d-block metal complexes: reaction...

Dr. Said El-Kurdi 5/15/2015

Dr. Said El-Kurdi 1

Inorganic Chemistry B Inorganic Chemistry B

Chapter 26

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d-Block metal complexes: reaction mechanisms

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26.1 Introduction

Mechanisms of ligand substitution and electron-transfer reactions in coordination complexes.

A proposed mechanism must be consistent with all experimental facts.

A mechanism cannot be proven, since another mechanism may also be consistent with the experimental data.

26.2 Ligand substitutions: some general points

In a ligand substitution reaction: MLxX + Y MLxY + X X is the leaving group and Y is the entering group.


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Kinetically inert and labile complexes

Metal complexes that undergo reactions with t1/2 1min are described as being kinetically labile. If the reaction takes significantly longer than this, the complex is kinetically inert.

There is no connection between the thermodynamic stability of a complex and its lability towards substitution.

For example, values of hydGo for Cr3+ and Fe3+ are almost equal, [Cr(OH2)6]3+ (d3) undergoes substitution slowly [Fe(OH2)6]3+ (high-spin d5) undergoes substitution rapidly.

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exchange of a water molecule in the first coordination sphere of [M(OH2)x]

n+ with one outside this coordination shell

average residence time (=1/k) of an H2O ligand in the first coordination sphere of a metal ion.


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For main group metal ions

On descending a given group, the rate of water exchange increases as:

the metal ion increases in size

the coordination number increases the surface charge density decreases

[Li(OH2)6]+ (least labile) to [Cs(OH2)8]+ (most labile).

Each group 13 M3+ forms a hexaaqua ion, and values of k range from 1 s1 for [Al(OH2)6]3+ to 107 s1 for [In(OH2)6]3+, consistent with the increase in ionic radius from 54pm (Al3+) to 80pm (In3+).


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The lanthanoid M3+ complex ions are all relatively labile with k >107?????????

Stoichiometric equations say nothing about mechanism

use of H218O as solvent shows that all the oxygen in the

aqua complex is derived from carbonate

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Types of substitution mechanism

In inorganic substitutions, the limiting mechanisms are dissociative (D), in which the intermediate has a lower coordination number than the starting complex and associative (A), in which the intermediate has a higher coordination number

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In most metal complex substitution pathways, bond formation between the metal and entering group is thought to be concurrent with bond cleavage between the metal and leaving group. This is the interchange (I) mechanism.

In an I mechanism, there is no intermediate but various transition states are possible.


dissociative interchange (Id), in which bond breaking dominates over bond formation; the rate shows only a very small dependence on the entering group.

associative interchange (Ia), in which bond formation

dominates over bond breaking. the reaction rate shows a dependence on the entering group.

It is usually difficult to distinguish between A and Ia D and Id Ia and Id processes.


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Activation parameters

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An Eyring plot


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Values of S‡ are particularly useful in distinguishing between associative and dissociative mechanisms.

A large negative value of S‡ is indicative of an associative mechanism, i.e. there is a decrease in entropy as the entering group associates with the starting complex.

Solvent reorganization can result in negative values of S‡ even for a dissociative mechanism

The pressure dependence of rate constants leads to a measure of the volume of activation, V‡


A reaction in which the transition state has a greater volume than the initial state shows a positive V‡, whereas a negative V‡ corresponds to the transition state being compressed relative to the reactants.


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A large negative value of V‡ indicates an associative mechanism; a positive value suggests that the mechanism is dissociative.


26.3 Substitution in square planar complexes

Complexes with a d8 configuration often form square planar

complexes, especially when there is a large crystal field: Rh(I),

Ir(I), Pt(II), Pd(II), Au(III).

4-Coordinate complexes of Ni(II) may be tetrahedral or

square planar.

The majority of kinetic work on square planar systems has

been carried out on Pt(II) complexes because the rate of

ligand substitution is conveniently slow.




The trans-influence

Consider a square planar complex which contains a trans LML’ arrangement:

Ligands L and L’ compete with each other for electron density because the formation of ML and ML’ bonds uses the same metal orbitals.


ground state trans-influence (i.e. the influence that L has on the ML’ bond)

for a series of square planar complexes trans [PtXH(PEt3)2]




for a series of square planar complexes trans [PtXH(PEt3)2]

Values of (Pt–H) show that the PtH bond is weakest for X=CN and the trans-influence of the X ligands follows the order CN > I > Br > Cl.

trans-influence may be observed wherever ligands are mutually trans, e.g. in octahedral species.


