Download unit iii kinetics and mechanism of reactions in metal complexes

M.Sc CHEMISTRY – SEMESTER 2 - INORGANIC CHEMISTRY
UNIT III
KINETICS AND MECHANISM OF REACTIONS IN METAL COMPLEXES
The kinetics and mechanisms of the reactions of transition metal complexes has not
been fully understood by chemists. This can be attributed to the inherent difficulties
involved in sytematising the reactions of a no: of elements, in contrast to organic chemistry.
Even attempts to predict from one element to another in the same group are not always
successful. However, different types of reactions have been classified for metal complexes.
THERMODYNAMIC AND KINETIC STABILITY
The kinetics and mechanisms of metal complexes can be better understood by
making a distinction between the thermodynamic terms stable and unstable and the kinetic
terms labile and inert. For example, consider the cyano complexes, [Ni(CN)4]2-, [Mn(CN)6]3and [Cr(CN)6]3-. Though these complexes are stable from thermodynamic point of view,
kinetically they are different. Rates of exchange of radiocarbon labeled cyanide (for cyanide
exchange reaction) vary much. [Ni(CN)4]2- exchanges cyanide ions rapidly (t1/2 ≈ 30 s),
[Mn(CN)6]3- exchanges moderately (t1/2 ≈ 1 h) and [Cr(CN)6]3- is somewhat inert (t1/2 ≈ 24
days).
Generally, complexes that react completely within one minute at 25°C can be
considered labile and those that take longer are considered inert. [Ni(CN)4]2- is a good
example of a thermodynamically stable complex that is kinetically labile. The lability of
four coordinate Ni2+ complexes can be associated with the ready ability of Ni2+ to form fiveor six- coordinate complexes. The additional bond energy of the fifth or sixth bond in part
compensates for the loss of ligand field stabilization energy. Example for a kinetically inert
complex that is thermodynamically unstable is the [Co(NH3)6]3+ cation in acid solution.
Several days are required at room temperature for degradation of the complex despite the
favourable thermodynamics. The inertness of the complex is considered to be from the
absence of a suitable low energy pathway for the acidolysis reaction. The reaction for
[Co(NH3)6]3+ must involve either an unstable seven coordinate species or five coordinate
species with concomitant loss of energy and LFSE.
Valence Bond Theory
Several explanations have been put forward for the explanation of lability and
inertness of complexes. According to Valence Bond Theory, the octahedral complexes are
of two types; 1) outer orbital complexes which use sp3d2 hybridisation and 2) inner orbital
complexes which use d2sp3 hybridisation. Outer orbital complexes are generally labile. This
weakness is correlated to the weakness of the bonds of sp3d2 as compared to d2sp3 bonds. In
the case of inner orbital complexes, if all the three t2g levels are filled either singly or
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doubly, then they are inert kinetically. If an inner orbital complex contains one or two
electrons in the t2g set, then atleast one level will be vacant. For example, in [V(H2O)]3+, two
of the three t2g levels are singly occupied. Hence the third level can be used to accept the
electron pair donated by the incoming ligand to form a 7-coordinated intermediate which is
less stable. To get itself stabilised, one of the original six ligands is expelled leading to a
substitution product and hence [V(H2O)]3+ is labile. But in [Cr(H2O)]3+, there is no d level
vacant to accept the electron pair donated by the incoming ligand, hence this complex is
inert.
Taube’s Explanation
According to Taube, the degree of lability or inertness of a transition metal complex can be
correlated with the d electronic configuration of the metal ion. If a complex contains
electrons in the antibonding eg* orbitals, the electrons are expected to be weakly bound and
easily displaced; it is labile. If the metal contains an empty t2g orbital, the four lobes of that
orbital correspond to directions from which an incoming ligand can approach the complex
with relatively little electrostatic repulsion. Therefore it can be concluded that a complex
with one or more eg* electrons or with fewer than three d electrons should be relatively
labile and that a complex with any other electronic configuration should be relatively inert.
Crystal Field Theory
In Crystal Field Theory, Crystal Field Activation Energy is defined as the change in
the Crystal Field Stabilisation Energy when the reacting complex is transformed into the
transition state.
CFAE = CFSE of intermediate – CFSE of reacting complex
Thus it infers that octahedral complexes having negative or zero CFAEs would therefore be
labile whereas those having positive CFAEs would be slow to react.
Octahedral complexes with metal ion configurations of d3 and spin paired d5 and d6 and to
some extent d4 would be slow to react by SN1 and SN2 mechanism (strong field causes
pairing of electron spins). Similarly, octahedral complexes with metal ion configuration of
d8 would be slow to react whatever be the strength of the ligand field and whatever be the
mechanism of substitution reaction.
Octahedral complexes with metal ion configurations of d0, d1, d2 spin free d4, d5,d6, d7, d9
and d10 configurations have negative or zero CFAEs and would therefore be labile.
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NUCLEOPHILIC
COMPLEXES
SUBSTITUTION
REACTIONS
IN
SQUARE
PLANAR
Square planar complexes with d8 configurations undergo substitution reactions of the
type,
in which Y is the entering nucleophilic ligand, X is the leaving ligand and T is the
ligand trans to X.
MECHANISM
Pt(II) complexes are widely used for studying the mechanism and kinetics since the
substitutions are comparatively slow and hence easier to study. From kinetic studies
scientists have arrived at an associative SN2 mechanism for substitution reactions in square
planar complexes. Consider a nucleophile Y attacking a d8 complex from either side of the
plane. In addition to being attracted to the electron deficient metal centre, the ligand
experiences repulsion from the filled metal d orbitals and from the bonding electrons.
However, it coordinates to the metal through an empty p z orbital to form a square pyramidal
species, though electronic repulsions as well as steric factors slow the attack. Once formed,
the square pyramidal species will undergo a transformation to a trigonal bipyramidal
structure. It will have three ligands (Y, T and X) in its equatorial plane and two of the
groups that were trans to each other in the original complex will occupy the axial positions.
As X departs from the trigonal plane, the T-M-Y angle opens up and the geometry will pass
through a square pyramid on its way to the square planar product.
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The trigonal bipyramidal species that forms during the reaction may exist either as an
activated complex or as a true intermediate. The distinction between the two depends on the
lifetime of the species. The term activated complex refers to the configuration of the
reactants and products at a peak in the reaction profile energy curve. i.e at the transition
state. The term intermediate implies that a species has a detectable lifetime (though short)
and that it is at least somewhat more stable than any activated complexes.
Reaction coordinate/energy profile for a square planar substitution reaction having a)
trigonal bipyramidal activated complex and b) a trigonal bipyramidal intermediate.
Evidences for the mechanism
The mechanism of substitution reactions of square planar complexes appears to be
associative SN2 rather than dissociative SN1. The evidences for this are,
1. In the case of square planar complexes of Ni(II), Pd(II) and Pt(II), five empty
orbitals of comparable energy can be made available for bonding of which four are
used up for bonding with ligands. The fifth orbital can easily accommodate electrons
from the attacking ligand forming a five coordinate intermediate. i.e an associative
SN2 mechanism.
2. There exists a parallelism between the reactivity of the square planar complexes of
Ni(II), Pd(II) and Pt(II) complexes (Ni(II) > Pd(II) > Pt(II)) and their ease with
which these expand their coordination no: which indicates the formation of an
intermediate with a higher coordination no:.
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3. The rates of aquation reactions of [PtCl4]2-, [PtCl3NH3]-, cis [Pt(NH3)2Cl2] and
[Pt(NH3)3Cl]+ changes only by a factor of 2 whereas the charge of the complex
changes from -2 to +1. This suggests that both bond breaking and bond making are
important, which is characteristic of an associative SN2 mechanism.
