Substitution Reaction Kinetics
2.2 Mechanistic Classification of Inorganic Substitution Reactions
A reaction mechanism illustrates the progress of different steps by which a chemical transformation occurs which culminates in the observed overall reaction.5 Langford and Gray6 used an operational way to classify inorganic reactions based on the concepts of stoichiometric mechanism and intimate mechanisms.2 The stoichiometric mechanism can be determined from the kinetic behaviour and classified in one of the three forms.2,7
a. Limiting associative (A)- a mechanism having an intermediate with higher coordination number. The ‘A’ term replaces the older SN2 term.
b. Limiting dissociative (D)- a mechanism having an intermediate with lower coordination number. The ‘D’ term replaces the older SN1 term.
c. Interchange (I)- a mechanism in which the bond breaking and bond forming take place in a pre-formed aggregate. No observable intermediate forms for this pathway.
The stoichiometric interchange mechanism is further classified based on intimacy:1-2 a. Associatively activated (IA) – an intimate mechanism which has a
transition state which involve the bonding between the incoming group and the reactive centre.
b. Dissociatively activated (ID) – an intimate mechanism where no direct interaction between the reactive centre and the entering group in the transition state, i.e. the reaction centre is more sensitive to changes in the leaving group,
For the intimate mechanism, the incoming group, Y, and the leaving group, X, are interchanged in the inner coordination sphere of the metal centre. Therefore, the rate is independent of the incoming nucleophile and the relative reaction coordinates from associative to dissociative can be represented as in Figure 2.1.8
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Figure 2.1 Reaction profiles for (a) associative A, (b) associative interchange, Ia, (c) dissociative interchange Id and (d) dissociative D.8
2.2.1 Limiting Associative Mechanism
In this mechanism, the intermediate is formed via two transition states in which bond formation is more dominant than bond breaking (Scheme 2.1). Here, the rate determining step is one which involves formation of a bond between the metal centre and the incoming nucleophile. When the entering group, Y, and the leaving group, X, are chemically identical, the bond making and the bond breaking transition states have the same energy. In a non-coordinating medium and in the presence of an excess of Y, the substitution is strongly dependent on the nature of the incoming nucleophile. Y participates in the early stages of the transition state and at the end stereochemistry of the complex is retained.9
In an associative mechanism, all the species involved such as the incoming group, the leaving group and the chelate ligand can influence the stability and the activation energy of the reaction. Thus, all the groups will influence the rate of substitution reaction of the complex. For this reason many ligand substitution reactions are performed by varying the character of the ligands.10
Y
T M
X1 X2
L
Y
T M
X1 X2
L T M
X1 X2
T M
X1 X2
Y + L Incoming group can attach from
top or bottom
L Y
transition state T = trans ligand, L = leaving group, X1
and X2 are the chelate ligands
Scheme 2.1 Associative mode of substitution at the metal centre.11
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It has been reported12 that Pt(II) complexes containing strong cis Pt―C bonds result in the mechanistic changeover in Pt(II) complexes of the form cis-[Pt(L)2R2] (where L2 = Me, Ph and R2 = thioethers or DMSO). The trans effect due to the strong σ-donor increases the electron density at the Pt(II) centre and weakens the Pt―S bond length in case of DMSO. The fourteen-electron transition state intermediate is stabilized by the strong σ-donor carbine ligands. However, when one of the thioethers were replaced by a strong π-acceptor ligand such as CO or CNˉ, an associative mechanism was favoured for the Pt(II) complexes. The stronger π-accepting ligands remove the electron density from the metal centre thereby increasing the electrophilicity of the Pt(II) centre. This enhances the acceptance of the electron density from the incoming nucleophile and removing the added electron density from the metal centre in a five-coordinate transition state. Thus, the type of the reaction mechanism depends on the nature of the chelate ligand at the Pt(II) centre.
2.2.2 Dissociative Mechanism
Depending on the electronic and the steric effects, a dissociative pathway may be favoured by weakening of the metal-leaving group bond. In dissociative (D) mechanism, the bond between the leaving group and the metal breaks completely before incoming group attaches to the metal centre resulting in an intermediate with lower coordination number. This allows the intermediate to discriminate the potential ligands in the surrounding medium before it reacts with the entering group.9a The rate of reaction with the entering group depends on the nature of the leaving group and is independent of the concentration and the nature of the incoming group. The leaving group moves from the coordination shell, which favours the solvent attack as the solvent is present in large access. Therefore, the formation of the product will occur while the leaving group is still in close proximity.