Rate equations, mechanism and the trans-effect

based on a large body of experimental work, nucleophilic substitution reactions in square planar Pt(II) complexes normally proceed by associative mechanisms (A or Ia). Negative values of S‡ and V‡ support this proposal

studied under pseudo-first order conditions, with Y (as well as the solvent, S) in vast excess.





Rate constant k2 depends on Y,

These lines pass through a common intercept, equal to k1. If the kinetic runs are repeated using a different solvent, a different common intercept is observed.

The k2 term arises from an associative mechanism involving attack by Y on PtL3X in the rate-determining step, and when Y is a good nucleophile, the k2 term is dominant.

The contributions of the two terms in equation 26.12 to the overall rate reflect the relative dominance of one pathway over the other.




The k1 term might appear to indicate a concurrent dissociative pathway. However, experiment shows that the k1 term becomes dominant if the reaction is carried out in polar solvents, and its contribution diminishes in apolar solvents.

When the solvent is a potential ligand (e.g. H2O), it competes with the entering group Y in the rate-determining step of the reaction, and X can be displaced by Y or S. Substitution of S by Y then occurs in a fast step, i.e. non-rate determining.


In the majority of reactions, substitution at square planar Pt(II) is stereoretentive: the entering group takes the coordination site previously occupied by the leaving group. An A or Ia mechanism involves a 5-coordinate intermediate or transition state and, since the energy difference between different 5-coordinate geometries is small, one would expect rearrangement of the 5-coordinate species

The choice of leaving group in a square planar complex is determined by the nature of the ligand trans to it;

this is the trans-effect and is kinetic in origin.

The choice of leaving group in a square planar complex is determined by the nature of the ligand trans to it;

this is the trans-effect and is kinetic in origin.




Initial attack by the entering group at a square planar Pt(II) centre is from above or below the plane. Nucleophile Y then coordinates to give a trigonal bipyramidal species which loses X with retention of stereochemistry.


If L2 is a strong -acceptor (e.g. CO), it will stabilize the transition state by accepting electron density that the incoming nucleophile donates to the metal center, and will thereby facilitate substitution at the site trans to it.

The general order of the trans-effect (i.e. the ability of ligands to direct trans-substitution) spans a factor of about 106 in rates and is:




Ligand nucleophilicity

Effect of entering group

for most reactions at Pt(II), the rate constant k2 increases in the order:

This is called the nucleophilicity sequence for substitution at square planar Pt(II)


A nucleophilicity parameter, nPt, is defined by equation 26.22 where k2’ is the rate constant for reaction 26.21 with Y = MeOH (i.e. for Y = MeOH, nPt = 0).

The nucleophilicity parameter, nPt, describes the dependence of the rate of substitution in a square planar Pt(II) complex

on the nucleophilicity of the entering group.

The nucleophilicity parameter, nPt, describes the dependence of the rate of substitution in a square planar Pt(II) complex

on the nucleophilicity of the entering group.




If we now consider substitution reactions of nucleophiles with other Pt(II) complexes, linear relationships are found between values of log k2 and nPt

where s is the nucleophilicity discrimination factor and k2’ is the rate constant when the nucleophile is MeOH.


For a given substrate, s can be found from the slope of a Line

Each complex has a characteristic value of s,




The relatively small value of s for [Pt(dien)(OH2)]2+ indicates that this complex does not discriminate as much between entering ligands as, for example, does trans-[PtCl2PEt3]2; i.e. [Pt(dien)(OH2)]2+ is generally more reactive towards substitution than other complexes in the table, consistent with the fact that H2O is a good leaving group.


The nucleophilicity discrimination factor, s, is a characteristic of a given square planar Pt(II) complex and describes how

sensitive the complex is to variation in the nucleophilicity of the entering ligand.

The nucleophilicity discrimination factor, s, is a characteristic of a given square planar Pt(II) complex and describes how

sensitive the complex is to variation in the nucleophilicity of the entering ligand.




26.4 Substitution and racemization in octahedral complexes

The popular candidates for study have been Cr(III) (d3) and low-spin Co(III) (d6) species. These complexes are kinetically inert and their rates of reaction are relatively slow and readily followed by conventional techniques.

Water exchange


for M2+ and M3+ ions of the d-block metals, data for reaction 26.24 indicate a correlation between rate constants and electronic configuration.

The change from negative to positive values of V‡ indicates a change from associative to dissociative mechanism, and suggests that bond making becomes less (and bond breaking more) important on going from a d3 to d8 configuration




For the M3+ ions, values of V‡ suggest an associative mechanism. Where data are available, an associative process appears to operate for second and third row metal ions, consistent with the idea that larger metal centers may facilitate association with the entering ligand.

