4As]3 [Ru(CN)6].2H2O. Most recently an X-ray structure determination on the above isolated compound [67] reveals that it contains discrete [Ru(CN)6]3- ions with an almost perfect octahedral coordination. In recent years one of the diagnostic tests is being widely applied for elucidating and classifying the substitutional mechanisms in solution involving the measurements of volume of activation ( V≠). Several monographs have appeared since 1986 [68-69]. The transition states arising form mechanisms, ranging form Id to D, are indicated by increasing positive values of V≠. The negative values of V≠ denote activation proceeding form Ia to A with increasing compactness and increasing negative magnitude. The activation volumes for dissociation of several complexes of the type [Fe(CN)5L](3-n)- are listed in Table. 1.5. - 22 - Table 1.5: Activation volumes (V) for some ligand substitution reactions on pentancyano(ligand)ferrate(II) complexes by various incoming ligands in aqueous and non-aqueous medium S. No. 1 1. 2. Reaction Solvent 2 Fe(CN ) 5 L CN Fe(CN ) 64 L 3 L = 4-(1-butylpentyl)pyridine L = 4-phenylpyridine L = N-(n-pentyl)pyrazinium (Na2 salt) L = pyrazine H2O-MeMeOH H2O H2O H2O 3– 4 5. Ref. 5 – +16 +10 +10 +13 70 Fe(CN)5 L3 CN Fe(CN)64 L L = 4-CNpy L = 4,4-bpy L = 4-tBupy 3. V (cm3 mol–1) 4 H2O H2O H2O +19.00.5 +130.7 +11.41.0 L = p-(CH3CH2CH2)2 CHC5H4N L = p-(C5H4N)2 L = p-(C6H5)(C5H4N) L = p-CH3(CH2)5NC4H4N+ L = p-C6H4N2C2H2 L = p-C6H4N2C2H2 L = p-(CH3)3CC5H4N L = p-NCC5H4N L = p-CH3C4H4N2 L = C4H4N2 L = p-CH3C4H4N2 20% MeOH H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O +16.31.4 +13.60.5 +10.40.5 +9.60.8 +17.90.4 +11.41.0 +19.01.0 +0.90.5 +12.51.2 +20.90.5 Fe(CN)5(4-CNpy)3– + CN– 4– Fe(CN) 6 +4CNpy H2O +20.90.5 73 H2O H2O H2O H2O. H2O +16.40.6 +24.01.0 +16.31.5 +17.41.4 +18.50.6 74 71 Fe(CN)5 L3 CN Fe(CN)64 L 72 Fe(CN ) 5 ( NH 2 R) 3– py Fe(CN ) 5 ( py) 3– NH 2 R R=H R = CH3 R = C2H5 R = PhCH2 R = iPr - 23 - 6. Fe(CN)5 H2 O3– Ln Fe(CN)5 L(3–n )– Ln– = imidazole Ln– = histidine Ln– = methionine Ln– = glutathione Ln– = glycine Ln– = -alanine 7. 8. H2O H2O H2O H2O H2O H2O +15.50.7 +17.00.4 +17.90.6 +14.10.4 +16.40.6 +16.80.2 75 Fe(CN ) 5 H 2 O 2 – L Fe(CN ) 5 L2 – H 2 O L = cytosine L = cytidine L = CMP H2O H2O H2O Fe(CN ) 5 ( NO2 ) 42 – H 2 O H2O +2.50.5 +9.51.2 +12.81.1 +20.11.0 76 77 Fe(CN ) 5 ( H 2 O) 3– NO2 The similarity in values of V≠ impress a common rate- determining step which does not depend on the nature of incoming ligand. The positive value of V≠ in above systems suggest either a dissociative mechanism or a mechanism involving charge dispersal in the transition state [78]. I.6.1.2 Multidentate by Unidentate The displacement of multidentate ligands by unidentate ligands is another example of unidentate ligand exchange reactions. The cyanide is a potential unidentate ligand having the capacity of displacing multidentate ligands viz., microcyclic ligand, polyamines, polyaminocarboxylates and thioligands from their metal complexes. The reactions involving [NiL](2-n) complexes (L=polyaminocarboxylates [79-80] and polyamines [81,82] microcyclic ligands [83-87] with cyanide ions have been studied extensively by many workers. Recently Nigam and coworkers have investigated the kinetics and mechanisms for replacement of aminocarboxylates from mono(aminocarboxylato) hydroxoferrate(III) complexes by cyanide ions - 24 - [88-90]. The general mechanistic scheme for [NiL](2-n)-–CN– replacement reactions requires that three cyanides are bonded to Nickel(II) ion while four cyanides are required in [FeL(OH)](2-n)-–CN– systems (n = charge on ligand L) in their respective are determining step. The last cyanide adds rapidly forming [Ni (CN)4]2– or [Fe(CN)5OH]3– as the case may be. The [FeL(OH)](2–n)-–CN– reaction presents some complications and takes place in three distinct stage [88-91]. The mechanism for the first stage i.e. the formation of [Fe(CN)5(OH)](2-n)- complexes has already been reported for may aminocarboxylates . I.6.1.3 Unidentate by Multidentate This type of reactions have been studied extensively in connection with the replacement of coordinated water during the formation reactions of multidentate ligand complexes [92]. The reactions of open chain and macrocyclic polyamines with Cu(II) in strongly basic solutions have been studied [93] to examine the kinetic behaviour of the unprotonated ligands. Some kinetic information is also available on the reaction of [Ni(CN) 4]2– with aminopolycarboxylate ligands. A mixed [Ni(CN)3L](n+1) complex is formed before the rate determining step in which an additional CN -is lost. The behaviour with triene is different, however, because the ratedetermining step is the reaction of the ligand directly with [Ni(CN)4]2- by an associative mechanisms [94]. Also the disappearance of [NI(CN)4]2- is greatly accelerated by the polyamine concentration in comparison to the much slower reaction of [Ni(CN)4]2- with aminocarboxylates [21,79]. Crouse and Margerum [95] have reported the reaction of [Ni(CN)4]2– with a few polyamines in presence and absence of I2 as a scavenger for cyanide ion. In presence of I2 the released cyanide has no effect on the forward reaction rate but in absence of I2 there is an inverse effect. - 25 - I.6.2 Multidentate Ligand Substition Reaction The mechanisms by which one multidentate ligand displaces another from a metal ion depends upon the ability of both ligands to coordinate with the metal ion simultaneously. The rate determining step of overall reaction is the cleavage of any one of the several bonds between metal and the leaving group which must be broken in the course of the reaction. For example Ni Tet2+ reacts with EDTA [96], TMDTA [97], PDTA or DTPA [98] and Nitrien2+ reacts rapidly with EDTA [99], HEDTA [100] or DTP [101] forming mixed ligand intermediates and give respective products by unwrapping of Tet or Trien. A review of on