Kinetics of Intramolecular Interconversion of Two Isomers of {μ-H)M₃{μ-Cme₂){CQ)₉L (M = Ru, L = PR₃, AsPh₃, or SbPh₃; M = Os, L = AsPh₃)

The clusters (μ-H)M₃(μ-CNMe₂)(CO)₉L (M = Ru, L = PPh₃, PMePh₂, PMe₂Ph, PBu₃, PCyc₃, P(OMe)₃, P(0-i-Pr)₃, P(OPh)₃, AsPh₃, SbPh₃; M = Os, L = AsPh₃) exist in solution as equilibrium mixtures of two isomers, L being coordinated to the nonbridged metal atom in one (n-e isomer) and to the bridged metal atom in the other (b-e isomer). The n-e isomer is the kinetic product formed by addition of L to (p-H)M₃(M-CNMe₂)(CO)₉I/ (M = Ru, L’ = py; M = Os, L’ = NCMe). The kinetics of the intramolecular rearrangements of the n-e isomers to the equilibrium mixtures have been determined. The equilibrium constant depends upon the identity of L, increasing in the order SbPh₃ (0.37) < PMe₂Ph (0.59) < PBu₃ (0.67) < PMePh₂ (1.75) < AsPh₃ (2.5) < P(OMe)₃ (3.2) < P(OPh)₃ (4.5) < P(0-i-Pr)₃ (6.9) < PCyc₃ (7.2) < PPh₃ (7.5), and values of the forward rate constant 10¹ (s^-1) vary slightly (at 5 °C): SbPh₃ (0.27) < AsPh₃ (0.87) < PMe₂Ph (1.1) < PBu₃ (1.4) < P(0-i-Pr)₃ (1.9) < PMePh₂ (2.7) ~ PPh₃(2.7) < P(OPh)₃ (2.8) < P(OMe)₃ (3.6) < PCyc₃. These trends indicate that both steric and electronic factors are important, K_eq and k_f increasing as the size and hardness of L increase. Activation parameters for k_ffor rearrangement of (μ-H)M₃(μ-CNMe₂)(CO)₉(AsPh₃) are ΔH* = 20.7 (M = Ru) and 15.5 kcal/mol (M = Os) and AS* = -3.2 (M = Ru) and -27 eu (M = Os). The deuterium isotope effect ^Hkf/^Dk_f for the Ru complex is 1.24. The proposed mechanism involves migration of hydride, methylidyne, and carbonyl ligands through pairwise bridge opening to form intermediates having only terminally bound ligands.