4. Nature of the entering ligand greatly affects the rate of substitution in square planar
complexes indicating that the entering ligand takes part in the rate determining step.
This is another evidence for SN2 mechanism.
5. The nature of the departing group, does not affect much the rate constants of these
reactions indicating that the dissociative SN1 mechanism in which the departing
ligand has a greater role is not operative.
6. It has been observed that substitution in Pt(II) square planar complexes occurs with
retention of configuration. i.e cis → cis and trans → trans. This is in agreement with
a trigonal bipyramidal intermediate. With a 3 coordinate intermediate (dissociative
mechanism), the entering group can lead to both cis and trans isomers.
7. As the bulkiness of the ligands other than the entering and departing ligands
increases, the reaction rate decreases, indicating an associative mechanism.
KINETICS
The kinetics of the reaction can be illustrated by the reaction,
[PtA3X]n+ + Y- → [PtA3Y]n+ + X- in the presence of water.
For this reaction, a two term rate law was found out.
Rate = k1[PtA3X n+] + k2[PtA3X n+] [Y-]
where k1 = first order rate constant for solvent controlled reaction and k2 = second order rate
constant for reaction with Y-. The analysis of the rate constants is made by conducting the
reactions with a large excess of nucleophile, Y-. Then the observed rate constant, kobs is
pseudo first order and is related to k1 and k2 as kobs = k1+ k2[Y-]. Thus for the same complex,
linear plots of kobs against different nucleophile concentration, [Y-] should be obtained,
having the same intercepts k1 and different slopes, k2.
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Plots of kobs for the reaction: [Pt(dien)Cl]+ + Y- → [Pt(dien)Y]+ + Cl- against concentration
of nucleophile, [Y-]
The obtained rate law indicates that the reaction of [PtA3X]n+ with Y- to yield
[PtA3Y]n+ is occurring by a two path mechanism, of which only one involves Y- in the rate
determining step.
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The upper path is the solvent path (also called Y- independent path) and the lower
path is the direct path (reagent path). In the solvent path, the solvent H2O replaces X- in a
slow step. It is subsequently replaced by Y- in a rapid step. Experiments show that Yindependent path is not an SN1 process, but is a direct SN2 displacement of leaving group by
nucleophile in probably second order path while the solvent path gives pseudo first order
kinetics.
The rate constant k1 is due to solvent path, while k2 is due to the direct displacement
of the leaving group by nucleophile. Hence solvent path k1 can be designated as ks and the
direct displacement path k2 as kY so that
kobs = ks+ kY[Y-]
Factors affecting the rates of substitution reactions in square planar complexes
1. Trans Effect
2. Effect of leaving group
For the reaction, [Pt(dien)X]+ + py → [Pt(dien)py]2+ + X-, the rates of the reaction
show that if the leaving group X- is replaced by NO3-, H2O, Cl-, Br-, I-, N3-, SCN-, NO2- and
CN-, the rates decrease in the order, NO3- > H2O > Cl- > Br- > I- > N3- > SCN- > NO2- > CN3. Solvent Effect
Since in the solvent path, the solvent replaces directly, as the coordinating ability of
the solvent increases, contribution made by this path to the overall rate of the reaction would
also increase. The solvents can be divided into two,
a) Good coordinating solvents like H2O, ROH etc which provide almost entirely a
solvent path for exchange. For the reaction of Cl- with [Pt(py)2Cl2], rate of exchange
does not depend on the nucleophile. i.e ks >> kCl [Cl-] or k1 >> k2.
b) Poor coordinating solvents like CCl4, C6H6 contribute little to the overall rate of the
reaction. Here rate of exchange depends on [Cl-]. i.e ks < kCl [Cl-] or k1 < k2. The
experimental results of the effect of solvent on the rate of exchange are,
Solvents in which rate is kobs (min-1)
independent of [Cl-]
H2O
2.1 x 10-3
Solvents in which rate is kobs (min-1)
dependent on [Cl-]
CCl4
1 x 10-4
C2H5OH
8.5 x 10-4
C6H6
2 x 10-4
m- C3H7OH
2.5 x 10-4
m-cresol
2 x 10-4
(CH3)2SO
2.3 x 10-2
tert-C4H9OH
1 x 10-3
CH3NO
1.9 x 10-3
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4. Effect of charge on the complex
The charge on the complex does not have appreciable effect on the rate of
nucleophilic substitution of square planar complexes. For example, the rates of aquation
reactions of [PtCl4]2-, [PtCl3NH3]-, cis [Pt(NH3)2Cl2] and [Pt(NH3)3Cl]+ changes only by a
factor of 2 whereas the charge of the complex changes from -2 to +1.
TRANS EFFECT-THEORY AND APPLICATIONS
The ability of an attached group to direct substitution into a position trans to itself is
called trans effect. Such a group has a marked influence (trans influence) on the rate of a
reaction. For example, in the substitution reaction,
Since Cl- has greater trans effect than NH3, the Cl- trans to Cl- and not the one trans to NH3
is replaced by C2H4. Also,
Since C2H4 has greater trans effect than Cl-, the Cl- trans to C2H4 and not the one which is
trans to Cl- is replaced by NH3. The approximate ordering of ligands in a trans directing
series is,
CN-, CO, NO, C2H4 > PR3, H- > CH3-, C6H5-, SC(NH2)2, SR2 > SO3H- > NO2-, I-, SCN- >
Br- > Cl- > py > RNH2, NH3 > OH- > H2O
Trans effect is used in synthesizing certain specific complexes. For example cis and
trans diamminedichloro Pt(II) complexes have been synthesized separately as,
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Another application of trans effect is in distinguishing cis and trans isomers of the
formula [PtCl2(NH3)2]. Addition of thiourea (tu) to the trans isomer results in the
replacement of the two trans Cl- ions. The reaction stops at this stage because the trans NH3
molecules do not labilise each other.
But the addition of thiourea to the cis isomer results in the displacement of all the
original ligands and gives [Pt(tu)4]2+ as the final product.
The trans effect of the ligands decreases in the order tu> Cl- > NH3. This method of
differentiating the geometrical isomers is called the Kurnakov effect.
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Theories of trans effect
Several theories have been proposed for the explanation of trans effect.
1) Polarisation theory
This is a thermodynamic approach. According to this theory, the primary positive
charge of Mn+ induces a dipole in all the four ligands. If all the four ligands are identical as
in (a), then the dipoles induced by the metal ions cancel each other and the resultant dipole
is zero. None of the four ligands show trans effect. But if the four ligands are not identical,
then the induced dipoles do not cancel each other. The two L ligands which are similar and
trans to each other balance each other. But the other two trans ligands T and D, which are
not similar do not balance each other. T is large and has greater polarisability than D.
Polarisation takes place in such a way that the positive charge of Mn+ central ion at a point
trans to T is reduced. Hence the attraction of D for Mn+ is also reduced and the bond trans to
T is weakened and hence lengthened. This facilitates the replacement of D by E (entering
ligand) at a point trans to T.
Evidences: 1) The theory predicts that trans effect is important only when the central metal
ion itself is polarisable and large in size. Actually, trans effect is observed predominantly in
Pt(II) complexes than in Pd(II) or Ni(II) complexes. 2) If the ligand T is highly polarisable
in [PtL2TD] complex, then Pt-D bond trans to T is longer than Pt-L bond cis to T. The
complex [Pt(C2H4)X3]- type where X= Cl-, Br- and C2H4 has large trans effect. The Pt-Cl or
Pt-Br bond trans to C2H4 is longer than that cis to C2H4.