multidentate ligand exchange reaction and their application to analytical chemistry has appeared for systems involving EGTA and 4-(2-Pyridylazo) resorcinol [102]. Other multidentate ligand exchange reactions of metal ions usch as Cu(II) [103-105], Zn(II) [106]. Hg(II) [107], Cd(II) [108-111] and Ni(II)[112-116] have been investigated by many workers. The elucidation of substitution mechanism of a coordinated ligand to Fe(III) centre by another incoming ligand has been the subject of considerable interest for many workers [117-122], including us [90]. Most of these studies are largely centred around exchange of a polydentate by a monodentate or monodentate by polydentate ligand. On the contrary, there have been limited reports on the kinetic and mechanistic studies involving the exchange of a polydentate ligand coordinated to Fe(III) by another polydentate ligand [123-125] - 26 - I.6.3 Metal substition Reaction Between Ligands Multidentate ligand transfer between two metal ions i.e. displacement of one metal cation from its complex with a multidentate ligand by another metal ion is representate by eq. ( 8 ) : ML + M ML + M (8) It has been proposed that the substitution proceeds through a dinuclear intermediate species, in which a multidentate ligand is partially unbound from the initially complexed metal ion and is partially bound to the entering metal ion, where the cleavage of one of the bonds between the ligand and the leaving metal ion constitutes the rate determining step. In some cases these reactions proceed via dissociation of initial complex with or without acid calalysis by complex formation between the new free ligand and the displacing cations. Most of the available data are related to the reactions of aminopolycarboxylato complexes [128], in particular the combination of various metal ions [129] with EDTA , according to eq. (8). A stopped flow study of cobalt(II) have been performed previously by Mentasti [126] and suggested the stepwise unwrapping mechanism followed by attack of Cu 2+ to give dinuclear intermediate for all reactions. I.6.4 Double Exchange Reactions An interesting situation exists when two multidentate complexes are mixed and the thermodynamics dictates a double exchange represented by eq.(9) ML + ML ML + ML (9) The reactions of this category are generally slow because they proceed through cleavage of a series of coordination bonds in succession [130, 131]. No evidence for the formation of a dicomplex intermediate has been - 27 - reported so far but the rates of such reactions have been found to increase sharply if small amount of either a free ligand or a metal ion is added to the reaction mixture [132]. The phenomenal increase occurs due to operation of a coordination chain mechanism and has been used for analysis of traces of metals or ligands [131-135]. I.7 Kinetic Aspects of Analytical Chemistry Based on Ligand Substitution Reaction The bulk of analytical chemistry is based on chemical reactions at metal ion centres in liquid media, particularly in aqueous solutions. Their study, understanding and applications constitute a large portion of todays tasks of analytical chemists. The use of kinetics by analytical chemists has been increased tremendously during the past few years. Reaction rate techniques have been used in the development of analytical methods for estimation of inorganic and organic compound present in industrial environmental and biological sample. A brief survey of the significant developments made in this field will be presented here though the discussion will be mainly centred around the recent advances in the applications of ligand substitution kinetics to the determination analysis of components in mixtures of catalytic species present in or added to suitable reaction system. Generally, kinetic method are classified into two broad categories: 1. Methods based on catalysed reaction. 2. Methods based on uncatalysed reaction. Here, methods only based on catalyzed reactions will be discussed. - 28 - I.7.1 Methods Based on Catalysed (Non-Enzymatic) Reactions The recent development of catalytic method of analysis/determination is a result of their high sensitivity combined with relatively simple procedures. A variety of catalytic effects on reactions have been employed in analytical determinations. Catalytic determinations can be broadly viewed as: a. Use of primary catalytic rates (determination of catalyst) and b. Use of modified catalytic rates (determination of modifiers) I.7.1.1 Kinetic Methods Based on Primary Catalytic Effects The following two facts must be accounted for catalytic determination: (a) The uncatalysed reaction proceeds simultaneously with the catalysed reaction, and (b) the rate of catalyzed reaction is proportional to the concentration of the catalyst. The latter is a consequence of cyclic regeneration of the catalyst so that its concentration remain constant. Another practical requirement for successful application is that the concentrations of reactants other than the catalyst or the species, whose change in concentration is monitored, must be such as to make the reaction rate pseudo-zero order. The species whose change in concentration is being monitored is adjusted to give first order dependence. Thus, for the generalized reaction: C S + R P+Y (10) Where S (monitored species) and R are reactants, P and Y are products and C is the catalyst, the general rate expression can be written as: Rate= - d[S] = kc [S] [C]o + ku [S] dt (11) - 29 - Here kc and ku are the rate coefficients containing some concentration terms for catalysed and uncatalysed reactions respectively and [C]o is the initial concentration of the catalyst in the reaction system. Focussing our attention on the term for the catalyzed path in eq.(11)eq.(12) and accounting for the presence of the catalytic cycle one can use a simplified two-step reaction scheme as represented by eqs.