First order rate constants, k, for reaction 26.24 vary greatly among the first row d-block metals (all high-spin Mn+ in the hexaaqua ions):


The Eigen-Wilkins mechanism

Water exchange is always more rapid than substitutions with other entering ligands

for most ligand substitutions in octahedral complexes, experimental evidence supports dissociative pathways.




The Eigen–Wilkins mechanism applies to ligand substitution in an octahedral complex.

An encounter complex is first formed between substrate and entering ligand in a preequilibrium step, and this is

followed by loss of the leaving ligand in the rate-determining step.

An encounter complex is first formed between substrate and entering ligand in a preequilibrium step, and this is

followed by loss of the leaving ligand in the rate-determining step.

The first step in the Eigen–Wilkins mechanism is the diffusing together of ML6 and Y to form a weakly bound encounter complex


Usually, the rate of formation of {ML6,Y} and the back reaction to ML6 and Y are much faster than the subsequent conversion of {ML6,Y} to products. Thus, the formation of {ML6,Y} is a pre-equilibrium.

The rate-determining step

estimated value




at low concentrations of Y where KE[Y ] 1


The fact that k varies so little is consistent with an Id

mechanism. If the pathway were associative, the rate would depend more significantly on the nature of Y.

At a high concentration of Y (e.g. when Y is the solvent), KE[Y] 1, and equation 26.32 approximates to equation 26.35, a first order rate equation with no dependence on the entering ligand. The value of k can be measured directly kobs = k.




An Id mechanism is supported in very many instances. From experimental trends

The rate of ligand substitution usually depends on the nature of the leaving ligand.

the rate of substitution increases with X in the following order:

This trend correlates with the MX bond strength


The stronger the bond, the slower the rate, and is consistent with the rate-determining step involving bond breaking in a dissociative step




Stereochemistry of substitution

most substitutions in octahedral complexes involve D or Id pathways, consider the stereochemical implications of the D mechanism this involves a 5-coordinate species


If the mechanism is limiting dissociative (D), a 5-coordinate intermediate must be involved . It follows that the stereochemistry of [Co(OH2)(en)2Y]2+ must be independent of the leaving group X, and will depend on the structure of the intermediate




mixture of cis- and trans-products in an approximately 2 : 1 ratio.


By comparing pairs of data for complexes with the same Y but different leaving groups (X = Cl or Br),

addition of water in the closing step of the reaction is faster than any internal square-based pyramidal–trigonal bipyramidal rearrangement.

Stereochemistry of aquation of cis- or trans-[CoX(en)2Y]+ is essentially independent of the leaving group.




Base-catalysed hydrolysis

because [OH] deprotonates a coordinated NH3 ligand.

pre-equilibrium is first established, followed by loss of X to give the reactive amido species


conjugate–base mechanism (Dcb or SN1cb mechanism).




Isomerization and racemization of octahedral complexes

octahedron is stereochemically rigid, loss of a ligand gives a 5-coordinate species which can undergo Berry pseudo-rotation

we discussed cases where the assumption is that such rearrangement does not occur


if the lifetime of the intermediate is long enough, it provides a mechanism for isomerization

For [Ni(bpy)3]2+ and [Ni(phen)3]2+, the rates of exchange with 14C-labelled ligands are the same as the rates of racemization.

This is consistent with a dissociative process (equation 26.47) in which the intermediate is racemic, or racemizes faster than recombination with LL.




Such a dissociative mechanism is rare

Two intramolecular mechanisms are possible


(a) the Bailar twist and

(b) the Ray–Dutt twist.




Cleavage and reformation of the ML bond of one end of the bidentate ligand.


26.5 Electron-transfer processes

The simplest redox reactions involve only the transfer of electrons, and can be monitored by using isotopic tracers

In an outer-sphere mechanism, electron transfer occurs without a covalent linkage being formed between the reactants. In an inner-sphere mechanism, electron transfer occurs via a covalently bound bridging ligand.




Inner-sphere mechanism

All the Cr(III) produced was in the form of [Cr(OH2)5Cl]2+, and tracer experiments showed that all the chloro ligand in [Cr(OH2)5Cl]2+ originated from [Co(NH3)5Cl]2+.

Since the Co center could not have lost Cl before reduction, and Cr could not have gained Cl after oxidation, the transferred Cl must have been bonded to both metal centers during the reaction.


Cl is transferred between metal centers; such transfer is often (but not necessarily) observed.




the intermediate 26.4 (which is stable enough to be precipitated as the Ba2+ salt) is slowly hydrolysed to products without transfer of the bridging ligand.

[Fe(CN)6]3 + [Co(CN)5]3 [Fe(CN)6]4 + [Co(CN)5(OH2)]2


Common bridging ligands in inner-sphere mechanisms include halides, [OH], [CN], [NCS], pyrazine and 4,4’-bipyridine.

