General mechanisms for substitution at a solvated metal ion

A generalized representation of the net reaction for metal—ion complex formation, i.e. substitution at a solvated metal ion, is 

For simplicity the back reaction in the second stage of each process is omitted. 

Kinetically, there are inherent difficulties in distinguishing the three mechanisms. As an illustration, is the anation reactions of the inert (d low-spin) complexes of cobalt(III), as typified by 

Ion-pairing is avoided by using anionic, rather than cationic, complexes and there is strong kinetic evidence [10] to suggest that a common five-coordinated intermediate Co(CN)s is involved in the substitution reactions of Co(CN)5 OH2 values of k 1 agreeing well with the rate coefficient for water exchange, viz. 

Predictably, the Co(CN)5 intermediate displays variations in reactivity towards nucleophiles as indicated by differences in the ratio (a 

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