Solution Structures and Dynamics of Complexes of H₂Os₃(CO)₁₀ with Lewis Bases

The unsaturated cluster H₂Os₃(CO)₁₀ reacts with a variety of Lewis bases L, including carbon monoxide, phosphines, phosphites, arsines, stibines, amines, nitriles, isocyanides, halides, and pseudohalides, to form 1:1 adducts H₂Os₃(CO)₁₀L. Each of these adducts has been shown to contain one axial, terminal hydride ligand and one bridging hydride ligand. Variable-temperature ¹H and ¹³C NMR studies reveal that the two hydride ligands and two carbonyl ligands undergo simultaneous fluxional exchange. A turnstile-like mechanism is proposed. Variation of the free energy of activation for the exchange process with the steric and electronic properties of L suggests that the transition state involves two equivalent hydride ligands bridging the same metal-metal vector.