Substitution and racemization in octahedral complexes

Values of Hpt vary considerably (Table 25.2) and illustrate the dependence of the rate of substitution on the nucleophilicity of the entering group. There is no correlation between npt and the strength of the nucleophile as a Bronsted base. 

Finally, we should note that a small civ-effect does exist, but is usually of far less importance than the iranv-effect. 

If one studies how the rate of substitution by Y in a given complex depends on the entering group, then for most reactions at Pt(II), the rate constant 2 (equation 25.9) increases in the order  

This is called the nucleophilicity sequence for substitution at square planar Pt(II) and the ordering is consistent with Pt(II) being a soft metal centre (see Table 6.9). A nucleophilicity parameter, Kpt, is defined by equation 25.19 where kf is the rate constant for reaction 25.18 with Y = MeOH (i.e. for Y = MeOH, npt = 0). 

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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... 

If one end of a chelate ring on an octahedral complex is detached from the metal, the five-coordinate transition state can be considered as a fluxional molecule in which there is some interchange of positions. When the chelate ring reforms, it may be with a different orientation that could lead to racemization. If the chelate ring is not symmetrical (such as 1,2-diaminopropane rather than ethyl-enediamine), isomerization may also result. For reactions carried out in solvents that coordinate well, a solvent molecule may attach to the metal where one end of the chelating agent vacated. Reactions of this type are similar to those in which dissociation and substitution occur. 

Any detailed description of the mechanism of an octahedral substitution must also account for the stereochemical changes that accompany reaction. Werner recognized this and made use of it in his discussions of the stereochemistry of reactions of cobalt(III) complexes. The available experimental results can be explained on the basis of possible molecular rearrangements and some cautious predictions can even be made. The base hydrolysis of cobalt III)ammines appears to be unique in that it often occurs with rearrangement it also affords the few known examples of optical inversion. These results can be explained by formation of a 5-coordinated species with a trigonal bipyramidal structure. Optically active metal complexes racemize by either an intramolecular or an in-termolecular process. Substitution reactions of platinum metal complexes often occur with retention of configuration. 

The use of activation volumes in the diagnosis of mechanism has continued to provide much valuable information. Activation volumes for substitution at octahedral complexes have formed the subject of a well-referenced review,in which the importance both of intrinsic and of solvation contributions is recognized. The topics of most relevance to this chapter include isomerization and racemization reactions of cobalt(III) complexes, aquation of cobalt(III) and of iron(II) complexes, and base hydrolysis of cobalt(III) complexes. Merbach s continuing investigations into the effects of pressure on rates of solvent exchange at 2-h and 3+ transition metal cations, while not being always strictly... 

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