Calculated volume and energy profiles for water exchange on t2g6 Rhodium(III) and Iidium(III) hexaaqualons:: Conclusive evidence for an Ia mechanism

An I-a mechanism was assigned for water exchange on the hexaaquaions Rh(OH2)(6)(3+) and Ir(OH2)(6)(3+) on the basis of negative DeltaV(double dagger) experimental values (-4.2 and -5.7 cm(3) mol(-1), respectively). The use of DeltaV(double dagger) as a mechanistic criterion was open to debate primarily because DeltaV(double dagger) could be affected by extension or compression of the nonparticipating ligand bond lengths on going to the transition state of an exchange process. In this paper, volume and energy profiles for two distinct water exchange mechanisms (D and I-a) have been computed using quantum chemical calculations which include hydration effects. The activation energy for Ir(OH2)(6)(3+) is 32.2 kJ mol(-1) in favor of the la mechanism (127.9 kJ mol(-1)), as opposed to a D pathway; the value for the la mechanism being close to DeltaH(double dagger) and DeltaG(double dagger) experimental values (130.5 kJ mol(-1) and 129.9 kJ mol(-1) at 298 K, respectively). Volumes of activation, computed using Connolly surfaces and for the I-a pathway (DeltaV(double dagger) (calc) = -3.9 and -3.5 cm(3) mol(-1), respectively, for Rh3+ and Ir3+), are in agreement with the experimental values. Further, it is demonstrated for both mechanisms that the contribution to the volume of activation due to the changes in bond lengths between Ir(III) and the spectator water molecules is negligible: -1.8 for the D, and -0.9 cm(3) mol(-1) for la mechanism. This finding clarifies the debate about the interpretation of DeltaV(double dagger) and unequivocally confirms the occurrence of an la mechanism with retention of configuration and a small a character for both Rh(III) and Ir(III) hexaaquaions.