(12) and (13). (12) (13) Depending upon the relative magnitudes of rate coefficients for reactions shown in eq.(12) and (13), two situations may arise: (a) pre-equilibrium case (b) steady state situation. After restoring to some valid approximations both these conditions demonstrate the proportionality between catalyst concentration and the ratio of reaction as given by eq. (14) initial rate = - d [s] k2 [C]o dt (14) where k2 is a composite rate constant made up of some rate constants and concentration terms. Thus under conditions and also considering the uncatalysed reaction is invariably taking place, eq.(35) can be written in a general form for both the above conditions as. - d[S ] = F[C]o + F dt (15) - 30 - Where F[C]o is a linear function of the catalyst concentration and F is the rate due to uncatalysed path. eq.(39) can be used to determine the concentration of catalyst ‘C’ using either a differential or an integral approach. The overwhelming majority of indicator reactions chosen for catalytic determinations involve redox systems. Howevever, sometimes a reaction involving the exchange of ligands in a complex can be catalysed homogeneously by a metal ion provided that the metal ion has an affinity for the leaving ligand and the experimental conditions are so selected that the catalyst can be regenerated. As an illustrated example, the metal ion catalysed replacement of CN- from [Fe(CN)6]-4 has been used for the determination of small amount of catalyst metal ions. Pavlovic and Asperger [137] found a method for determining as low as 2.7 × 10 -7 M of Hg2+ in biological materials with relative standard error of about 20%. The substitution of CN- from [Fe(CN6)]4- by 2,2-bipyridyl and 1,10phenanthroline has also been used for the determination of Hg2+ and Ag+ions [138-139] Nigam et al. have determined Hg2+ down to 1±×10-7 M using pNDA [140] and Mpz+ [141] as the entering ligand. Table 1.6 gives a summary of analytical applications of catalysed ligand substitution reactions of [Fe(CN)6]4- and [Fe(CN)5NH3]3However, on contrary only a single preliminary report is available in literature involving ligand substitution of corresponding [Ru(CN) 6]4- with Nitrosobenzene [66] catalysed by Hg2+. Thus the metal catalysed ligand substitution reaction between [Ru(CN)6]4- and various nitrogen heterocylic ligands provides more scope to determine trace concentration of toxic metal - 31 - Table 1.6: Reactions involving exchange on hexacyanoferrate(II) and pentacyano(ligand)ferrate(II) in presence of catalysts Indicator Reaction 1 4[Fe(CN)6] + PhNO Analyte 2 2+ Hg Remark* 3 528 nm (2.7×10 M), pH=4.1, 20% Ref. 4 137 [Fe(CN)6]4- + 1,10-Phen Hg2+ (10-4M), pH=3.0 138 [Fe(CN)6]4- + Bipy Ag2+ 522 nm (10-7 )M 139 [Fe(CN)6]4- + p-NDA Hg2+ 640 nm (2.0×10-8M), pH=5.0, 5% 140 [Fe(CN)6]4- + Mpz+ Hg2+ 655 nm (3.6×10-8M), pH=5.0, 1.5% 141 [Fe(CN)5NH3]3- + Ferrozine Ag2+, 162 nm (0.02 ppm Ag2+ 0.1 ppm 142 Au2+, Au2+, 0.01 ppm Hg2+ ), pH=4-6, 4.7 Hg2+ %, 4.3 %, 2.7 % [Fe(CN)5NH3]3- + nitroso-R-salt Pd2+ 720 nm (0.04 ppm), pH=5.0 143 [Fe(CN)5NH3]3- + nitroso-R-salt Hg2+, 625 nm (0.01 ppm Hg2+, 0.005 144 Ag2+, ppmAg2+, 0.05 ppm Au3+), pH=4.5- Au3+ 6.0, 3.75%, 2.15 %,5.44 % Ag2+, 520 nm (1.67×10-7 M Ag2+, 1.65×10- 145 Au3+ 5 [Fe(CN)6]4- + Bipy Hg2+ 485 nm (1×10-5 M) 146 [Fe(CN)6]4- + PhNO Hg2+ 525 nm (0.5µg/2ml), pH=4.1, 7 % 147 [Fe(CN)6]4- +PhNO2 Hg2+ - 148 [Fe(CN)6]4- + Phen Au3+ (10-4M), pH=3.5 149 [Fe(CN)6]4- + L(in presence of Phen, (5×10-4M), pH=2-4 150 catalyst) Bipy [Fe(CN)6]4- + Phen(in presence CN- 528 nm (1×10-5M), pH=6, 6.68 % 151 Hg2+ 482 nm or 462 nm (0.05 ppm), 152 [Fe(CN)6]4- + Bipy -7 M Au3+) of Hg2+ ) [Fe(CN)5NH3]4- + Bipy or Phen pH=4.5, 3 % [Fe(CN)6]4- + nitroso-R-salt Ag2+ 720 nm (0.05 ppm), pH=4.0, 5 % [Fe(CN)6]4- + NN Hg2+ 630 nm (1×10-6M), pH=3.5, 6 153 pH=4.5, 3 % * detection limit is given in parenthesis and error in % - 32 - Catalysed substitution reactions involving displacement of one ligand by another in a metal complex have been widely used to determine microgram quantities of catalysts [155]. Tabata and Tanaka have reported kinetic methods for determination of nanogram amounts of Hg 2+ and Cd2+ by their catalytic effects [156-157] on the metal ion incorporation into porphyrins to form metalloporphyrins viz. the complex formation of Mn2+ with α, β, γ, δ- TPPS. Cu2+ catalysed metal-metal exchange reaction of Zn2+ with [NiEDTA]2- has been exploited by Bydalek and Margerum [158] for determination of Cu2+ at concentration 10-5M. Double exchange reaction are very sensitive to trace catalysis and inhibition. These properties have led to the development of sensitive analytical methods for concentrations down to 10-8M of metal ion or ligands [133-137]. I.8 Micelle A micelle (rarely micelle, plural micellae) is an aggregate of surfactant molecules dispersed in a liquid colloid. A typical micelle in aqueous solution forms an aggregate with the hydrophilic "head" regions in contact with surrounding solvent, sequestering the hydrophobic tail regions in the micelle centre. This type micelle is known as anormal phase micelle (oil-in-water micelle). Inverse micelles have the head groups at the centre with the tails extending out (water-in-oil micelle). Micelles are approximately spherical in shape. Other phases, including shapes such as ellipsoisds, cylinders and bilayers are also possible. The shape and size of a micelle is a function of a micelle geometry of its surfactant molecules and solution condition such as surfactant concentration, temperature, pH, and ionic strength. The process of forming micelle is known as micellisation and - 33 - forms part of the phase behaviour of many lipids according to their polymorphism. I.8.1 Critical Micelle Concentration, C.M.C Amphiphillic molecules contain two distinct components, differing in their affinity for solutes. The hydrophilic part of the molecules has polar solutes, such as water, and