Defect: The theory can well explain the ligands at the low end of the trans effect series like
H2O, OH-, NH3 etc. However, this theory cannot explain the high trans effect of the pi
bonding ligands like C2H4, CN-, CO etc which lie at the other end of trans effect series.
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2) Pi bonding theory
According to this theory, the vacant π or π* orbitals of the pi bonding ligands accept
a pair of electrons from the filled d orbitals of the metal (dxz or dyz) to form M-L pi bond. In
the case of Pt(II) square planar complex, [PtL2TD] (T is the pi bonding ligand, D is the
departing ligand trans to T), the filled orbital of Pt(II) overlaps with the empty orbital of the
ligand T to form M-T pi bond. The formation of this pi bond increases the electron density
in the direction of T and diminishes it in the direction of the ligand trans to T. The electron
shift towards T facilitates the approach of the entering ligand E with its lone pair in the
direction of trans directors.
The formation of dπ – pπ bond between Pt(II) and the π- bonding ligand, T in the five
coordinated transition complex.
3) Molecular Orbital approach
In this approach, there is the formation of a five coordinated intermediate with
trigonal bipyramidal arrangement in which the more electronegative atoms occupy the axial
positions. The loss of the ligand L from the triangular plane will take place from trans
position to the least electronegative group T so that the entering group E is trans to T.
Cis Effect
Certain ligands such as thiocyanate and hydroxide ions greatly accelerate the
hydrolysis of a complex when they are cis to the leaving group as compared to the
analogous reaction in which the leaving group is trans to these ligands. For example,
[Co(en)2XCl]n+ + H2O → [Co(en)2X(H2O)]n+ + Clwhere X is a cis activating ligand like OH-, SCN- etc. When OH- is cis to the leaving Cl-, the
reaction rate is about ten times as great as that when it is in the trans position. The ligands
that possess a strong cis effect are those that have unshared pairs of electrons in addition to
the pair used in the sigma dative bond.
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KINETICS AND MECHANISM OF OCTAHEDRAL SUBSTITUTION
Substitution reactions involve either replacement of ligands by other ligandsnucleophilic substitution (SN) or replacement of central metal ion by other metal ionselectrophilic substitution (SE) which is rare.
Two main types of nucleophilic substitution mechanisms are,
1) Unimolecular nucleophilic substitution (SN 1) or Dissociative mechanism
2) Bimolecular nucleophilic substitution (SN 2) or Associative or Displacement
mechanism
Unimolecular Nucleophilic Substitution (SN 1) or Dissociative mechanism
According to this mechanism, the complex first undergoes dissociation losing the
ligand to be replaced, X and changes into a five coordinated intermediate which then readily
adds the new ligand, Y.
[MX6] → [MX5] + X
[MX5] + Y → [MX5Y]
-slow step
-fast step
The characteristics of this reaction are,
1. Only one species, [MX6] is involved in the formation of the activated species in
the slow step.
2. In the second step, the activated species undergoes fast reaction with incoming
ligand, Y.
3. The activation energy for the first step is high and that for the second step is low.
4. The rate of the overall reaction depends on [MX6] and not on [Y].
5. The reaction is first order with respect to MX6 and is zero order with respect to
Y.
6. In the formation of the activated complex, the coordination no: of the metal is
decreased by one.
7. The rate law for the substitution is ν = k1[MX6]
Example for a reaction undergoing this mechanism is,
[Cr(H2O)6]3+ → [Cr(H2O)5]3+ + H2O (slow)
[Cr(H2O)5]3+ + CN- → [Cr(H2O)5CN]2+
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Types of intermediates formed during SN1 reaction
Consider an octahedral complex MA5X being attacked by a nucleophile, Y. If the
reaction proceeds through a dissociative SN1 mechanism, two types of intermediates can be
formed.
1) The bond M-X dissociates causing least disturbance to the remaining MA5
intermediate which has a square pyramidal geometry. The intermediate MA5 is
then attacked by Y to produce MA5Y.
2) The bond M-X dissociates and the remaining MA5 species immediately adjust the
bond angles to produce a trigonal bipyramidal intermediate which is then
attacked by Y to produce MA5Y.
It is evident that the formation of a trigonal bipyramidal intermediate involves the
movement of atleast two metal ligand bonds whereas no such movement is required during
the formation of a square pyramidal intermediate. SN1 reactions thus proceed generally
through the more stable square pyramidal intermediate unless the trigonal bipyramidal
intermediate is stabilized by pi bonding.
Bimolecular Nucleophilic Substitution (SN 2) or Associative or Displacement
mechanism
According to this mechanism, the new ligand first adds on to the complex to form a
seven coordinated activated or intermediate complex which then readily undergoes
dissociation to yield the final product.
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[MX6] + Y → [MX5Y]
[MX5Y]
→ [MX5Y] + X
-slow step
-fast step
In this case, the reaction rate depends upon the first step. i.e upon the concentration
of the complex as well as the concentration of the incoming ligand Y.
1.
2.
3.
4.
5.
6.
The characteristics of this reaction are,
In this reaction, two species (MX6 and Y) are involved in the formation of the
activated species; Y is added to the reactant complex to form the activated species.
In the second fast step, the activated complex loses a ligand and the new ligand Y
becomes a permanent part of the molecule.
The rate of the overall reaction depends on both [MX6] and [Y].
The reaction is first order with respect to MX6 and first order with respect to Y; the
overall order is 2.
In the formation of the activated species, the coordination no: of the metal is
increased.
The general rate law for the substitution is ν = k1[MX6] [Y].
Example for a reaction undergoing this mechanism is,
Cl
[Co(NH3)5Cl]2+ + H2O → [Co(NH3)5]2+ → [Co(NH3)5H2O]3+ + ClOH2
Types of intermediates formed during SN2 reaction
If the reaction proceeds through an associative SN2 mechanism, there are two types
of intermediates.
1. If the nucleophile Y attacks through one of the edges of the octahedron, a pentagonal
bipyramidal intermediate is formed. The formation of pentagonal bipyramidal
intermediate requires the movement of atleast four ligands to adjust the nucleophile
Y. The ligand- ligand repulsions also increase the energy of pentagonal bipyramidal
intermediate because the decrease in A-M-A bond angles brings the electron pairs of
metal bonds nearer to one another in this intermediate.
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2. The second type of intermediate is formed if the nucleophile Y attacks through the
middle of one of the triangular faces of the octahedron. As soon as Y starts
approaching M, the outgoing ligand X starts moving towards the middle of another
triangular face so that the octahedral wedge intermediate formed has both X and Y
ligands in equivalent positions. The formation of an octahedral wedge intermediate
requires minimum movement of ligands and the ligand- ligand repulsions are less
than the repulsions in pentagonal bipyramidal intermediate. i.e it requires less energy
than pentagonal bipyramidal intermediate. Therefore SN2 reactions generally proceed
through an octahedral wedge intermediate.
SN1 Vs SN2 mechanisms
SN1 and SN2 mechanisms can be differentiated from each other by the following
points.
1) In SN1 process, the rate determining slow step is a metal ligand bond breaking step,
and the coordination no: of the complex is reduced from 6 to 5. In SN2 process, the
rate determining step involves a metal ligand bond making step and the coordination
no: is increased to 7.
2) The rate of SN1 mechanism is first order with respect to MX6. i.e rate determining is
unimolecular. On the other hand, the rate determining step for SN2 mechanism is
bimolecular. i.e its reaction rate is second order: first order with respect to MX6 and
first order with respect to Y.
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SOLVOLYTIC REACTIONS
Consider a substitution reaction of octahedral complex [MX6]n+ in presence of a
ligand Y- in aqueous medium. Since water also acts as a ligand and is much more abundant
than Y-, aquation occurs.
