Rhodium(III) and iridium(III) solvates of the form [(eta(5)-C5Me5)M(S)(3)](2+) as models for the reactivity of half-sandwich compounds

Solvent exchange on the half-sandwich organic solvates [(eta(5)-C5Me5)M(S)(3)](2+) (M = Rh, S = MeCN (1) or Me2SO (3); and M = Ir, S = MeCN (2) or Me2SO (4)) has been investigated as a function of temperature, pressure, and concentration of free solvent by H-1 NMR line-broadening techniques in CD3CN and/or CD3NO2. The exchange rates span several orders of magnitude, from k(ex)(298) = 8.8 x 10(-2) s(-1) for 2 to 3.6 x 10(3) s(-1) for 3, as a result of changes in the electronic and steric properties of the ligands. Nevertheless, the volume of activation remains consistently positive for compounds 1-4 with values ranging from +0.8 to +3.3 cm(3) mol(-1). In combination with the positive activation entropies obtained and the first-order rate law established for these systems, it was concluded that regardless of the nature of the ligand the solvent exchange process on 1-4 proceeds via a dissociative D mechanism. Of note, the intermolecular exchange with free Me2SO on 4 takes place exclusively from a conformational isomer of 4 (structure 4.2), which is itself in equilibrium with a second, more compact conformer (structure 4.1).