the hydrophobic part of the molecule has an affinity for non-polar solutes, such as hydrocarbons. An molecules display distinct behavior when interacting with water. An amphiphillic molecule can arrange itself at the surface of the water polar interacts with the water and the non-polar part is held above the surface (either in the air or in a nonpolar liquid). The pre molecules on the surface disrupts the cohesive energy at the surface and thus lower the surface tension. The proportion of molecules present at the surface or as aggregate in the bulk of the liquid depends on the concentration of the amphiphiles. Amphiphiles will favour their arrangement on the surface, as the surface becomes crowded with amphiphiles more molecules aggregates. At some concentration the surface becomes completely loaded with amphiphile and any further additions leads to arrange aggregates. This concentration is called the Critical Micelle Concentration(C.M.C.). I.8.2 Surfactant The term surfactant is a blend of "surface active agent". It is usually organic compounds that are amphiphillic in nature they containing both hydrophoic (their "tails") and hydrophilic groups (their "heads"). Surfactants, are also known as tensides and wetting agents, that lower the surface tension of a liquid, allowing easier spreading, and also lower the interfacial tension between two liquids. - 34 - I.8.3 Classification A surfactant can be classified by the presence of formally charged groups in its head. A nonionic surfactant has no charge groups in its head. The head of an ionic surfactant carries a net charge. If the charge is negative, the surfactant is more specifically called anionic. If the charge is positive, it is called cationic. If a surfactant contains a head with two charged groups, it is termed as zwitterionic. Ionic Anionic Sodium dodecyl sulfate (SDS), ammonium lauryl sulfate and other alkyl sulfate salts. Sodium lauryl ether sulfate (SLES) Soaps, fatty acid salts Cationic (quaternary ammonium cations) Cetyltrimethylammonium bromide (CTAB) and other alkyl ammonium bromide. Cetylpyridinium chloride (CPC) Zwitterionic (amphoteric) Dodecyl betaine Dodecyl dimethylamine oxide Nonionic Alkyl poly (ethylene oxide) Fatty alcohals The ligand substitution reaction was studied in aqueous and micellar medium. The effect of ionic micelles on the rate of reactions can be disscused by considering that the free energy of activation controlling the - 35 - observed rate constant, has two components i.e. an intrinsic one, independent of the influence of the interfacial electrical potential and an electric component representing the contribution of the electrical component. Micellar catalysis, which is much simpler than enzyme catalysis [159], involves cations, anions and coordinating compounds. The types of kinetic catalytic methods have proved to be very appropriate for trace analysis. The best advantage of kinetic catalytic determinations (catalymetry) is the combination of very low determination limit (in some cases-in pictogram range), with a simple and available experimental technique, especially with spectrophotometric monitoring of the reaction rate [160]. Another way of improving the analytical features of kinetic methods is the use of organic micro heterogenous systems- surfactants. In the last two decades, surfactant molecules and their aggregates have been used increasingly in analytical techniques in order to alter the properties, including the reactivity, of the analytes [161]. Surfactant aggregates often accelerates or "catalyse" chemical reactions, but they also inhibit reactions. The term, "micellar catalysis",was first applied to the increase in rate of reactions produced by association colloids, in particular micelles [162]. Surfactants micelles can enhance the sensitivity and can bring about changes in solubility, pKa, chemical equilibria, reaction rates and intensities and the stereoselectivity of some chemical processes. There are a host of analytical methods involving micellar media. Most are equilibrium methods based on molecular absorption or emission spectroscopy; although micelle aggregates have also been utilized successfully with other techniques such as phosphorescence, atomic absorption and chemiluminescence [163,164]. - 36 - Surfactants increase the absorptivity of the analytes and some of them also facilitate solubilisation of the analytical system [163,165]. I.7.4 Kinetic approaches in micellar catalysis Numerous kinetic and catalytic kinetic methods involving micelles with the objective of determining trace metal ions have been investigated. Since a host of analytical kinetic methods are based on catalyzed reactions, it is important to determine the effect of micelles on these processes in order to be able to assess the analytical potential of these media. The variety of surfactants have been used for changing the kinetics of reactions and various kinetic models have been developed to describe micellar catalysis on chemical reactions [164-171]. Kinetic treatments have been developed most extesively for reactions in the presence of aqueous micelles. In the pseudo-phase kinetic model proposed by Berezin [162], reactions are assumed to take place in the bulk aqueous and micellar pseudophases. The micellised surfactant is in equilibrium with solutes throughout the reaction, and observed rates can be treated as the step of rates of concurrent reactions in each pseudo-phase. The pseudo-phase model in its various guises explains many features of micellar rate effects and it can be applied, atleast qualitatively, to reactions in a variety of colloidal assemblies such as reverse micelles and microemulsions. A large amount of kinetic data has been explained successfully in terms of the pseudo-phase model [168]. However, this model fails to explain some of the kinetic data obtained in the presence of charged micelles and it is