[MX5Y]n+ + H2O →
[MX5(H2O)](n+1)+ + Y-
The reaction in aqueous medium in which a water molecule replaces a coordinated
ligand from the complex species is termed as aquation reaction or acid hydrolysis. The
reaction in aqueous medium in which the anion of water molecule i.e OH- ion replaces a
coordinated ligand from the complex species is termed as base hydrolysis.
[MX5Y]n+ + OH- →
[MX5(OH)]n+ + Y-
Since some OH- ions are always present due to auto ionization of H2O, some
[MX5(OH)]n+ is always formed along with [MX5(H2O)](n+1)+ during the hydrolysis of
[MX5Y]n+ even in neutral aqueous medium.
ACID HYDROLYSIS OF OCTAHEDRAL COMPLEXES
Considering SN1 mechanism for aquation reactions, rate = k[MX5Yn+].
Considering SN2 mechanism, rate = k[MX5Yn+][H2O]. Since H2O is present in large excess,
rate = k[MX5Yn+]. Thus, kinetic studies cannot predict the mechanism of aquation reactions.
However, many other factors that affect the rate of aquation reactions suggest a
dissociative SN1 mechanism for aquation reactions. These factors are,
1) Charge on the substrate: It has been observed that increase in positive charge on the
reacting species decreases its rate of aquation. For example,
cis[Co(en)2Cl2]+ + H2O → cis[Co(en)2Cl(H2O)]2+ + Clcis[Co(en)2Cl(H2O)]2+ + H2O → cis[Co(en)2(H2O)2]3+ + ClThe first reaction is hundred times faster than the second reaction. Similarly for the aquation
of [RuCl6]3-, [RuCl3(H2O)3] and [RuCl(H2O)5]2+, rate constants are 1.0 s-1, 2.1 x 10-6s-1 and
10-8s-1 respectively. The observations favour a dissociative SN1 path since the increase in
positive charge render the dissociation of leaving group from metal M more difficult
resulting in slower reaction by this mechanism. For SN2 mechanism, no significant change
in rate is expected since increase in charge would not only make the breaking of M-Y bond
more difficult but would also make the formation of M-H2O bond more easy.
2) Strength of M-Y or Metal- Leaving group bond: The rate constants for acid hydrolysis of
[Co(NH3)Y]2+ is,
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Leaving group, YCF3COOCCl3COOCHCl2COOCH2ClCOOCH3CH2ClCOO-
Dissociation Constant
2.0 x10-14
5.0 x10-14
2.0 x10-13
7.1 x10-12
6.6 x10-10
kaquation
5.5 x10-3
5.4 x10-3
1.6 x10-3
0.6 x10-3
0.3 x10-3
It is evident that the rate of aquation goes on decreasing with increase in basicity of the
leaving group, Y-. Since the strength of M-Y bond is directly proportional to the basicity of
the leaving group, Y-, it can be inferred that the rate determining step involves the
dissociation of M-Y bond which supports a dissociative SN1 mechanism.
3) Inductive effect of inert ligand, X: It has been found that rate of aquation increases with
basicity of inert ligand, X. This is because, greater the basicity or electron donating
power of the inert ligand, greater would be the accumulation of negative charge on the
central metal ion which result in easier heterolytic dissociation of M-Y bond. This in
turn supports a dissociative mechanism.
4) Stability Constants: A study of stability constants of [Co(NH3)Y]2+ with different leaving
groups reveals that during aquation of a complex, the leaving group is present in the
product as well as in the intermediate in the form of a solvated anion. i.e the intermediate
is formed by the dissociation of M-Y bond which gets solvated immediately. Thus the
aquation of [Co(NH3)Y]2+ occurs through a solvation assisted dissociative mechanism.
5) Solvation effects: The rate of aquation is found to decrease as the extent of chelation in
the complex goes on increasing. This can be explained by using solvation theory.
According to solvation theory, in aqueous medium, the reacting species, the
intermediates and the products are all present in the hydrated state. The hydration of any
species decreases its energy and thus stabilises it. The extent of stabilisation is related to
the extent of hydration of the species. The greater the charge and smaller the size of the
species, greater would be the extent of its hydration and hence the extent of stabilisation.
The five coordinate intermediate formed during aquation through SN1 mechanism, is
smaller in size than the seven coordinated intermediate formed during SN2 mechanism.
Hence SN1 mechanism is more feasible. Another interesting fact is that the presence of
chelating ligand increases the size of the complex and hence that of the intermediate
compared to complexes with unidentate ligand. Therefore, it requires more energy for its
formation since it is stabilised by hydration to a lesser extent. As a result, the rate
determining step for aquation would be slower.
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M.Sc CHEMISTRY – SEMESTER 2 - INORGANIC CHEMISTRY
6) Steric effects: An increase in the steric overcrowding around the metal ion favours a
dissociative SN1 mechanism. For example, a six membered chelate ring produces more
steric strain than a five membered ring. Therefore a complex with six membered ring
should be aquated faster than a complex containing five membered ring if it proceeds
through dissociative mechanism. This has been proved experimentally using propylene
diammine ligated complexes and ethylene diammine ligated complexes. The former
aquates faster than the latter.
Thus from the discussion on the various factors, it follows that acid hydrolysis of
octahedral complexes which contains no pi donor or pi acceptor occurs through a
dissociative SN1 mechanism involving a square pyramidal intermediate.
Mechanism of acid hydrolysis when the inert ligand is a pi donor
The rate constants for the aquation reactions of [Co(en)2(OH)Cl]+ and
[Co(en)2(NH3)Cl]2+ are 1200 x 10-5 s-1 and 0.05 x 10-5 s-1. This difference in rate constants
cannot be accounted by the difference in basicities of OH- and NH3 ligands alone.
The coordinated OH- ligand has filled p orbitals which are capable of forming pi
bond with empty orbitals of the central metal ion which NH3 has not. The square pyramidal
intermediate formed during the aquation of cis [Co(en)2(OH)Cl]+ has an empty d2sp3 hybrid
orbital of the central metal ion which can overlap with a filled p orbital of coordinated OHligand forming a pi bond that stabilizes the intermediate.
For trans complexes of [Co(en)2(OH)Cl]+ and [Co(en)2(NH3)Cl]2+, the rate constants
for the aquation reactions are 160 x 10-5 s-1 and 0.034 x 10-5 s-1. Formation of a stable square
pyramidal intermediate is ruled out in the case of trans complexes due to the absence of lack
of symmetry. However, a trigonal bipyramidal intermediate can be stabilized by pi bonding
involving the overlapping of empty d orbital of the metal with filled p orbital of the
coordinated OH- ligand. The formation of a trigonal bipyramidal intermediate from a square
pyramidal intermediate requires some energy since some bond angles have to change from
900 to 1200. The extra energy required is more than compensated by the energy released as a
result of pi bonding. The trigonal bipyramidal intermediate can explain the mixture of cis
and trans products during aquation in this case.
Mechanism of acid hydrolysis when the inert ligand is a pi acceptor
The mechanism of aquation of complexes having an inert pi acceptor ligand cis or
trans to the leaving group is different. For example in the aquation of trans
[(NO2)Co(en)2Cl]+, one of the filled t2g orbitals of Co can overlap with an empty p orbital of
NO2 group to form a pi bond. The electron charge of t2g orbital would shift more towards
NO2 group to maximize pi overlap which results in a decrease of the electron charge from
around the leaving Cl- group. The lone pair of electrons of the attacking water ligands would
18
M.Sc CHEMISTRY – SEMESTER 2 - INORGANIC CHEMISTRY
experience lesser repulsion from t2g electrons. As a result, the attack of water on central Co
atom in a position cis to the leaving group becomes easier. i.e an associative SN2 mechanism
is favoured. A dissociative mechanism is unlikely since, due to the electron withdrawing
inductive effect of NO2 group, the bonding electrons of Co-Cl bond are pulled towards Co,
making the dissociation of Co-Cl bond difficult.
