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(μ3-O)(μ-CH3COO)6(py)3]+ (1) (Ru3(III, III, III)), Ru3(μ3-O)(μ-CH3COO)6(py)3 (2) (Ru3(II, III, III)), and Ru3(μ3-O)(μ-CH3COO)6(CO)(py)2 (3) (Ru3(II, III, III)), have been studied in CD3CN by following the change in 1H NMR spectra. The rates are practically independent of [py-d5] (0.05–1.0 mol dm−3). The first-order rate constants are 3 × 10−5 s−1 at 55 °C (ΔH≠ = 123 ± 6 kJ mol−1, ΔS≠ = +41 ± 19 J K−1 mol−1), 5.9 × 10−4 s−1 at 50 °C (ΔH≠ = 122 ± 14 kJ mol−1, ΔS≠ = +69 ± 44 J K−1 mol−1), and 3 × 10−5 s−1 at 55 °C (ΔH≠ = 126 ± 9 kJ mol−1, ΔS≠ = +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(μ3-O)(μ-CH3COO)6(H2O)3]+.