inappropriate for accounting electrolyte effects on the rates of micelle-catalysed reactions [161,168]. Ion distributions have been modeled in different ways. One, proposed by Romsted [166], includes counterion distributions in the micellar Stern layer in buffered solution. - 37 - Romsted et al. developed pseudo-phase ion exchange (PIE) model [167169]; in that micellar surfaces are treated as selective ion exchangers saturated with counter ions. The number of assumptions and restrictive conditions required is larger in the PIE model than in the pseudo-phase model [168]. Jimenez et al. in their pseudo-phase model [170-175] or other models derived from it, such as the ion-exchange model [171,176-179], have been frequently and successfully applied to the interpretation of kinetic effects in micellar systems. These models describe the effect of micelles on reaction kinetics by fitting the experimental data to some equations containing several parameters, whose meanings are defined in the model. Thus, as it is well known, the pseudophase model for a first-order reaction (or for a second-order reaction if only one reactant is partitioned between the bulk and micellar pseudo-phases) results in the following equation: k obs k w k m K[M] 1 K[M] (16) kw being the rate constant of the process in the bulk (aqueous pseudophase), km the rate constant at the micellar pseudophase, and K the equilibrium constant for the binding of the reactant, S, to the micelles. Sw + M K Sm (17) Here, M is the micellized surfactant and S is the reactant being partitioned between the aqueous (w) and micellar (m) pseudo-phases. So, K is given by K [Sm ] [Sw ][M] (18) - 38 - The basic pseudo-phase model provided excellent qualitative and often good quantitative interpretation for many reactions [180]. Analysis of various micelle-catalysed reactions on the basis of pseudo-phase kinetic model ended with the conclusion that two principal factors are responsible for the efficiency of miceller catalysis i.e. concentration effects in the miceller pseudo-phase due to the hydrophobic, electrostatic and specific interactions of reactants with the micelles and changes in the reactivity of reagents on transfer from water to the miceller pseudo-phase. Surfactants are widely used in kinetic determination of metal ions given in Table 1.7. - 39 - Table I.7: Kinetic determination of metal ions in the presence of surfactants. S. N. Reaction Metal ion Surfactant Method 1 PGR + S2O82- Pb(II) 2 SA + KIO4, Activator Phen Fe(II) cationic DTAB Cationic CPC SA + KIO4 Mn(II) 3 4 5 6 7 8 cationic CTAB V(V), CPC cationic CPC Ni(II) + 5octyoxymethyl-8quinolinol Co(II) + 5octyoxymethyl-8quinolinol PGR + Cr(IV) PGR + V(IV) PGR + Ti(IV) Ni(II) Non-ionic Triton X100 Cr(II) V(II) Ti(II) cationic DTAB spectrophotom etric I-+BrO3-+6H+ 3I2+Br-+3H2O I2 + I- [I3][I3]- + H2O2 As(II) cationic DTAB indirectphoto metric cationic CPC catalytic determination spectrophotom of Mo in etric water soil extracts catalytic spectrophotom etric Catalytic FIA - Co(II) Mo(VI) Hexacyanoferrate (II) + Phenol Hg(II) Anionic SDS 10 12phosphomolybdate + ascorbic acid SA + KIO4, Activator Phen Sb(III) Non-ionic Triton X100 cationic CTAB Nuclear Fast Red + KIO4 Cu(II) 12 catalytic photometric catalytic determination spectrophotom of serum iron etric catalytic spectrophotom etric catalytic spectrophotom etric spectrophotom etric - Catechol +pphenetidine 9 11 Application Mn(II) nonionic Tween-20 Ref 181 182 183 184 185 catalytic spectrophotom etric - 186 187 188 - 189 190 191 192 - catalytic Serum and spectrophotom urine etric 193 - 40 - 13 MBTH +DAOS; H2O2; activator pyridine Abse + B +S2- Cu(II) BDTAC catalytic FIA Cu(II) 15 o-Toluidine blue + KIO4 Rh(III) cationic CTAB cationic CTAB 16 PGR + IO4- Pd(II) anionic 17 Au(II) + PDR Pd(II) + PDR Au(II) Pd(II) nonionic catalytic FIA catalytic spectrophotom etric catalytic spectrophotom etric HPSAM 14 inj-35 18 Fe(III) + Co(II) Fe(II) + Phen Fe(II) + Phen Fe(III) Fe(II) cationic CTAB HPSAM 19 Chrome Azurol S + Be(II) Chrome Azurol S + Be(II) Be(II) Al(III) cationic CTAB HPSAM determination of Cu in pepperbush Water 194 Water 196 three different hydrogenation catalysts simultaneous determination of Au(III) and Pd(II) in jewellery and synthetic samples determination of Fe(II) and Fe(III) in synthetic mixtures determination of Be(II) and Al(III) in environmental , geochemical and alloy samples 197 195 198 199 200 PGR – Pyrogallol Red; SA sulfanilic acid; Phen – 1, 10 phenahthroline; DTAB – dodecyltrimethylammonium bromide; CTAB - cetyltrimethylammonium bromide; BDTAC – benzyldimethyltetradecylammonium chloride; CPC - cetylpyridinium chloride; SDS – sodium dodecyl sulfate; MBTH – 3-methyl-2-benzothiazolinone hydrazone; DAOS – n-ethyl-n-(2-hydroxy-3-sulfopropyl)-3,5dimethoxyaniline; PDR – 5(p-dimethlaminobenzylidene)rhodanine; HPSAM – H-point standard addition methyl. - 41 - I.9 Electron Transfer Reactions or Redox Reactions There have been number of studies on the electron transfer reaction on [Fe(CN)6]4-, [Fe(CN)5L]3- and [Rh(NH3)5L]2+ and [S2O8]2- [201-204]. The reducing agent and the oxidizing agent would simply bump into each other and electron transfer would take place. Reactions in solution are complicated. However, by the fact that the oxidized and educed species are often metal ions surrounded by shields of ligands and solvating molecules. Electron transfer reactions involving transition metal complexes have been divided into two broad mechanistic classes called outer sphere and inner sphere. Here we discuss the detailed study of electron transfer reactions through outer sphere mechanism. I.9.2 Outer-Sphere Mechanism The coordination sphere of both metals remains intact and the electron transfer occurs when the two complexes come in close proximity. Widely studied was the self-exchange reaction, where the left and right hand sides of the equation are identical; only