The steps of an inner-sphere mechanism are bridge formation, electron transfer and bridge cleavage.




Most inner-sphere processes exhibit second order kinetics overall.

Any one of bridge formation, electron transfer or bridge cleavage can be rate-determining.

For the following reaction with a range of ligands X,

the rate-determining step is electron transfer, and the rates of reaction depend on X


Second order rate constants for reaction 26.55 with different bridging X ligands.

The increase in k along the series F, Cl , Br , I correlates with increased ability of the halide to act as a bridge




Outer-sphere mechanism

When both reactants in a redox reaction are kinetically inert, electron transfer must take place by a tunnelling or outersphere mechanism.

In a self-exchange reaction, the left- and right-hand sides of the equation are identical; only electron transfer, and no net chemical reaction, takes place.

In a self-exchange reaction, the left- and right-hand sides of the equation are identical; only electron transfer, and no net chemical reaction, takes place.

The Franck–Condon approximation states that a molecular electronic transition is much faster than a molecular vibration.

The Franck–Condon approximation states that a molecular electronic transition is much faster than a molecular vibration.


This reductant–oxidant pair is called the encounter or precursor complex.




Consider a self-exchange reaction of the type:

There is no overall reaction and therefore Go = 0, and K = 1.

Why do reactions of this type have widely differing reaction rates? Why do reactions of this type have widely differing reaction rates?

It is usually the case that the M–L bond lengths in the M(III) complex are shorter than those in the corresponding M(II) complex.


How can a reaction with Go = 0 continually lose energy as the electron is transferred between [ML6]2+ and [ML6]3+? The answer, of course, is that it cannot.

The electron transfer can only take place when the M–L bond distances in the M(II) and M(III) states are the same, i.e. the bonds in [ML6]2+ must be compressed and those in [ML6]3+ must be elongated

Franck–Condon restriction

The activation energy required to reach these vibrational excited states varies according to the system, and hence the self-exchange rate constants vary.




The outer-sphere mechanism: when the reactants have differing bond lengths, vibrationally excited states with equal bond lengths must be formed in order to allow electron transfer to occur.


[Fe(bpy)3]2+ and [Fe(bpy)3]3+, low-spin Fe–N bond distances are 197 and 196 pm, respectively. Electron transfer involves only a change from t2g

5 to t2g6 (Fe2+ to

Fe3+) and vice versa. k > 106 dm3 mol 1 s1

The greater the changes in bond length required to reach the encounter complex, the slower the rate of electron transfer. The greater the changes in bond length required to reach the encounter complex, the slower the rate of electron transfer.

For example, the rate of electron transfer between [Ru(NH3)6]2+ (Ru–N= 214 pm, low-spin d6) and [Ru(NH3)6]3+ (Ru–N = 210 pm, low-spin d 5) is 104 dm3 mol 1 s1.




Electron transfer between [Co(NH3)6]2+ (Co–N=211 pm) and [Co(NH3)6]3+ (Co–N=196 pm) requires not only changes in bond lengths, but also a change in spin state: [Co(NH3)6]2+ is high-spin d7 (t2g

5eg2) and

[Co(NH3)6]3+ is low-spin d6 (t2g6eg

0). Transfer of an electron between the excited states shown in Figure 26.10 leads to a configuration of t2g

5eg1 for

{[Co(NH3)6]3+}* and t2g6eg

1 for{[Co(NH3)6]2+}*. These are electronically excited states,


The activation energy for the self-exchange reaction therefore has contributions from both changes in bond lengths and changes in spin states.

self-exchange between [Co(phen)3]2+ and [Co(phen)3]3+ is much faster than between [Co(NH3)6]2+ and [Co(NH3)6]3+ or [Co(en)3]2+ and [Co(en)3]3+ (all three exchange processes are between high-spin Co(II) and low-spin Co(III)).

This is consistent with the ability of phen ligands to use their -orbitals to facilitate the intermolecular migration of an electron from one ligand to another, and phen complexes tend to exhibit fast rates of self-exchange.




The rates of self-exchange reactions involved cationic species in aqueous solution are typically not affected by the nature and concentration of the anion present in solution.

The rate of electron transfer between anions in aqueous solution generally depends on the cation and its concentration.


For example, the self-exchange reaction between

[Fe(CN)6]3 and [Fe(CN)6]4 with K+ as the counter-ion. The

rate constant of self-exchange reaction is of the order of

104 dm3 mol1 s1

by adding the macrocyclic ligand 18-crown-6 to complex

the K+ ions, the rate constant of self-exchange is 2.4102

dm3 mol1 s1

d-block metal complexes: reaction...

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