One of the important aspects of dissociative mechanism in square planar Pt(II) complexes is the non-stereo specific products formed. During dissociation mechanism, the complex forms a T-shaped three coordinate intermediate, which undergoes intermolecular rearrangements of ‘cis-like’ configuration to a ‘trans-like’ configuration.
The mechanism is rare for square planar Pt(II) complexes because increase of electron density at the Pt(II) centre hinders the approach of the nucleophiles and stabilization of the coordinatively unsaturated 14-eletron intermediate.12d
5 2.2.3 Interchange Mechanism
In between the limiting associative and limiting dissociative mechanisms, there exists a set of mechanisms which involves a single activated complex in which bond formation between the metal centre and the incoming nucleophile and bond breaking between the metal and the leaving group are concurrent. These mechanisms are considered as interchange mechanisms. They are:
i. Associatively Activated Interchange Mechanism: In this mechanism, the rate of reaction dependents on the nature of the incoming species since the rate limiting step involves bond formation between the entering group and the reactive centre in the transition state. The bond breaking is less important even though the two steps co-exist leading to a single activated complex.13
ii. Dissociatively Activated Interchange Mechanism: In a dissociatively activated mechanism, the leaving group starts to break away from the inner coordination sphere to the outer coordination sphere at the same time while the entering group moves from the outer coordination sphere to the inner coordination sphere. In this mechanism, if there is a reagent whose concentration is much less than that of the solvent which is already in the inner coordination sphere when the dissociation takes place, then the probability of the solvent attaching to the reactive metal centre is higher. Thus, the discrimination between the entering group and the solvent molecules is minimized.
2.3 Substitution Reactions of Square Planar Platinum(II) Complexes
A full understanding of four coordinate substitution reactions dates back to 1920s.13 Square planar complexes often contain d8, low-spin, 2+ oxidation state metal centres.9 Common examples of square planar complexes include coordinated complexes commonly formed by Ni(II), Pd(II), Au(III), Rh(I) and Pt(II).9b Of these, substitution reactions of platinum are the most well studied and understood in inorganic reaction mechanisms due to its redox stability and moderately slow reactivity.14 This allows the synthesis of specifically designed platinum complexes and investigation of their kinetic behaviour.2,9b,13,14b To date, appreciable amount of research has been reported on the kinetic and the mechanistic behaviour of substitution reactions of Pt(II) complexes.6,7b,13,14b Since the mechanism of substitution behaviour of Pt(II) is similar to other square planar d8-metal complexes, the information collected on Pt(II) complexes can therefore be useful to the other d8-metal complexes. For example, substitution
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reactions of Pd(II) complexes are very similar to Pt(II)’s. However, Pd(II) complexes are often five order of magnitude faster than the Pt(II) analogues.15
During substitution, the coordinatively unsaturated square planar d8 Pt(II) metal complexes often undergo an associative mechanism which proceeds via a five coordinate, trigonal bipyramidal transition state having eighteen electrons in the valence shell.16 Pt(II) d8 complexes have vacant 6pz orbitals which can easily accommodate the extra electrons from the incoming nucleophile.2,9b,14b Moreover, square planar complexes being four coordinate, are not sterically hindered. Thus, the incoming ligand can approach the metal centre both from above and below the plane, retaining the stereochemistry of the complex as shown in Figure 2.2(e).7b
Figure 2.2 Schematic representation of the energy profile and possible steric changes during an associative substitution of leaving group, X by the entering group, Y of a square planar complex: energies at 2, 4, 6, and 8 represent the transition states and the reaction intermediates would have energies shown at 3, 5 and 7.7b,17
Therefore, the incoming ligand binds to the metal centre before the leaving group leaves, resulting in a trigonal bipyramidal complex7b,9b,13 (Figure 2.2, c). Although an associative mechanism is more common for square planar complexes, recently, some dissociative mechanisms for square planar complexes have been also reported.12a,12d,18
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2.3.1 Kinetics and Mechanism of Substitution Reactions
The nature of the reactants and the solvent play an important role during the substitution reactions of Pt(II) complexes. The substitution kinetics of square planar complexes may take place in any one of the three pathways shown in Figure 2.3.2 This involves an associative pathway either by direct attack of the nucleophile or the solvent and a dissociative pathway which involves a three coordinate intermediate.2
Figure 2.3 Schematic representation for substitution in d8 four coordinate square planar complexes showing the alternative D and A or Ia solvolysis.2
2.4 Determining the Rates of Ligand Substitution Reactions