electron transfer, and no net chemical reaction takes place. The values of ∆G for such reaction is zero but actvation energy is needed. The reaction is in certain respect analogous to the water exchange reaction in octahedral complexes. I.9.2 The Frank-Condon principal A molecular electronic transition is much faster than a molecular vibration. During electron transfer, the nuclei are essentially stationary and so electron transfer between redox partners with differing bond lengths can only occur between vibrationl excited states with identical structures. This - 42 - reductant-oxidant pair is called counter or precurser complex . The greater the changes in the bond lenghts required to reach the encounter complex, that is, the greater the structural differences between the two complexes containing the metal in different oxidation states, the higher the oxidation activation energy and the slower the rate of electron transfer. The general reaction scheme involved three step mechanism: (19) (20) (21) The precursor complex formation (equilibrium constant K) is followed by a rate-determining step electron transfer step (rate constant ket), followed by rapid diffusion apart of the successor complex partners. - 43 - References 1. Atkins, P. W. Physical Chemistry 4thEdn, Freeman New York(1990). 2. Cotton F.A., Wilkinson G. and Gaus Paul L. “Basic Inorganic Chemistry” IIIrd Edition (1995), 187. 3. Basolo, F. Coord. Chem. Rev. (1990), 100, 47 and references there in. 4. Van Eldik, R. Coord. Chem. Rev. (1999), 182, 373 and references there in. 5. Van Eldik, R. and Hubbard Collin D. J.Coord. Chem. (2007), 60(1), and references there in. 6. Longford, C. H.; Gray, H. B. “Ligand Substitution Processes,” Bejamin Menlo Park, CA, (1984). 7. Twigg, M. V.; Ed. “Mechanisms of Inorganic and Organometallic Reactions”, Vols. 1and 2, Plenum, New york, (1982) and (1984). 8. Hughes, E. D.; Ingold, C. K. “Structure and Mechanism in organic Chemistry,” Cornell Univ. Press, Ithca, New York, (1953), Chapter. V. 9. Longford, C. H.; Gray, H. B. “Ligand Substiution Processes,” Benjamin, W. A. Inc., New York 1965, Chapter, 1. 10. Hue, J. E.; Keiter, E. A. and Keiter, R. L. “Inorganic Chemistry” 4th Ed., (2002), 548. 11. Eigen, M. and Tamm, K. Electrochem Z. (1962), 66, 93. 12. Eigen, M. and Tamm, K. Electrochem Z. (1962), 66, 107. 13. Eigen, M. Pure and Applied. Chem. (1963), 6, 97. 14. Wilkins, R. G. The study of kinetics and Mechanism of Transition Metal Complexes, Allyn and Bacon, Boston, (1974). - 44 - 15. Chaturvedi, D. D. Ph.D. Thesis, Lucknow University, Lucknow, India (1996). 16. Read Rita, A. and Margerum, D. W. Inorg. Chem. (1981), 20, 3143. 17. Bannister, C. E. and Margerum, D. W. Inorg. Chem. (1981), 20, 3149. 18. Raychebra, J. M. T. and Margerum, D. W. Inorg. Chem. (1981), 19, 497. 19. Burgess, J. Chem. Soc., Dalton. Trans. (1974), 2032. 20. Lucie, J. M. and Stranks, D.R. J .Chem. Soc., Dalton Trans. (1975), 245. 21. Coombs, L. C.; Margerum, D. W. and Nigam, P. C. Inorg. Chem. (1970), 9, 2081. 22. Swift, T. J. and Connick, R. E. J. Chem. Phys. (1962), 37, 307 and (1964), 41, 2553. 23. Hunt, J. P.; Dodgen, H. W. and Klanberg, F. Inorg. Chem. (1963), 2, 478. 24. Basolo, F. and Pearson, R. G. "Mechanism of Inorganic Reactions", John Wiley, New York, (1967), 2nd Ed., Chap.3. 25. Armor, J. N. and Taube, H. J. Am. Chem. Soc. (1970), 92, 6170. 26. Allen, R. J. and Ford, P. C. Inorg. Chem. (1972), 11, 679. 27. Johnson, C. R. and Sheperd, R. E. Inorg. Chem. (1983), 22, 1117. 28. Gentil, L. A.; Navaza, A,; and Olabe, J. A. Inorg. Chem. (1984), 23, 4297. 29. Gentil, L. A; Zerga, H. O. and Olabe, J. A. J .Chem. Soc., Dalton Trans. (1986), 2731. 30. Hoddenbagh, J. M. A. and Macartney, D. H. Inorg. Chem. (1986), 25, 2099. - 45 - 31. Tokman, A. L.; Gentil, L. A. and Olabe, J. A. Polyhedron (1986), 8, 2091. 32. Olabe, J. A.; Zerga, H. O. and Gentil, L. A. J. Chem. Soc., Dalton Trans. (1987), 1267. 33. Lergos, J. C. R. Hebd Seances Acad .Sci. (1959), 248, 1339. 34. Baran, Y. Trans. Met. Chem. (2000), 25, 41. 35. Toma, H. E. and Batista, A. A. J. Inorg. Chem. (1984), 20, 53. 36. Toma, H. E. and Malin, J. M. Inorg. Chem. (1973), 12, 2080. 37. Crean, F. M. and Schug, K. Inorg. Chem. (1984), 23 , 853. 38. Toma, H. E.; Malin, J. M. and Giesbrecht, E. Inorg. Chem. (1973), 12, 2084. 39. Macartney, D. H. and McAuley, A. Inorg. Chem. (1981), 20, 748. 40. Toma, H. E.; Martins, J. M. and Giesbrecht, E. J .Chem. Soc., Dalton Trans. (1978), 1610. 41. Bradic, Z.; Pribanic, M. and Asperger, S. J .Chem. Soc., Dalton Trans. (1975), 353. 42. Katz. N. E.; Blesa, M. A. Olabe, J. A. and Aymonino, P. J. Inorg.Chim. Acta, (1978), 27, L65. 43. Blesa, M. A.; Olabe, J. A. and Aymonino, P. J. J. Chem. Soc., Dalton Trans. (1976), 1196. 44. Bowers, M. L. Kovacs, D. and Shpherd, R. E. J. Am. Chem. Soc.(1977), 99, 45. 6555. Blesa, M. A.; Funai, I. A.; Morand, P. J.; Olabe, J. A.; Aymonino, P. J. and Ellenriender, G . J. Chem. Soc., Dalton Trans. (1977), 845. 46. Katz. N. E.; Blesa, M. A.; Olabe, J. A. and Aymonino, P. J. J. Chem. Soc., Dalton Trans. (1978), 1603. - 46 - 47. Coelho, A. L.; Moreia, I S.; Miyuel, A. B. and Aranj, D. E. Polyhedron (1994), 13, 1015. 48. Johnson, C. R. and Shepherd, R. E. Inorg. Chem. (1983), 22, 2439. 49. Henderson, W. W. and Shepherd, R. E. Inorg. Chem. (1985), 24, 2439. 50. Davies, G; Garafalo, A. R. Inorg. Chim. Acta (1976), 19, L3. Inorg. Chem. (1976), 15, 1101. 51. Hoddenbagh, J. M. A. and Macartney, D. H. Inorg. Chem. (1986), 25 380. 52. Zwickel, A. M. and Creutz, C. Inorg. Chem. (1971), 10, 2395. 53. Bernhard, P.; Helm, L.; Repaport, I.; Ludi, A. and Merbach, A. E. J. Chem. Soc. Chem. Commun. (1984), 302. 54. Surifi, J. J. and Conrick, R. E. J. Chem. Phys. (1962), 37, 307. 55. Matsubara, J. and Creuz, C. Inorg. Chem (1979), 18, 1956. 56. Shepherd, R. E.and Toube, H. Inorg. Chem (1973), 12, 1392. 57. Ised, S. S. and Taube, H. Inorg. Chem. (1976), 15, 3070. 