Anation reactions Water exchange rates

The rate constant for the base-catalysed hydrogen exchange of [Ir(NH3)6] at 298 K in D2O (reaction 64) has been determined (fc = 1.5 x 10 M s" ). The chloride anation of [Ir(H2)(NH3)5] + is three times faster than the water exchange rate, and has been attributed to an associative interchange mechanism. ... 

The kinetics of the anation of [Rh(OH2)6] by Br in aqueous solution have been investigated. Since the rate-limiting process has apparently identical characteristics to the Cl reaction and closely resembles water exchange, a D mechanism is indicated. 

Langford and others " have clearly described the role of pre-association in anation reactions of this type and the principles we have discussed already (p. 693). That the anion concentration term in the rate law arises as a consequence of a pre-equilibrium and not as a result of a bimolecular nucleophilic attack, has been established by a comparison of the solvent exchange rate with the saturation rate of anion entry. Such results had been accumulated previously in water, for example by Schmidt and Taube, and more recently by Duffy and Earley and Murray and Barraclough. Muir and Langford have recently measured anation rates of cw-[CoN02(DMSO)(en)2] in DMSO and compared these with the rate of DMSO exchange obtained from deuterium NMR experiments. ... 

Throughout our discussion, we still have not answered the fundamental question What is the preferred mechanism of substitution reactions of octahedral coordination compounds The answer is that dissociative mechanisms are preferred. In this section, we investigate some of the evidence that supports such a conclusion. We cite three different types of reactions (1) the rates of exchange of water molecules, (2) anation reactions, and (3) aquation reactions. 

To conclude this section, we have seen thatvarious pieces of data from (1) rates of water exchange, (2) anations, and (3) various aquation reactions all seem to generally favor a dissociative mechanism for substitution reactions of octahedral compounds. 

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