The extent of pi overlap when NO2 group is cis to the leaving group is less than
when it is trans to the leaving group. Consequently, the aquation of cis isomers would be
slower than the trans complex.
Experiments show that aquation of cis complex always give 100% cis aquated
product and trans complex, the trans product when the inert ligand is a pi acceptor like NO2,
CO, CN- etc. This can be satisfactorily explained by taking an octahedral wedge
intermediate.
BASE HYDROLYSIS OF OCTAHEDRAL COMPLEXES
For ammine complexes of Co(III) containing N-H bonds, it has been found that the
rate of base hydrolysis is about 106 times faster than the corresponding rate of acid
hydrolysis. An example for the reaction is,
[Co(NH3)5Cl]2+ + OH-
→
[Co(NH3)5(OH)]2+ + Cl-
Garrick suggested a Substitution Nucleophilic Unimolecular (Conjugate Base)
(SN1(CB)) mechanism for base hydrolysis. The essential characteristics of this mechanism
are,
1. In this mechanism, one of the ligands in the complex is converted into its conjugate base
by the action of a base (OH-). This first step is fast.
2. In the second step which is rate determining, the conjugate base dissociates releasing the
ligand to be replaced.
3. The new ligand is added to the remaining part of the complex species leading to the
product.
4. This mechanism is called unimolecular because in the slow step, only one molecule is
involved.
5. The kinetics is second order because the concentration of the complex and concentration
of base are involved upto the slow step.
1) The first step involves the production of conjugate base of the complex, since the
ammine complexes have removable hydrogens.
[Co(NH3)5Cl]2+ + OH↔
[Co(NH3)4(NH2)Cl]+ + H2O -fast step
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M.Sc CHEMISTRY – SEMESTER 2 - INORGANIC CHEMISTRY
2) The conjugate base produced contains a coordinated NH2 ligand which being a pi
acceptor forms pi bond with Co(III) forming a five coordinate intermediate by the
dissociation of Cl- from Co(III).
[Co(NH3)4(NH2)Cl]+ →
[Co(NH3)4(NH2)]2+ + Cl-slow step
3) The five coordinated intermediate then quickly reacts with H2O giving the final product
of hydration.
[Co(NH3)4(NH2)]2+ + HOH →
[Co(NH3)5(OH)]2+
-fast step
The rate determining step is the second slow step.
Rate
= k[Co(NH3)4(NH2)Cl+]
= kK[Co(NH3)5Cl2+] [OH-]
(Since [Co(NH3)4(NH2)Cl+] = K[Co(NH3)5Cl2+] [OH-] where K is the equilibrium constant
for the first step).
= k’[Co(NH3)5Cl2+] [OH-]
Thus in SN1 (CB) mechanism, although it has an SN1 mechanism, it is consistent with
second order: first order with respect to complex and first order with respect to base.
Evidences for SN1 (CB) mechanism
1) SN1(CB) mechanism requires that the reacting complex should have atleast one protonic
hydrogen atom on a nonleaving ligand so that H+ may transfer to OH- to form its
conjugate base. It has been found experimentally that complexes containing ligands with
no proton like [Co(CN)5Br] and trans [Co(py)4Cl2]+ undergo hydrolysis much more
slowly in basic solution at a rate independent of [OH-].
2) The reaction, [Co(en)2NO2Cl]+ + Y- →[Co(en)2NO2Y]+ + Cl- is slow in DMSO solvent
(t1/2 in hours). However, when trace amount of OH- is added, the reaction rate is faster
(t1/2 reduced to minutes). It has been observed that the rate depends on [OH-] and not on
the nature of Y-. The mechanism for the reaction is,
[Co(en)2NO2Cl]2+ + OH↔
[Co(en)(en-H)(NO2)Cl] + H2O
[Co(en)(en-H)(NO2)Cl]
→
[Co(en)(en-H)(NO2)]+ + Cl-
[Co(en)(en-H)(NO2)]+ + NO2[Co(en)(en-H)(NO2)2] + H2O
→
→
[Co(en)(en-H)(NO2)2]
[Co(en)2(NO2)2]+ + OH-
3) The reaction of [Co(NH3)5Cl]2+ and OH- in aqueous solution at 25oC in presence of
H2O2 gives supporting evidence for SN1(CB) mechanism. The addition of H2O2 to the
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M.Sc CHEMISTRY – SEMESTER 2 - INORGANIC CHEMISTRY
reaction mixture reduces the rate of base hydrolysis compared to OH- ions since OH- is
removed by the reaction, OH- + H2O2 → H2O + HO2-.
4) Complexes which contain removable hydrogens which are difficult to be removed due to
high negative charge on complex ion like [Fe(CN)5NH3]3- do not form their conjugate
base easily. The base hydrolyses of these complexes are independent of [OH-]
experimentally.
5) The isotopic exchange studies on base hydrolysis using
the SN1(CB) mechanism.
18
OH- unambiguously support
Nature of intermediate in base hydrolysis
The five coordinated intermediate formed during base hydrolysis can be square
pyramidal or trigonal bipyramidal. If the intermediate is square pyramidal, the
stereochemistry of the product would be unchanged since the water molecule tends to attack
the site vacated by the outgoing ligand. However with trigonal bipyramidal intermediate, a
mixture of cis and trans forms can be formed irrespective of the geometry of the reacting
complex. Experimental studies on base hydrolysis of Co(III) complexes show that the
products formed in each case is a mixture of cis and trans forms which clearly proves that
the intermediate is a trigonal bipyramidal one.
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M.Sc CHEMISTRY – SEMESTER 2 - INORGANIC CHEMISTRY
WATER EXCHANGE REACTIONS
Two types of water exchange reactions are possible. 1) Exchange of coordinated
water with solvent water 2) exchange of coordinated water with other ligands present in the
system, also termed as anation reactions.
Based on the rate of exchange of coordinated water, metal ions can be classified into
four categories.
Class 1: The exchange of water is extremely fast. First order exchange rate constants are of
the order of 108 s-1. These metal ions are characterized by low charge and large size. The
complexes are bound by essentially electrostatic forces and include the complexes of alkali
metals and larger alkaline earth metals.
Class II: The exchange of water is fast. First order exchange rate constants range from 10 5 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 those of Class I ions, but LFSEs are relatively small.
Class III: The exchange of water is relatively slow compared with Classes I and II. First
order exchange rate constants range from 1 to 104 s-1. Metal ions belonging to this group are
the tripositive transition metal ions stabilized by LFSEs and Be2+ and Al3+.
Class IV: The exchange of water is slow. These are the only inert complexes. First order
exchange rate constants range from 10-1 to 10-9 s-1. These ions are comparable in size to
Class III ions and exhibit considerable LFSE. Eg: Cr3+, Ru3+, Pt2+.
Mechanism
Consider the replacement of water by ligand L under neutral conditions.
M-OH2 + L → M-L + H2O
If the reaction proceeds by a dissociative mechanism, the first step is breaking the metalwater bond followed by formation of the M-L bond.
M-OH2 ↔ M + H2O
M + L → M-L
Let k1 and k-1 be the rate constants for the forward and backward reactions of the first step
and k2 be the rate constant of second step. The rate law obtained from these reactions shows
a dependence on [L], even though it is derived for a dissociative mechanism.