58. Malin, J. C. and Koch, R. C. Inorg. Chem. (1978), 17, 752. 59. Hicks, K. W. and Chapple, C. A. Inorg. Chem. (1980), 19, 1623. 60. Jordan, J. and Ewing, G. J. Inorg. Chem. (1962), 1, 587. 61. George, P.; Hanina, G. I. H. and Irvine, D. H. J. Chem. Soc. (1959), 2548. 62. Peteron, S. H. and Demas, J. H. J. Am. Chem. Soc.(1976), 98, 7880. 63. Sundberg, R. J.; Bryan, R. F.; Taylor,I. F. and Taube, H. J. Am. Chem. Soc.(1974),96, 381. 64. McCleverty, J. A. Chem. Rev. (1979), 79, 53 and refrences therein. 65. Lunak, S.; Veprek-Siska, Coll. Czech. Chem. Commun. (1974), 39, 2719. - 47 - 66. Frances, M. C. and Kenneth, S. Inorg. Chem. (1984), 23, 853 and refrences therein. 67. Bendix, J.; Steenberg, P. and Sotofte, I. Inorg. Chem. (2003), 42, 4510. 68. Uan-Eldix, R., Ed. Inorganic High Pressure Chemistry : Kinetics and Mechanisms, Elsevier, Amsterdam, (1986). 69. Drljaca, A.; Hubbard, C.D.; van Elddik R.; Asane, T.;Basilevsky, M. V. and LeNoble, W. J. “Activation Volume in Solution”, Chem. Rev. (1998), 98, 2178. 70. Alsheri, S. and Burgess, J. Inorg. Chem. Acta. (1991), 181, 153. 71. Al-Alousy, A.; Alsheri, S. Burgess, J.; Del Mar Gracini M.; Moya, M. L.; Munoz, E.; Rodriquez, A. and Sancchez, F. Trans. Met. Chem. (1993), 18, 179. 72. Alsheri, S.; Burgess, J.; van Eldik R. and Hubbard, C. D. Inorg. Chim. Acta. (1995), 240, 305. 73. Barrios, M. D. M.; Gracini, J. R.; Munoz, E,; Sanchez, F.; Moya, M. L.; Alsheri, S. and Burgess, J. Trans. Met. Chem. (1992), 17, 231. 74. Keddy and van Eldik R. Inorg. Chem. (1991), 30, 596. 75. Stochel, G.; Challas, J.; Marinez, P. and van Eldik R. Inorg. Chem. (1992), 31, 5480. 76. Stochel, G. and van Eldik R. Inorg. Chim. Acta. (1991), 30, 596. 77. Stochel, G. and van Eldik R.; Hejmo, E. and Stasicka, Z. Inorg. Chem. (1988), 27, 2767. 78. Blandamer, M. J. and Burgess, J. Pure and App. Chem. (1983), 55, 55. 79. Kumar, K. and Nigam, P. C. J. Phys. Chem. (1980), 84, 140 and references therein. - 48 - 80. Naik, R. M. and Nigam, P. C. Trans. Met. Chem. (1986), 11, 11. 81. Bajaj, H. C.; Phull, M. and Nigam, P. C. Bull. Chem. Soc. Jpn. (1984), 57, 564. 82. Pleskowicz, J. C. and Billo, E. J. Inorg. Chim. Acta. (1985), 99, 149. 83. D'amiello, M. A.; Mocella, M. T.; Barefield, E. K. and Paul, J. C. J. Am. Chem. Soc.(1975), 97, 192. 84. Steinman, W. and Kaden, T. A. Helv. Chum. Acta (1975), 58, 1358. 85. Hinz, H. P. and Margerum, D. W. Inorg. Chem. (1974), 13, 2941. 86. Hertle, L. and Kaden, T. A. Chima. Switerzerland (9750), 29, 304. 87. Barefield, E. K. and Mocella, M. T. J. Am. Chem. Soc.(1975),97, 4238. 88. Naik, R. M. and Nigam, P. C. Inorg. Chim. Acta. (1986), 114, 55. 89. Mishra, P.; Naik, R. M. and Nigam, P. C. Inorg. Chim. Acta. (1987), 127, 71 and refrences therein. 90. Naik, R. M.; Chaturvedi, D. D.; Srivastava, N.; Verma, A. K.; Tiwari, A. K. and Agarwal, A. Ind. J. Chem. (2004), 43A, 2307. 91. Prasad, S.; Nigam, P. C. and Naik, R. M. Trans. Met. Chem. (1990), 15, 58. 92. Cross, R. J. Chem. Soc. Rev. (1985), 14, 197. 93. Lin, L. T.; Rorabacher, D. B.; Cayley, G. R. and Margerum, D. W. Inorg. Chem. (1975), 14, 197. 94. Kokski, G. B. and Margerum, D. W. Inorg. Chem. (1969), 8, 1129. 95. Crouse, W. C. and Margerum, D. W. Inorg. Chem. (1974), 13, 1437. 96. Edmund, K. and Waldermar, N. Trans. Met. Chem. (1987), 12, 546. 97. Rorabacher, D. B. and Margerum, D. W. Inorg. Chem. (1964), 3, 382. 98. Kumar, K. and Nigam, P. C. Inorg. Chem. (1981), 20, 1623. - 49 - 99. Naik, R. M. and Nigam, P. C. Ind. J. Chem. (1987), 26A, 205. 100. Bajaj, H.C. and Nigam, P. Ind. J. Chem. (1981), 19A, 1070. 101. Bajaj, H.C. and Nigam, P. Ind. J. Chem. (1984), 23A, 8. 102. Funahashi, S. and Tanaka, M. Chem. Abst. (1974), 80, 33411g. 103. Tabata, M. and Tanaka, M. Inorg. Chem. (1978), 17(10), 2779. 104. Hittoshi, K. Chem. Abst. (1977), 87, 123307j. 105. Carr, J.D. and Olson, V.K. Inorg. Chem. (1975), 14(9), 2168. 106. Kodama, M. and Hagiya, K. Bull. Chem. Soc., Jpn, (1973), 46(10), 3151. 107. Funahashi, S.; Tabata, M. and Tanaka, M. Bull. Chem. Soc., Jpn, (1971), 44, 1586. 108. Fujisawa, T. and Tanaka, M. Chem. Abst. (1972), 76, 63982m. 109. Kodama, M. and Oyama, N. Bull. Chem. Soc., Jpn, (1972), 45, 2169. 110. Rabenstein, D.L. and Fuhhr, B.J. Inorg. Chem. (1972), 11, 2430. 111. Katsuyama T. and Kumai T. Bull. Chem. Soc., Jpn, (1978), 51, 1072. 112. Kodama, M. Chem. Abst, (1970), 73, 18953C. 113. Kodama, M.; Fujii, Y. and Uede, T. Bull. Chem. Soc., Jpn, (1970), 43, 2085. 114. Funahadhi, S. and Tanaka, M. Inorg. Chem. (1970), 9, 2092. 115. Kodama, M.; Karsawa, S. and Watanabe, T. Bull. Chem. Soc., Jpn, (1971), 44, 1815. 116. Steinhaus, R.K. Inorg. Chem. Acta.(1982), 63, 1. 117. Gerhard, G. and Horst, E. Inorg. Reac. Mech. (2002), 3, 221. 118. Gerhard, G. and Horst, E. Inorg. Chim. Acta, (2003), 342, 97. 119. Burmazovic, I.; Hamza, I.; Mohammed, S.A. and Eldik, R.van, Inorg. Chem. (2002), 40, 5150. 120. Gabor, I. and Istvan, F. Inorg. Chem.(2002),41, 1306. - 50 - 121. Thorsten, S., Sabina, S., Achim, Z., Peter T. and Eldik, R. Van, Inorg. Chem. (2001), 40, 3670. 122. Tiwari, A.K., Ph.D. Thesis, Lucknow University, Lucknow, India (2005). 123. Mentasti, F., Pelizzetti, E., Talanta (1975), 22, 930. 124. Gupta, N. and Nigam, P.C., J. Inorg. Chem. (1989), 5, 61. 125. Naik, R. M. Int. J. Chem. Kinet. (2005), 37,333. 126. Mentasti, E. J. Chem. Soc., Dalton Trans. (1984),903. 127. Bydalek, T. J.; Stokich, T.M.; and Coleman, D. M. Inorg. Chem. (1970), 9, 29. 128. Steinhaaus, R.K.; and Erikson, S. H. Inorg. Chem. (1980), 1913 and references therein. 129. Bruker, E.; and Laurenczy,G. Inorg. Chem. (1983), 22, 338. 130. Mentasti, E. J. Chem. Soc., Dalton Trans (1980), 958. 131. Stara, V., kopanica, M.Coll. Czech. Chem.Commun. (1973), 38, 2581. 132. Stehl, R. H.; Margerum, D. W.; and Latterell, J. L. Anal. Chem. (1967), 39, 1346. 133. Margerum, D. W.; Steinhaaus, R. K.; Anal. Chem. (1965), 37, 222. 134. 