Can octahedral t(2g)(6) complexes substitute associatively? The case of the isoelectronic ruthenium(II) and rhodium(III) hexaaquaions

For the low-spin t(2g)(6) Ru(OH2)(6)(2+) (DeltaV(double dagger) = -0.4 cm(3 ) mol(-1)) and Rh(OH2)(6)(3+) (Vi(double dagger) = -4.2 cm(3) mol(-1)) hexa aquaions, the respective I-d and I-a water exchange mechanisms had been ass igned, mainly on the basis of activation volumes DeltaV(double dagger) and entering ligands effects for water substitution. For Ru(II) the near-zero D eltaV(double dagger) was supposed to be due to the compensation between a p ositive contribution (the loss of a water molecule) and a negative one (the contraction of the bonds of the five spectator ligands at the transition s tate). Recently, it has been suggested that Rh(III), because of its higher positive charge, could promote further spectator ligands bond contraction s ufficient to change the sign of DeltaV(double dagger) to a negative value. If true, this would be an example of Limitation in the use of DeltaV(double dagger) for a direct diagnosis of the mechanism. Quantum chemical calculat ions including hydration effects show that the activation energies for the water exchange on Rh(OH2)(6)(3+) via the I-a (114.8 kJ mol(-1)) and the D p athways is 21.8 kJ mol(-1) in favor of the former. In the case of Ru(OH2)(6 )(2+) all attemps to compute a transition state for an interchange mechanis m failed, but the calculated DeltaE(double dagger) for the D mechanism (71. 9 kJ mol(-1)) is close to both experimental DeltaG(298)(double dagger) and DeltaH(298)(double dagger) values. The calculated Delta Sigmad(M-O) values of -0.53 Angstrom for rhodium(III) and +1.25 Angstrom for ruthenium(II) agr ee with the experimented DeltaV(double dagger) values and suggest I-a and D (or I-d) mechanisms, respectively. In the case of Ru(OH2)(6)(2+) the short ening of the bonds of the five spectator ligands to reach the transition st ates corresponds to a volume change of -1.7 cm(3) mol(-1). For Rh(OH2)(6)(3 +) these spectator ligands' volume decrease is much smaller (maximum of -0. 8 cm3 mol(-1)) and the bond lengths of the two exchanging ligands at the tr ansition state are characteristic of an interchange pathway with a small "a " character. Because of the strong Rh-III-O bonds, water exchange on Rh(OH2 )(6)(3+) proceeds via the I-a pathway with retention of the configuration, whereas the same reaction of Ru(OH2)(6)(2+), which has considerably weaker Ru-II-O bonds, follows the Icl or the D mechanism.