Reaction of (μ-H)2Os3(CO)9(PPh3) with Acetylene and Ethylene. Structures, Dynamics, and Interconversion of Two Isomers of the Vinyl Complex (μ-H)Os3(CO)9(PPh3)(μ-CHCH2)

As reported previously by Brown and Evans, the reaction of (μ-H) 2 Os 3 (CO) 9 (PPh 3 ) with acetylene provides two isomers of the hydrido vinyl compound (μ-H)Os 3 (CO) 9 (PPh 3 )(μ-η 2 -CH CH 2 ) ( 2 ). The solid-state structures of these two isomers now have been defined by single-crystal X-ray crystallography. The major isomer 2a contains a triangular array of osmium atoms with the vinyl ligand bridging the Os(2)−Os(3) edge, forming a σ-bond to Os(3) and a π-bond to Os(2), and the triphenylphosphine ligand coordinating to Os(2) in an “in-plane” position. The presence of the hydride ligand also bridging the Os(2)−Os(3) edge was deduced indirectly. The minor isomer 2b also contains a triangular Os 3 array with the vinyl ligand bridging the Os(2)−Os(3) edge and forming a σ-bond to Os(2) and a π-bond to Os(3). The triphenylphosphine ligand is bound to Os(1). The presence of the bridging hydride on the Os(1)−Os(3) edge was not directly detected but was supported by analysis of the structure; the hydride ligand therefore does not bridge the same edge as the vinyl moiety. The solution structures and dynamics of 2a and 2b have been investigated by variable-temperature 13 C NMR and 13 C spin saturation transfer experiments; isomer 2a exhibits evidence for nondegenerate σ/π bond interchange in the bridging vinyl group. Separation of 2a and 2b can be effected by thin-layer chromatography (silica; CH 2 Cl 2 /C 6 H 14, 1:3); subsequent 1 H NMR spectra reveal slow equilibration of 2a and 2b ( t 1/2 = 85 min, 2a: 2b = 2.2:1 at equilibrium). The same mixture of isomers is formed from the reaction of 1 with ethylene at high pressure (500 psig), with the previously characterized complex (μ-H) 2 Os 3 (CO) 9 (PPh 3 )(μ-CHCH 3 ) as an intermediate.