134. Margerum, D. W. Stehl, R. H. Anal. Chem. (1967), 39, 1351. 135. Margerum, D. W. and Carr, J. D. J. Am. Chem. Soc. (1966), 88, 1639, 1645. 136. Mattols, H. A. “Kinetic Aspects of Analytical Chemistry” Wiley Interscience, New York (1988). 137. Pavlovic, D. and Asperger, S. Anal. Chem. (1967), 39, 1351. 138. Kars, U. and Pinter, T. Croat. Chem. Acta (1958), 30, 141. 139. Kraljie, J. Mikrochim Acta (1960), 586. - 51 - 140. Phull, M.; Bajaj, H.C. and Nigam, P. C. Talanta (1981), 28, 610. 141. Prasad, S. AnalLett. (2004), 37, 2851. 142. Gadia, M. K.; Mehra, M. C. Microchem J. (1979), 68, 17. 143. Mehra, M. C.; Garvie, R. J. Less-Common Metals (1979), 68, 17. 144. Mehra, M. C.; Satake, M. and Katyal, M. Indian J. Chem. (1984), 23A, 860. 145. Datta, K. and Das, J. J. Indian Chem. Soc. (1974), 51, 553. 146. Raman, S. Indian J. Chem. (1975), 13, 1229. 147. Hadjiioannou, T. P. Anal Chim. Acta (1966), 35, 351. 148. Yatsimirskii, K. B. and Orlova, M. N. Z. Neorg. Chim. (1959), 4, 741. 149. Kars, U. and Pinter, T. Croat. Chim. Acta (1961), 33, 69. 150. Kars, U. and Pinter, T. Croat. Chim. Acta (1962), 34, 249. 151. Kraljie, J. Maye, M. Croat. Chim. Acta (1956), 28, 259. 152. Lis, N. D. and Katz, N. E. Anals. Asoc. Quim., Argentina (1981), 23A, 616. 153. Reddy, B. R. and Raman, S. Ind. J. Chem. (1984), 23A, 616. 154. Srikantamurthy, B. R. M. Phil. Dissertation, I.I.T., Kanpur, India (1981). 155. Funahanshi, S.; Tabata, M. and Tanaka, M. Anal Chim. Acta (1971), 57, 31. 156. Tabata, M. and Tanaka, M. Anal. Chem. (1960), 13(A6), 427. 157. Tabata, M. and Tanaka, M. Mikrochim Acta II (1982), 149. 158. Bydalek, T. J. and Margerum, D. W. Inorg. Chem. (1962), 1, 852. 159. Stoyanova, A. and Aexiev, A. Tarkia J. Sci. (2005), 3(2), 1. 160. Prez-Bendito, D. and Silva, M. Kinetic Methods in Analytical Chemistry, Hor-ood Chichester (1998). - 52 - 161. Carreto, M. L.; Rubio, S. and Perez- Bendito, D. Analyst, (1990), 121, 33R. 162. Berezin, I.V.; Martinek, K. and Yatsimirski, A. K. Russ. Chem. Rew., (1973), 42, 787. 163. Pramauro, E. and Pelizzetti, E. Wilson and Wilson’s, Comprehensive Analytical Chemistry, (1996), 31. 164. Prez-Bendito, D. and Rubio, S. Trends Anal. Chem. (1993), 12, 9. 165. Burguera, J. L. and Burguera, M. Talanta (2004), 64, 1099. 166. Romsted, L. S. Mittal, K. L. (Ed.); Plenum Press: New York, Vol. 2, (1997). 167. Chaimovich, H.; Aleixo, R. M. V. Cuccovia, J. Zanettie, D. and Quina F. H. Plenum Press, New York, (1977),Vol. 2. 168. Khan, M.N. Encyclopedia of Surface and Collid Science. Marcel Dekker, (2002), 3178. 169. Bunton, C. A.; Nome, F.; Quina, F. H.and Romested, L. S. Acc. Chem. Res. (1991), 24,357. 170. Jimenez, R.; Bueno, E.; Cano, I.; Corbacho, E.; Fernandez, M. E.; Gomez, L.; Graciani, J.; Hernandez, M.; Matitos, M. T.; Ortiz, J.; Loepez-Cornejo, p. and Prado-Gotor, R. Int. J. Chem. Kinet. (2004), 36, 627. 171. Bunton, C. A. J. Mo.l Liq. (1997), 72, 231. 172. Bunton, C. A.; Romsted, L. S. and Sepulveda, L. J. Phys Chem. (1980), 84, 2611. 173. Gonsalves, M.; Probst, S.; Rezenede, M. C.; Nome, F.; Zucco, C. and Zanette, D. J. Phys. Chem. (1985), 89, 1127. 174. Ortega, F. and Rodenas, E. J. Phys. Chem. (1986), 90, 2408. - 53 - 175. Marin, M. A. B.; Nome, F.; Zanette, D.; Zucco, C. and Romested, L. S. J. Phys Chem. (19950), 99, 10879. 176. Romested, L. S. In Micellization, Sloubilization and Microemulsions; Mittal, K. L. (Ed.); Plenum: New York, (1977), Vol.2. 177. Martinek, K.; Yatsimirski, A. K.; Levashov, A. and Berezin, I. In Micellization, Sloubilization and Microemulsions; Mittal, K. L. (Ed.); Plenum: New York, (1977), Vol. 2. 178. Quina, F. H. and Chaimovich, H. J. Phys Chem. (1979), 83, 1844. 179. Romested, L. S. In Surfactants in Solutions; Mittal, K. L.; Lindman, b. (Ed.); Plenum: New York, (1984), Vol. 2, 1015. 180. Bunton, C. A.; Romsted, L. S. and Yao, Curr. Opin. J. Colloid Interface Sci. (1997), 2, 622. 181. Sicilia, D.; Rubio, S. and Perez-Bedito, D. Anal Chim. Acta (1992), 266, 43. 182. Alexiev, A.; Rubio, S.; Deyanova, M; Stoyanova, A.; Sicilia, D. and Perez-Bedito, D. Anal Chim. Acta (1994), 295, 211. 183. Stoyanova, A. M. and Alexiev, A.A. Analytical Laboratory (1997), 6, 79. 184. Lunar, L.; Rubio, S. and Preez-Bendito, D. Anal. Chim. Acta, (1990), 237, 207. 185. Tagashira, S.; Onoue, K.; Murakami, Y. and Sasaki, Y. Anal. Sci. (1992), 8, 307. 186. Sicilia, D.; Rubio, S.and Preez-Bendito, D. Anal.Chim. Acta, (1993), 284, 149. 187. Sicilia, D.; Rubio, S.and Preez-Bendito, D. Anal.Chim. Acta, (1994), 297, 453. 188. Sicilia, D.; Rubio, S.and Preez-Bendito, D. Anal. Chem, (1994), 342, 327. - 54 - 189. Lunar, M.L.; Rubio, S. and Preez-Bendito, D., Analyst (1993), 118(6), 715. 190. Sicilia, D.; Rubio, S.and Preez-Bendito, D. Talanta (1991), 38, 1147. 191. Sicilia, D.; Rubio, S. and Preez-Bendito, D. Anal. Chem (1992), 64, 1490. 192. Stoyyanova, A. and Alexiev, A. Sciencetific Work of the Medical University of Eleven, XVI (1996), 3. 193. Bin Du, Yan Liu, Yulong Ding, Caihong Duan and Oin Wei Anal. Lett. (2005), 38,711. 194. Kawashima, T.; Itabashi, H.; Teshima, N.; Kurihara, M. and Nakano, S. Anal. Sci. (1999), 15, 835. 195. Safavi,A.; Mirzaee, M.; Hormozi Nezhad, M. R. and Saghir, N. Spectrosc. Lett. (2005), 38, 13. 196. Ensafi, A. A. and Keyvanfard, M. J. Anal. Chem. (2003), 58,106. 197. Ensafi, A.A. and Keyvanfard, M. Anal. Lett. (2002), 35, 423. 198. Pourreza, N. and Rastegarzadeh, S. Anal. Chim Acta (2001), 437, 273. 199. Safavi,A.; Abdollahi, H.and Hormozi Nezhad, M. R. Talanta (2002), 56, 699. 200. Afkhami, A. and Zarei, A. R. Anal. Sci. (2004), 20, 1711. 201. Miralles, A. J.; Armstrong, R. E. and Haim, A. J. Am. Chem. Soc. (1977), 99, 1416. 202. Sutin N in Inorganic Chemistry, vol.2, Ed. Eichhorn, G. L. Elsevier Science, Amsterdam. (1973), 611. 203. Furholz, U. Haim, A. Inorg.Chem. (1987), 26, 3243. 204. Xiang-Li, M.; Cai-Xia, Y.; Qing-Hua, Z.; Miao-Yu, L. Yang, P. Chinese J. Chem. (2004), 22, 841. - 55 -