Preparation, X-ray structural analysis, and method of interconversion of two isomeric forms of [Os6(CO)16{P(OMe)3}2]

Abstract

Treatment of the labile cluster [Os₆(CO)₁₆(NCMe)₂](1) with excess trimethyl phosphite, at room temperature, affords two isomers of [Os₆(CO)₁₆{P(OMe)₃}₂](2a) and (2b) as the major products, together with small amounts of [Os₆(CO)₁₅{P(OMe)₃}₃](3). The ¹H n.m.r. spectrum of (2a) indicates that the two P(OMe)₃ groups are equivalent, and an X-ray analysis of this product shows that these two ligands occupy equivalent sites on the two capping Os atoms of the bicapped tetrahedral metal framework, as has been observed previously in other disubstituted derivatives of [Os₆(CO)₁₈]. In isomer (2b), ¹H n.m.r. data show that the two phosphite environments are inequivalent. This is confirmed by the X-ray crystal structure in which the bicapped tetrahedral hexaosmium framework is retained, but while one P(OMe)₃ group occupies a site on one of the capping Os atoms, the other is co-ordinated to one of the Os atoms in the central tetrahedron. In solution isomer (2b) converts to (2a). Possible mechanisms by which this interconversion may take place are discussed.