Rate = -d[M-OH2]/dt
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M.Sc CHEMISTRY – SEMESTER 2 - INORGANIC CHEMISTRY
= k1k2[M-OH2][L]/(k-1[H2O] + k2[L])
At high concentrations of L, k2[L] > k-1[H2O].
Then rate = kobs[M-OH2]
At low concentrations of L, both L and H2O compete for M and the rate shows a dependence
on [L]. For example, the reaction of SCN- with a Co(III) hematoporphyrin complex shows
the expected dependence of the rate on [SCN-] for lower concentration of SCN-.
If some M-L bond making takes place before the M-OH2 bond is completely broken
(Id- Interchange dissociative), the process can be described in three steps.
M-OH2 + L ↔ M……….OH2………L
M……….OH2………L → M………L …….OH2
M………L ………OH2 →
M-L + H2O
Let K and k2 be the equilibrium constants for first and second steps.
Then rate = kK[M-OH2][L]/(1 + K[L])
This simplifies to kobs = kK[L] under pseudo first order conditions.
The rate law remains the same if bond making becomes more important than bond
breaking (Ia- Interchange associative). Thus, the mechanism cannot be distinguished with
certainty. However, the most discussions center around Id and Ia mechanisms.
For reactions of Co(III) complexes like
[Co(NH3)5(H2O)]3+ + Xn- → [Co(NH3)5X](3-n)+ + H2O, most evidence supports the Id
mechanism for substitution. First, there is little dependence of reaction rates on the nature of
the incoming ligand. If bond making is significant (Ia), the opposite would be expected. The
Id mechanism is further supported by steric arguments.
For Cr(III) complexes, there is a strong dependence of reaction rates on the nature of
the entering group which supported the Ia mechanism. High pressure 17O NMR
spectroscopy has been recently used to calculate volume of activation, ∆V‡ for water
exchange reactions. The data obtained for solvent exchange with [M(NH3)5(H2O)]3+
complexes show a positive ∆V‡ for Co3+ (+1.2 cm3/mol-1), but negative values for Cr3+ (-5.8
cm3/mol-1), Rh3+ (-4.1 cm3/mol-1) and Ir3+ (-3.2 cm3/mol-1) suggesting an Id mechanism for
Co3+, but Ia for Cr3+, Rh3+ and Ir3+ ions.
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M.Sc CHEMISTRY – SEMESTER 2 - INORGANIC CHEMISTRY
ELECTRON TRANSFER REACTIONS
Transition metals can undergo several oxidation- reduction reactions where in there
is a change in oxidation state. There are formally two types of reactions of this type.

Reactions involving simple electron transfer
[Fe(CN)6]4- + [Ir(Cl)6]2- →[Fe(CN)6]3- + [Ir(Cl)6]3-

Reactions that can be considered as atom transfer reactions that occur with electron
transfer
[Co(NH3)5Cl]2+ + [Cr(H2O)6]2+ → [Co(H2O)6]2+ + [Cr(H2O)5Cl]2+
Simple electron transfer reactions involving transition metal complexes in solutions
are complicated by the fact that the oxidised and the reduced species are often metal ions
surrounded by shields of ligands and solvating molecules. No heat change is associated with
the reaction. The reactions can be between two metal ions or between single element in
different oxidation states (self exchange reactions). The electron transfer can be broadly
divided into two mechanistic classes called outer sphere mechanism and inner sphere
mechanism.
OUTER SPHERE MECHANISM
For transition metal complexes an outer sphere mechanism is established when rapid
electron transfer occurs between two substituent inert complexes. (A substituent inert
complex is one that undergoes substitution at a rate substantially less than the rate of
electron transfer.) In this mechanism, the coordination shell of the reductant and oxidant
stays intact. i.e the bonds are neither broken nor made. The electron effectively hops from
one species to the other (also called tunnelling) and the ligands act as electron conduction
media. An outer sphere electron transfer may be generally represented as,
O + R → [O………R] - formation of precursor complex
[O………R] → [O………R]* → [O-………R+] – chemical activation of precursor complex
followed by electron transfer and relaxation to successor complex.
[O-………R+] → O- + R+ -dissociation of separated products
Here O is oxidant and R is reductant. First the oxidant and the reductant come
together to form a precursor complex. Activation of the precursor complex which include
reorganisation of solvent molecule and changes in metal –ligand bond length occur. Final
step is the dissociation of the ion pair into products.
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M.Sc CHEMISTRY – SEMESTER 2 - INORGANIC CHEMISTRY
The salient features of electron transfer by outer sphere mechanism are,
1. Minimum electronic interactions by chemical bonding between the reactants.
2. The inner coordination spheres of the reactants remain unaffected.
3. The ligands in the two reactants remain as such and the bonds are neither made nor
broken.
4. Rate is first order with respect to reactants.
5. Electron transfer from one stable complex to another with no breaking of M-L bond.
6. Rate depends on the size of the cations present in the solution.
The exchange of [Fe(CN)6]3- and [Fe(CN)6]4- denotes a typical example of a process
that takes place by direct electron transfer through an outer sphere activated complex. The
rate of exchange can be studied by isotopic labelling of Fe.
[Fe(CN)6]4-
+
Fe2+ → d6
Fe-C bond longer
[Fe(CN)6]3-
→
[Fe(CN)6]3- + [Fe(CN)6]4-
Fe3+ → d5
Fe-C bond shorter
The electron is transferred from a t2g orbital of Fe2+ to t2g orbital of Fe3+. The bond
lengths are unequal. i.e the energies of these orbitals are not equivalent. The probability of
the reaction can be given by Franck Condon principle which states that there can be no
appreciable change of atomic rearrangement during the time of electronic transition- the
energies of the participating electronic orbitals must be same. Hence the ion- ligand bond
length adjusts to intermediate value and then electron transfer takes place. Thus the actual
process occurs with shortening of the bonds in Fe2+ and lengthening of bonds in Fe3+
complexes until participating orbitals are of same energy. Vibrational stretching and
compression along M-L bonds allow this. In specific cases, there may be angular distortion
of the ligand as well as solvent reorganisation to accommodate the precursor structural
changes.
We assume that metal ligand stretching motion resemble a harmonic vibration and so
the potential energy curve drawn as parabolas can be approximated as a harmonic potential
well. The activated complex is located at the intersection of the two curves. However then
the noncrossing rule states that, molecular potential energy curves of states of same
symmetry does not cross but instead split into an upper and lower curve as shown.
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M.Sc CHEMISTRY – SEMESTER 2 - INORGANIC CHEMISTRY
Potential energy diagram for a homonuclear electron transfer reaction. The activated
complex is situated at I and ∆E is the activation energy (Gibb’s energy of activation). A
general potential energy diagram for a heteronuclear electron transfer reaction can be drawn
as,
Correlation of rates with structure and electronic configuration
In outer sphere mechanism, the bond distortion magnitudes have an important role in
the rates of the reaction. The rate seems to be relatively great for two ions of similar
geometry so that little rearrangements are needed to symmetrise the transition state
enhancing a strong coupling interaction. i.e low ∆Gi‡. This will generally be the case for
complexes differing by one electron in low energy t2g orbitals used in ligand bonding.
Eg:
[Mn(CN)6]4-/[Mn(CN)6]326
M.Sc CHEMISTRY – SEMESTER 2 - INORGANIC CHEMISTRY
[Fe(phen)3]2+/[Fe(phen)3]3+
The rates will be greatest if the ligands are unsaturated and pi bonding. This
stabilises the lower valency state more than the higher and hence the geometries will be
nearly equal. Thus [Fe(CN)6]4-/[Fe(CN)6]3- is faster than [Fe(H2O)6]2+/[Fe(H2O)6]3+
If the geometry of the two ions is very different, the coupling interaction is very
weak since large bond distortion is needed. This is the case for ions where the electrons to
be transferred occupies one of the eg orbitals used to hold a ligand.
