Planar substitution mechanisms

The ki term in the rate law for square planar substitution is very clearly connected with an associative mechanism. The k term may be also. Consider the pathway (S = solvent)... 

Also tetracarbonylnickel which isn t sterically hindered but is d10, exchanges with radiocarbon monoxide by dissociative mechanism, and I think it could go to the same trigonal bipyramid without much steric difference. I wouldn t want these comments to swing everybody over to steric effects and say that really there is nothing interesting in substitution reactions—all the six-coordinate ones go one way, all the planar ones go the other way, and the electronic structure doesn t matter. This will drive people out of substitution mechanisms into the more interesting field of electron transfer mechanisms where electronic effects must be important. 

Next, I would like to speak on the mechanism of planar substitution, which hasn t been brought out too well today. 

If my memory serves me correctly, two different mechanisms have been suggested for planar substitution the first one, by a trigonal bipyramidal intermediate, was suggested in 1954 and 1955 by Chatt and Orgel. In 1958, an alternative mechanism for planar substitution was suggested in which 7r-bonding is not very... 

In this chapter, elimination reactions were presented both independently and in association with their related nucleophilic substitution mechanisms. Furthermore, the processes by which molecules undergo both El and E2 eliminations were presented and explained using bonding and nonbonding orbitals and their required relationships to one another. While much emphasis was placed on the planar relationships of orbitals required for both elimination reaction mechanisms, the special case of frans-periplanar geometries were described as necessary for efficient E2 eliminations to occur. 

Complexes with a configuration often form square planar complexes (see Section 20.3), 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 ligand 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. 

Figure 1. Possible mechanism for formation of acyl complex. Application of substitution mechanism on square planar d complexes.

All of these results support an A or 1 mechanism for square planar substitution. But why are there two terms ... 

Proposed competing mechanisms for square planar substitution reactions to explain the observed rate law shown in Figure 17.6 and given by Equation (17.40). 

For an associative mechanism, one would expect that the rate of substitution would show a strong dependence on the nucleophile, as the nucleophile directly participates in the RDS. This is indeed the case for square planar substitution reactions, as illustrated by the data in Table 17.12, which show that the rates of reaction vary by five orders of magnitude with the nature of the incoming ligand Y. 

In Equation (19.8), a similar mechanism occurs for the 18-electron [(> 6 6H6) Mo(CO)3] molecule, with the coordinated benzene ring slipping to an i/ -linkage to allow room for the initial PR3, where the intermediate [(>/4-C6H6)Mo(CO)3(PR3)] has been isolated and characterized in the solid state. Sometimes an associative mechanism will also involve a second term in its kinetics rate law involving a competition between addition of Y and addition of solvent, as was observed in Chapter 17 for square planar substitution reactions. In this case, the solvent term will follow pseudo-first-order kinetics. 

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