Synthesis and solution studies of Os₃(CO)₉L(R₂C₂) (L = CO, MeCN, P(OMe)₃ or PPh₃); crystal and molecular structure of Os₃(CO)₉P(OMe)₃(Ph₂C₂)

Reaction of Os₃(CO)₁₀(R₂C₂) (R = Ph, Et or Me) with 1.1 equivalent of Me₃NO in the presence of MeCN at - 78°C produces a labilized cluster, Os₃(CO)₉(NCMe)-(R₂C₂), which upon reaction with alkyne (R′₂C₂) affords the known violet complex of formula Os₃(CO)₉(R₂C₂)(R′₂C₂), in which both the alkyne molecules are linked via a carbon-carbon bond to produce an osmacyclic ring. With simple phosphorus donor ligands L (where L = P(OMe)₃, PPh₃), the cluster Os₃(CO)₉(NCMe)(R₂C₂) affords the corresponding substituted complex Os₃(CO)₉L(R₂C₂) in yields of up to 80% yields. The structure of Os₃(CO)₉P(OMe)₃(Ph₂C₂) has been established by a single crystal X-ray study. The structure is based on a triangular metal framework with the diphenylacetylene ligand bonded in a μ₃-η²-|-bonding mode, forming two σ- and a π-bond with the osmium atoms, a P(OMe)₃ ligand being attached to one osmium bearing a σ-bonded acetylenic carbon and occupying a pseudo equatorial position. ¹³C NMR studies have been carried out on the complex Os₃(CO)₉L(Me₂C₂) (L = CO, P(OMe)₃ or PPh₃) to give information about the structure in solution. The carbonyl ligands in all of these complexes are fluxional, and an intramolecular mechanism by which the carbonyls interchange between the three osmium atoms, in each complex, is discussed in detail.