Substitution chemistry of pentaosmium carbonyl clusters: crystal structure of H₂Os₅(CO)₁₃P(OMe)₃PEt₃

Thermolysis of H₂Os₅(CO)₁₄P(OMe)₃ with an excess of L′ (L′ = P(OMe)₃ or PEt₃) in toluene at 80°C initially affords an addition product with a formulation H₂Os₅(CO)₁₄LL′, which undergoes decarbonylation on prolonged heating to give H₂Os₅(CO)₁₃LL′. The latter cluster can be obtained under much milder conditions by the direct reaction of H₂Os₅(CO)₁₄P(OMe)₃ with Me₃NO in the presence of the appropriate phosphorus donor ligand. A single crystal structure of H₂Os₅(CO)₁₃P(OMe)₃PEt₃ has revealed a trigonal bipyramidal metal framework with a P(OMe)₃ ligand attached to an apical osmium atom and a PEt₃ ligand attached to an equatorial osmium atom of the bipyramid. Both of the hydride ligands have been shown by potential energy calculations to be edge-bridging. In solution, these hydride ligands are fluxional at room temperature; the fluxional process can be frozen by cooling the sample to 213 K, where the ¹H NMR spectrum shows a pair of doublets of doublets.