REACTION-KINETICS AND THERMODYNAMICS OF THE (MU-2-H)2OS3(CO)10-CO SYSTEM

The kinetics of the reversible addition of CO to the electronically unsaturated cluster (mu2-H)2Os3(CO)10 in decalin have been studied at 25.0-40.0-degrees-C. The kinetic parameters for addition and loss of CO are k1(25.0-degrees-C) = 4.10(0.07) X 10(-2) M-1 s-1, DELTAH1 double dagger = 15.7(0.4) kcal mol-1, and DELTAS1 double dagger = -12.1(1.3) cal K-1 mol-1 and k-1(25.0-degrees-C) = 7.0(0.8) X 10(-5) s-1, DELTAH-1 double dagger = 25.9(0.4) kcal mol-1, and DELTAS-1 double dagger = +9.3(1.4) cal K-1 mol-1, respectively. The reactions are most probably simple concerted, one-step reactions. Equilibrium constants for addition of CO to (mu2-H)2Os3(CO)10 have been determined over the temperature range 25.0-90.0-degrees-C and K1(25.0-degrees-C) = 5.9(0.7) X 10(2) M-1, DELTAH-1-degrees = -9.5(0.3) kcal mol-1, and DELTAS1-degrees = -19.2(0.9) cal K-1 mol-1. The kinetics of loss of H-2 from equilibrium mixtures of (mu2-H)2Os3(CO)10 and (mu2-H)(H)Os3(CO)11 in the presence of CO to form Os3(CO)12 at 65.0-95.0-degrees-C have also been studied. An unexpected feature of the reductive elimination of H-2 is that it can occur via a first-order reaction of (mu2-H)2Os3(CO)10 (DELTAH3 double dagger = 14.2(4.5) kcal mol-1, DELTAS3 double dagger = -35(13) cal K-1 mol-1, hence k3(25.0-degrees-C) = ca. 7 X 10(-6) s-1) as well as from (mu2-H)(H)Os3(CO)11 (DELTAH2 double dagger = 27.7(2.3) kcal mol-1, DELTAS2 double dagger = +9.5(6.5) cal K-1 mol-1, and k2(25.0-degrees-C) = ca. 4 X 10(-6) s-1). Combination of these results with others reported earlier has enabled a complete energy profile to be obtained for the system (mu2-H)2Os3(CO)10 + 2CO half arrow right over half arrow left (mu2-H)(H)Os3(CO)11 + CO half arrow right over half arrow left H-2 + Os3(CO)12 for which the overall parameters K(25.0-degrees-C) = ca. 5 X 10(7) M-1, DELTAH-degrees = -19.1(2.4) kcal mol-1, and DELTAS-degrees = -27.0(6.7) cal K-1 mol-1 apply. The standard heats of formation of (mu2-H)2Os3(CO)10 and (mu2-H)(H)Os3(CO)11 are estimated to be 709 and 672 kcal mol-1, respectively, and an estimate of the Os-H bond strengths in (H)2Os3(CO)11 of ca. 62 kcal mol-1 can be derived. This estimate is compared with other data for M-H bond strengths found in the literature, and the importance of effects due to the presence of different ligands and other metals attached to the metal in the M-H bond is emphasized.