Substitution in Square-Planar Complexes

Complexes with a.d configuration often form square planar complexes (see Section 21.3), especially when there is a large crystal field Rh(I), Ir(I), Pt(II), Pd(II), Au(III). However, 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 hgand substitution is conveniently slow. Although data for Pd(II) and Au(III) complexes indicate similarity between their substitution mechanisms and those of Pt(II) complexes, one cannot justifiably assume a similarity in kinetics among a series of structurally related complexes undergoing similar substitutions. 

The consensus of opinion, based on a large body of experimental work, is that nucleophilic substitution reactions in square planar Pt(II) complexes normally proceed by associative mechanisms (A or 4). Negative values of AS and AV support this proposal (Table 26.1). The observation 

Reaction 26.11 shows the substitution of X by Y in a square planar Pt(II) complex. 

The usual form of the experimental rate law is given by equation 26.12 indicating that the reaction proceeds simultaneously by two routes. 

Reaction 26.11 would usually be studied under pseudo-first order conditions, with Y (as well as the solvent, S) in vast excess. This means that, since [Y] a [Y]q, and [S] [S]q (where the subscripts represent time t and time zero), we can rewrite equation 26.12 in the form of equation 26.13 where fegbs is the observed rate constant and is related to and 2 by equation 26.14. 

Reaction 25.8 would usually be studied under pseudo-first order conditions, with Y (as well as the solvent, S) in vast 

The contributions of the two terms in equation 25.9 to the overall rate reflect the relative dominance of one pathway over the other. The 2 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 2 term is dominant. The term might appear to indicate a concurrent dissociative pathway. However, experiment shows that the term becomes dominant if the reaction is carried out in polar solvents, and its contribution diminishes in apolar solvents. This indicates solvent participation, and equation 25.9 is more fully written in the form of equation 

The majority of square planar complexes are those that contain d8 metal ions, of which the most common examples are Ni2+, Pd2+, and Pt2+, although some complexes containing Au3+ have also been studied. As a general trend, the rate of substitution in these complexes is 

Because most complexes of platinum are quite stable, substitution reactions are generally slow. For the reaction 

Therefore, the observed first-order rate constant, kobs, is given by 

There is a considerable difference in rate of substitution depending on the nature of the leaving group. For the reaction 

I FIGURE 20.8 Substitution in a square planar complex in which the solvent participates in a second-order step. 

Although in the previous section the basic concepts related to substitution reactions were explained with reference to octahedral complexes, substitution reactions are also common in square planar complexes. Studies on these complexes have resulted in a great deal of knowledge of the mechanisms of these reactions, so a brief description of the topic is presented next. 

As explained in Chapter 19, the most common complexes having square planar geometry are those of the ds ions Pt(II), Pd(II), Au(III), and, to a lesser extent, Ni(II). Substitution reactions of complexes containing Pt(II) have been the subject of a large number of kinetic studies. 

It can easily be seen that the only product possible when one X ligand is replaced is [PtX3Y]. The first of the limiting mechanisms possible for such a substitution reaction is described as X leaving the coordination sphere of the metal ion before Y enters. This is the SN1 or dissociative mechanism described earlier, and it can be shown as 

If Y substitutes for X in [PtX4] by an Sn2 mechanism, the process involves an increase in coordination number of the metal as the transition state forms. The entering group, Y, becomes coordinated before the leaving group, X, departs, giving a transition state that can be shown as 

In this associative pathway, the rate law depends on the concentrations of both the complex ion and the entering ligand because both are present in the transition state. Therefore, the rate law for the reaction is 

The pressure dependence of rate constants leads to a measure of the volume of activation, AV (eq. 26.10). 

A reaction in which the transition state has a greater volume than the initial state shows a positive A, whereas a negative AV corresponds to the transition state being compressed relative to the reactants. After allowing for any change in volume of the solvent (which is important if solvated ions 

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

As an alternative to eq. 26.9, the following linearized form of the Eyring equation can be derived from eq. 26.8  

What graph would you constmct to obtain a Unear plot How would you use this plot to obtain values of AH and AS l [Am. See G. Lente et al. (2005) New. J. Chem., voL 29, p. 759] 

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One of the fascinating characteristics of substitution in square planar complexes is illustrated by the following equations ... 

A completely empirical LFER can also be constructed with recourse only to kinetic data. This has been the case in the setting up of a scale of nucleophilic power for ligands substituting in square-planar complexes based on the Swain-Scott approach. The second-order rate constants Ay for reactions in MeOH of nucleophiles Y with tra 5-Pt(py)2Cl2, chosen as the standard substrate... 

Fig. 4.9 Simplified reaction profiles for various situations in the associative mechanism for substitution in square planar complexes, focusing attention on the replacement M-X-l-Y —> M-Y + X(4.93).

This problem requires a modified approach which Gray 16) has solved in the case of substitution in square planar complexes. He uses the fact that bases, like hydroxide, substitute very slowly but will immediately deprotonate, and hence stabilize, a protonic solvento intermediate. This elegant approach cannot be applied to the octahedral cobaltammines whose reaction rate with such bases is very high. 

Mechanism of Nucleophilic Substitution in Square Planar Complexes... 

Initially a solvent molecule attacks a vacant coordination site on Pd in (77-C3H5)Pd(77-C5H5) to give a 20-electron intermediate (XIV) having a distorted square-planar configuration. Probably the latter then becomes stabilized by the Tr-CjHjPd bond changing to a localized <7-bond. Subsequent reactions proceed as ordinary ligand substitution in square-planar complexes (1). 

The detection of a reaction intermediate is usually not possible in coordination chemistry because lifetimes of intermediates are commonly extremely short. The simple mechanisms of reaction are commonly designated as an associative mechanism (A, with an intermediate of expanded coordination number formed) or a dissociative mechanism (D, with an intermediate of reduced coordination number formed). Intermediates of expanded coordination number are important in ligand substitution in square-planar complexes and in a few cases can actually be detected. For example, NifCNls " is known from exchange reaction of Ni(CN)4 with CN (288). Even in octahedral complexes, some evidence for associative processes exists indirectly. The [RulNHsle] " ion reacts with NO in acid to form [RuINHslsNO] and NH4 much more rapidly than can be explained by aquation of the hexaamine as the initial step, and a bimolecular mechanism with a 7-coordinate intermediate has been proposed (11, 226). 

Kinetically labile and inert complexes Dissociation, association and interchange Activation parameters Substitution in square planar complexes Substitution in octahedral complexes Racemization of octahedral complexes Electron-transfer processes... 

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