KINETIC-STUDIES ON THE TERMINAL PYRIDINE EXCHANGE-REACTIONS OF SOME OXO-ACETATE BRIDGED TRINUCLEAR RUTHENIUM COMPLEXES - INFLUENCE OF THE METAL OXIDATION-STATE AND NON-LEAVING TERMINAL LIGAND (CO)

Substitution reactions of py-d5 for the terminal pyridines of the three triruthenium complexes, [Ru3(mu-3-O)(mu-CH3COO)6(py)3]+ (1) (Ru3(III, III, III)), Ru3(mu-3-O)(mu-CH3COO)6(py)3 (2) (Ru3(II, III, III)), and Ru3(mu-3-O)(mu-CH3COO)6(CO)(py)2 (3) (Ru3(II, III, III)), have been studied in CD3CN by following the change in H-1 NMR spectra. The rates are practically independent of [py-d5] (0.05-1.0 mol dm-3). The first-order rate constants are 3 x 10(-5) s-1 at 55-degrees-C (DELTA-H not-equal = 123 +/- 6 kJ mol-1, DELTA-S not-equal = + 41 +/- 19 J K-1 mol-1), 5.9 x 10(-4) s-1 at 50-degrees-C (DELTA-H not-equal = 122 +/- 14 kJ mol-1, DELTA-S not-equal = + 69 +/- 44 J K-1 mol-1), and 3 x 10(-5) s-1 at 55-degrees-C (DELTA-H not-equal = 126 +/- 9 kJ mol-1, DELTA-S not-equal = + 52 +/- 27 J K-1 mol-1) for 1, 2, and 3, respectively. Rate constants appear to depend on the apparent oxidation state of the metal center. While the three Ru ions in 2 are equivalent and each ion has average oxidation number +2(2/3), Ru ions with coordinated py in 3 are in +3 oxidation state as the +2 oxidation state is localized at carbonyl-Ru ion. A dissociative mechanism is proposed for these reactions on the basis of the absence of [py-d5]-dependence, the activation parameters, and the comparison of the rate constants with those of the substitution of methanol-d4 for the terminal water ligands in [Ru3(mu-3-O)(mu-CH3COO)6(H2O)3]+.