Interchange associative substitution reactions

Pressure-accelerated water exchange reactions and evidence for associative interchange in corresponding net substitution reactions ... 

Langford and Gray proposed in 1965 (13) a mechanistic classification for ligand substitution reactions, which is now generally accepted and summarized here for convenience. In their classification they divided ligand substitution reactions into three categories of stoichiometric mechanisms associative (A) where an intermediate of increased coordination number can be detected, dissociative (D) where an intermediate of reduced coordination number can be detected, and interchange (I) where there is no kinetically detectable intermediate [Eqs. (2)-(4)]. In Eqs. (2)-(4), MX -i and... 

When an associative mode of activation is indicated it is instructive to examine the ways in which reactivity, as measured by the second-order rate constant k2, depends upon the nature of the nucleophile and if a large number of substrates show the same pattern of preference it is useful to consider scales of nucleophilicity. A systematic study of the substitution reactions of fra/is-[Pt(py)2Cl2] (it appears that pyridine and piperidine were used interchangeably) in methanol at 30 °C (displacement of chloride) led to the establishment of an n scale, nm d= log,o(k2/kI) for the standard reaction,447 and later the more dimensionally correct Up,0 scale448-449 ( p,° = log,0(li2/ki)[MeOH]) so that npt° = npt+1-41 (unfortunately this distinction has not been strictly adhered to in the literature).449 A collection of n 0 values will be found in Table 14. 

Associative reactions are also possible in octahedral substitution, but are much less common. Table 12-5 gives data for both dissociative and associative interchanges for similar reactants. In the case of water substitution by several different anions in [Cr(NH3)5(H20)] ", the rate constants are quite similar (within a factor of 6), indicative of an Id mechanism. On the other hand, the same ligands reacting with [Cr(H20)6] show a large variation in rates (more than a 2000-fold difference), indicative of an mechanism. Data for similar Co(lll) complexes are not conclusive, but their reactions generally seem to have mechanisms. 

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