Can octahedral t2g6 complexes substitute associatively? The case of the isoelectronic ruthenium(II) and rhodium(III) hexaaquaions.

Abstract

For the low-spin t2g6 Ru(OH2)6(2+) (delta V++ = -0.4 cm3 mol-1) and Rh(OH2)6(3+) (delta V++ = -4.2 cm3 mol-1) hexaaquaions, the respective Id and Ia water exchange mechanisms had been assigned, mainly on the basis of activation volumes delta V++ and entering ligands effects for water substitution. For Ru(II) the near-zero delta V++ was supposed to be due to the compensation between a positive contribution (the loss of a water molecule) and a negative one (the contraction of the bonds of the five spectator ligands at the transition state). Recently, it has been suggested that Rh(III), because of its higher positive charge, could promote further spectator ligands bond contraction sufficient to change the sign of delta V++ to a negative value. If true, this would be an example of limitation in the use of delta V++ for a direct diagnosis of the mechanism. Quantum chemical calculations including hydration effects show that the activation energies for the water exchange on Rh(OH2)6(3+) via the Ia (114.8 kJ mol-1) and the D pathways 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 mechanism failed, but the calculated delta E++ for the D mechanism (71.9 kJ mol-1) is close to both experimental delta G298++ and delta H298++ values. The calculated delta sigma d(M-O) values of -0.53 A for rhodium(III) and +1.25 A for ruthenium(II) agree with the experimented delta V++ values and suggest Ia and D (or Id) mechanisms, respectively. In the case of Ru(OH2)6(2+) the shortening of the bonds of the five spectator ligands to reach the transition states corresponds to a volume change of -1.7 cm3 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 transition state are characteristic of an interchange pathway with a small "a" character. Because of the strong RhIII-O bonds, water exchange on Rh(OH2)6(3+) proceeds via the Ia pathway with retention of the configuration, whereas the same reaction of Ru(OH2)6(2+), which has considerably weaker RuII-O bonds, follows the Id or the D mechanism.