Reaction of [Os₅H₂(CO)₁₅] with pyridine: Crystal and molecular structures of [Os₅H₃(CO)₁₄(C₅H₄N)] and [Os₅H₂(CO)₁₄(C₅H₅N)]

The reaction of the hydrido cluster [Os₅H₂(CO)₁₅] with pyridine, in n-octane under reflux, affords three neutral products [Os₅H₃(CO)₁₄(C₅H₄N)] (1), [Os₅H₂(CO)₁₄(C₅H₅N)] (2), and [Os₅H₂(CO)₁₅(C₅H₅N)] which exists in two isomerisc forms (3a) and (3b), together with the monoanion [Os₅H(CO)₁₅]^-. The complexes were characterized by spectroscopic, and in the case of (1) and (2) by single-crystal X-ray crystallographic techniques. Within the series of neutral compounds the pyridine ligand occupies a variety of co-ordination sites and displays different bonding modes. In the case of complexes (1), (3a), and (3b) the reaction with pyridine also results in a change in metal framework geometry from that present in the starting material. The structure of (1) consists of an edge-bridged tetrahedron of Os atoms with an orthometallated pyridyl ligand linking the apex Os atom of the tetrahedron to the Os atom which bridges the tetrahedron. In (2) the five Os atoms define a trigonal bipyramid. The pyridine ligand bonds to the cluster via nitrogen, and occupies a pseudo-'axial' site on one of the equatorial Os atoms. The spectroscopic data for (3a) and (3b) are consistent with the presence of an edged-bridged tetrahedral metal geometry, with the pyridine terminally bound to a co-ordination site on the edge-bridgine Os atom. In (3a) the pyridine occupies an equatorial site and in (3b) an axial site. A possible mechanism for the reaction is discussed.