Eg. Co(II)/Co(III), Cr(II)/Cr(III)
Qs: Why the electron transfer reaction of [Fe(CN)6]4-/[Fe(CN)6]3- is faster than
[Co(NH3)6]2+/[Co(NH3)6]3+
MARCUS THEORY
Marcus gave a quantitative relation for the calculation of rate constant for electron
transfer by outer sphere mechanism.
ket = νNkee-∆G‡/RT where ket is rate for electron transfer and ∆G‡, the Gibb’s energy of
activation, given by ∆G‡ = [λ (1 + ∆rG0/λ)2]/4 where ∆rG0 is standard reaction Gibb’s energy
(obtained from the difference in standard potentials of the redox partners) and λ the
reorganization energy, the energy required to move the nuclei associated with the reactant to
the position they adopt in the product immediately before the transfer of the electron. This
energy depends on the changes in metal-ligand bond lengths (innersphere reorganization
energy) and orientation of solvent molecules around the complex (outersphere
reorganization energy).
The pre exponential factor has two components, the nuclear frequency factor νN, and
the electronic factor, ke. The former is the frequency at which the two complexes having
already encountered each other in the solution, attain the transition state. The electronic
factor gives the probability on a scale from 0 to 1 that an electron will transfer when the
transition state is reached; its precise value depends on the extent of overlap of the donor
and acceptor orbitals.
A small reorganization energy and a value of ke close to 1 corresponds to redox
couple capable of fast electron self exchange. The first requirement is achieved if the
electron is removed from or added to a nonbonding orbital, as the change in metal-ligand
bond length is then least. It is also likely if the metal is shielded from the solvent. Simple
metal ions such as aqua species typically have λ well in excess of 1 eV, whereas buried
redox centres in enzymes, which are well shielded from the solvent can have values as low
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M.Sc CHEMISTRY – SEMESTER 2 - INORGANIC CHEMISTRY
as 0.25 eV. A value of ke close to 1 is achieved if there is good orbital overlap between the
two components of the precursor complex.
For a self exchange reaction, ∆rG0 = 0 and therefore, ∆G‡ = λ/4 and the rate of
electron transfer is controlled by reorganization energy. For example in [Cr(H 2O)6]3+/2+ self
exchange reaction, an electron is transferred between antibonding σ* orbitals which result in
large innersphere reorganisation energy and hence a slow reaction. With Ru, V, Fe etc aqua
complexes, the electron is transferred between weakly antibonding or nonbonding π orbitals,
the innersphere reorganization is less extensive and the reactions are faster.
The bulky hydrophobic ligands like bipyridyl acts as a solvent shield decreasing the
outersphere reorganization energy. Bipyridyl and other pi acceptor ligands allow electrons in
an orbital with pi symmetry on the metal ion to delocalize on to the ligand which lowers the
reorganization energy.
If we suppose that the reorganization energy for a redox reaction is the average for
the two self exchange processes, we can write λ12 = (λ11 + λ22)/2, then Marcus equation
breaks down to k12 = [k11k22K12f12]1/2 where k12 is the rate constant of overall heteronuclear
reaction, k11 and k22 are rate constants for the two self exchange reactions, K12 is equilibrium
constant for the overall reaction and log f12 = (log K12)2/4 log (k11k22/Z2). Here Z is the
collision frequency for the hypothetically uncharged reactant ions in solution. The factor f12
is described as a correction for the difference in free energies of the two reactants and is
often close to unity. This relation called Marcus cross relation gives a relation for predicting
the rate constant for heteronuclear outersphere redox reaction from self exchange rate
constants for each partner and overall equilibrium constant.
The Marcus cross relation connects thermodynamics and kinetics as shown by the
dependence of k12 on K12. As K12 increases, the reaction rate increases. i.e outersphere
reactions that are thermodynamically more favourable tend to proceed faster. This
relationship however breaks down when K12 becomes large.
For the reaction,
[Fe(CN)6]4-
+
[Mo(CN)8]3-
→
[Fe(CN)6]3- +
[Mo(CN)8]4-
The self exchange reactions are,
[Fe(CN)6]4-
+
[Fe(CN)6]3- →
[Mo(CN)8]4- + [Mo(CN)8]3- →
[Fe(CN)6]3- + [Fe(CN)6]4- k11 = 7.4 x 102 M-1s-1
[Mo(CN)8]3- + [Mo(CN)8]4- k22 = 3.0 x 104 M-1s-1
f12 can be calculated as 0.85 and K12 = 1.0 x 102
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M.Sc CHEMISTRY – SEMESTER 2 - INORGANIC CHEMISTRY
From Marcus equation, k12 = 4 x 104 M-1s-1. The experimental value is very close, 3
x 104 M-1s-1.
The confidence in Marcus equation is so high that departures from Marcus equation
is taken as a special feature in outer sphere mechanism like the barrier created by change in
high spin to low spin or a change in symmetry. Alternatively, departure from Marcus
equation indicate that the reaction is innersphere rather than outersphere.
INNER SPHERE MECHANISM- TAUBE MECHANISM
Inner sphere reactions are more complicated that outer sphere reactions because in
addition to electron transfer, bonds are broken and made. In this mechanism, the
coordination spheres of the reactants shares a ligand temporarily or transitorily and forms a
bridged intermediate activated complex and the electron is transferred across the bridging
group. In order for the electron transfer to occur, the molecular orbital from the reducing
agent from which the donated electron originate and the molecular orbital in the oxidizing
agent into which the electron is transferred must be of the same type (σ*).
The salient features of electron transfer by inner sphere mechanism are,
1.
2.
3.
4.
5.
Oxidant and reductant attach to one another at some stage of the reaction.
At least one ligand should be capable of binding two metals.
The attachment between oxidant and reductant occurs through a bridging ligand.
The bridging helps in the transfer of electrons from reductant to oxidant.
The rate determining step may be the bridge formation or electron transfer process.
The elementary steps in this mechanism can be generalised as,
1. Formation of precursor complex
MIIL6 + MIIIL’5 ↔ L5MII……….X……….MIIIL’5 +
L
2. Activation of precursor complex and electron transfer
L5MII……….X……….MIIIL’5 → L5MIII……….X……….MIIL’5
3. Dissociation to separated products
L5MIII……….X……….MIIL’5 →
products
The first step involves substitution by bridging group X into the coordination sphere
of the labile reactant (usually reductant) to form the precursor complex. This, then
undergoes some kind of reorganisation followed by electron transfer to give the successor
complex. In the last step, the successor complex breaks up to give the product.
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M.Sc CHEMISTRY – SEMESTER 2 - INORGANIC CHEMISTRY
Example for such a mechanism is the Taube reaction.
[CoCl(NH3)5]2+ + [Cr(H2O)6]2+ ↔ [Co(NH3)5.Cl.Cr(H2O)5]4+ →[Co(NH3)5]2+
[Cr(H2O)5Cl]2+
+
[Co(NH3)5]2+ + 6H2O → [Co(H2O)6]2+ + 5NH4+
The role of the bridging ligand in an innersphere mechanism is dual. It brings the
metal ions together (thermodynamic contribution) and mediates the transfer of electron
(kinetic contribution). The thermodynamic contribution arises from factors important to the
stability of the intermediate complex and the kinetic contribution arises from factors like
oxidant reductant reorganisation and matching of donor and receptor molecular orbital
types. The bridging ligands can be organic and inorganic in nature.
Two types of mechanisms have been proposed for the transfer of electron density
from the reductant to the oxidant, once the bridged binuclear intermediate has been formed.
