Download INTRODUCTION

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)
102.402.1
S (JK–1
mol–1)
20.98.3
Ref.
100.31.7
8.34.2
27
6.79
92.80.8
-12.54.2
27
Me2SO
1.77
93.67.1
-20.920.9
N–
1.57
27
27
Mepyz+
107.01.3
29.216.7
27
92.82.9
–8.38.3
27
104.91.2
16.74.2
27
1021.7
–4.24.2
27
11.40
832
335
34
tri
51.20
781
181
34
PCI
tri
10.10
922
161
34
PyCN
tri
18.10
902
151
34
isox
tri
20.60
861
192
mtr
tri
52.20
801
212
tt
tri
68.40
881
222
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
kd104,
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 25C
Complex
kex,s–1
Ref.
Ru(H2O)62+
1.410–2
53
Fe(H2O)62+
4.4106
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+
2.010–2
57
Cis-Ru(NH3)4(CO)OH22+
4.010–6
57
Ru(terpy)(bpy)OH22+
7.010–5
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
+19.00.5
+130.7
+11.41.0
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
+16.31.4
+13.60.5
+10.40.5
+9.60.8
+17.90.4
+11.41.0
+19.01.0
+0.90.5
+12.51.2
+20.90.5
Fe(CN)5(4-CNpy)3– + CN– 
4–
Fe(CN) 6 +4CNpy
H2O
+20.90.5
73
H2O
H2O
H2O
H2O.
H2O
+16.40.6
+24.01.0
+16.31.5
+17.41.4
+18.50.6
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
+15.50.7
+17.00.4
+17.90.6
+14.10.4
+16.40.6
+16.80.2
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
+2.50.5
+9.51.2
+12.81.1
+20.11.0
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  ML + 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 + ML  ML + ML
(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]
 k2 [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- + NN
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 -
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