The First Measurements of Activation Volumes for Hydride Fluxionality on Metal Clusters. Intramolecular Exchange of Hydrides on H(μ−H)Os₃(CO)₁₀(PPh₃) and (μ−H)₂Ru₃(μ₃−CHC0₂Me) (CO)₉

The first volumes of activation for hydride fluxionality on transition-metal clusters have been determined by analysis of ¹H NMR line shapes recorded at pressures up to 200 MPa. For exchange of the two bridging hydrides of (μ−H)₂Ru₃(μ₃−CHCO₂Me)(CO)₉ the value for ΔV^‡ of +4.1 (±0.3) cm³/mol is consistent with an exchange pathway involving migration of one hydride from a doubly bridging coordination mode to a terminal coordination site in the transition state. For exchange of bridging and terminal hydride ligands of H(μ−H)Os₃(CO)₁₀(PPh₃) the value for ΔV^* of −0.8 (±0.4) cm³/mol is consistent with a transition state in which both hydrides are bridging.