1) Chemical radical stepwise mechanism in which an electron is transferred from reductant
metal to the bridging ligand reducing it to a radical anion. In a subsequent step, in an
electron hopping process, the electron is transferred from reductant metal to the oxidant
metal ion. 2) tunnelling resonance or exchange mechanism where the bridge acts simply as a
mediator of electron flow. The electron simply passes by quantam mechanical tunnelling
through the barrier constituted by the bridging ligand.
Though the bridging ligand is frequently transferred from oxidant to reductant in the
course of electron transfer, this is not a requirement. The transfer or nontransfer of the
bridging ligand depends on the relative stabilities of the products possible from the
intermediate.
[Cr(H2O)6]2+ + [Ir(Cl)6]2- → [Cr(H2O)6]3+ + [Ir(Cl)6]3There are two prerequisites for an innersphere mechanism to follow.
1) One reactant (usually the oxidant) should possess at least one ligand capable of bonding
simultaneously to two metal ions temporarily. If the bridging ligand contains only one atom
(Cl-) then both metal ion bond to it. However, if the bridging ligand contains more than one
atom (eg. SCN-), the two metal atoms may or may not be bound to the same bridging ligand
atom. The former is called adjacent attack and the latter, remote attack. Remote attack leads
to linkage isomers.
2) One ligand of one reactant (usually the reductant) be substantially labile. i.e it must be
capable of being replaced by a bridging ligand in a feasible substitution process. Thus the
reduction of hexamine Co(III) by hexa aqua Cr(II) occurs slowly by outersphere mechanism.
However when one NH3 ligand is substituted by Cl-, the reaction occurs with a substantially
greater rate. (Taube reaction)
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M.Sc CHEMISTRY – SEMESTER 2 - INORGANIC CHEMISTRY
Correlation of rates with structure and electronic configuration
For an innersphere mechanism, a simple substitution reaction could be the rate
determining step. i.e the rate of formation of the bridged species decides the rate of
innersphere reaction to a greater extent. Furthermore, the rate of loss of coordinated water
would put a limit on the rate of formation of a bridge. The order of reactivity for halides is I > Br- > Cl- > F- for the known examples. The strength of the bridge formed could usually
increase in the opposite order with F- giving the strongest bridge. i.e the ability to transmit
an electron and to undergo homolytic bond breaking could be greater for I- and least for F-.
The rates of redox reactions are very sensitive to the presence of other ions in the
solution. The mere presence of an anion should enable the cation to approach each other
more easily. This will be most effective if the anion is between the two cations in the
transition state. Thus the small anions may act as pseudo bridges even for reactions which
go by electron transfer. In addition, if the anion complexes with reducing agents first, it will
stabilise the oxidised form thus speeding up the rate of oxidation. Conversely, if the
oxidising agent is complexed first, this may stabilise it and slow down the rate of reaction.
For example, the rate of reaction between [Ru(bipy)3]2+ and Ce4+ is reduced strongly by
SO42- present in solution by the formation of Ce(SO4)2+
Doubly bridged inner sphere transfer
Only a few examples are known for this type of reactions. Eg: reduction of
cis[Co(en)2(N3)2]+ as its tetrammine analogue by Cr2+. Approximately 1.2 to 1.4 azide
ligands per Cr(II) are found in the respective products suggesting a doubly bridged
intermediate.
Two electron transfer reactions
In this case only one bridging ligand is involved but two electrons are transferred.
This type of reactions occurs for elements having stable oxidation states which differ by two
electrons without a stable one in between.
Sn2+ + Tl3+ → Sn4+ + Tl+
The two electron transfer reactions can be further categorised into complimentary
and noncomplimentary reactions.
Complimentary reactions are those in which reductant loses and oxidant gains two
electrons. For eg: in Tl+-Tl3+ system in aqueous medium containing ClO4- ions, the two
electrons are transferred from Tl+ to Tl3+. Other examples are,
Sn2+ + Tl3+ → Sn4+ + Tl+
Sn2+ + Hg2+ → Sn4+ + Hg0
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M.Sc CHEMISTRY – SEMESTER 2 - INORGANIC CHEMISTRY
Noncomplimentary reactions are those in which the no: of electrons released by the
reductant is not equal to the no: of electrons accepted by the oxidant. They generally
proceed through a multistep path.
Fe2+ + Tl3+ → Fe3+ + Tl2+
Fe2+ + Tl2+ → Fe3+ + Tl+
2Fe2+ + Tl3+ → 2Fe3+ + Tl+
RACEMIZATION REACTIONS
The most interesting rearrangements involve cis-trans (geometrical) and dextroleavo (optical) transformations. For example, when an HCl solution of violet dcis[Co(en)2Cl2] is evaporated, the green trans form was obtained. Conversely when an
aqueous solution of the green trans form was concentrated on a steam bath, violet crystals of
cis form separate. Here the rate of loss of optical activity was faster than rate of formation of
trans form. Thus optical transformation is faster than geometrical transformation.
There are several mechanisms that are possible for such an inversion that broadly
falls into two classes. a) those with bond rupture and b) those without bond rupture.
One mechanism with bond rupture involves complete dissociation of one chelating
ligand with formation of a square planar or trans diaqua complex as first step.
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M.Sc CHEMISTRY – SEMESTER 2 - INORGANIC CHEMISTRY
The dissymmetry would thus be lost and when the chelate ring reforms, it would
have a 50-50 chance of producing either the λ or δ isomer. Since the rate determining step is
the dissociation step, the rate of racemisation, kr would be equal to the rate of dissociation,
kd. Example for complexes that undergo rearrangement by this mechanism is tris
phenanthroline Ni(II).
In another mechanism that involves dissociation, only one end of the chelate
detaches with formation of a five coordinated complex which can be square pyramidal or
trigonal bipyramidal. Either of these could undergo pseudo rotation with scrambling of
ligand sites.
Reattachment of the dangling end of the bidentate ligand to reform the chelate ring would
give a racemic mixture of λ and δ isomers. Here also the rate determining step is the
dissociation step. i.e. kr ≈ kd. Considering the steric factors, the trigonal bipyramid leads to
racemisation but not trans isomer formation.
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M.Sc CHEMISTRY – SEMESTER 2 - INORGANIC CHEMISTRY
Since the loss of optical activity takes place by both pathways and trans isomerisation occurs
only through one path, the loss of optical activity is more rapid than cis-trans isomerisation.
Example for complexes that undergo rearrangement by this mechanism is [Co(en)2Cl2].
For many complexes kr > kd. i.e racemisation occurs without bond rupture. Therefore
an intramolcular pathway must be operative. There are four symmetry allowed pathways for
this. Of these, the push through (six coplanar ligands) and the cross over (four coplanar
ligands) mechanisms both require large metal ligand bond stretches to relieve steric
hindrance and are hence energetically unfavourable.
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M.Sc CHEMISTRY – SEMESTER 2 - INORGANIC CHEMISTRY
The other two mechanisms (twist) require much less bond stretches and are believed
for racemisation. The earliest twist mechanism proposed by Ray and Dutt is known as
rhombic twist. This involves reflecting one eclipsed ligand along parallel triangular edges
and then into the mirror image for the other two sets of ligands. The other twist mechanism
is called Bailar twist or trigonal twist mechanism. This involves twisting a complex about a
threefold axis into its mirror image.
Calculations show that the trigonal twist is favoured when the bite, b defined as the
distance between donor atoms in the same chelate ligand is substantially smaller than l, the
distance between donor atoms on neighbouring chelate ligand. On the other hand, rhombic
twist is favoured when b is much greater than l. When b and l are not significantly different,
both twist mechanisms may operate simultaneously.
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