Chemistry of the unsaturated cluster compound [Os3Pt(µ-H)2(CO)10{P(cyclo-C6H11)3}]; X-ray crystal structures of [Os3Pt(µ-H)2(CO)11{P(cyclo-C6H11)3}], [Os3Pt(µ-H)4(CO)10{P(cyclo-C6H11)3}], and [Os3Pt(µ-H)2(µ-CH2)(CO)10–{P(cyclo-C6H11)3}](two isomers)

Abstract

Reactions of the unsaturated cluster compound [Os₃Pt(µ-H)₂(CO)₁₀{P(cyclo-C₆H₁₁)₃}] with hydrogen and with diazomethane have been investigated, and the structures of the products determined by X-ray diffraction. The structure of the previously prepared carbon monoxide adduct [Os₃Pt(µ-H)₂(CO)₁₁{P(cyclo-C₆H₁₁)₃}] has also been established. The latter species, which is formed in a readily reversed reaction, has a ‘butterfly’ arrangement of the metal atoms with the platinum atom at a wing-tip position. One hydrido-ligand bridges the Os–Os vector of the body of the butterfly, and the other an Os–Pt edge. The latter is cis to the P (cyclo-C₆H₁₁)₃ group on the platinum, which also carries a CO ligand. The osmium atom at the other wing-tip is ligated by four CO groups, and the remaining two Os are each terminally bonded to three carbonyls. The hydrogen adduct [Os₃Pt(µ-H)₄(CO)₁₀{P(cyclo-C₆H₁₁)₃}] is also formed reversibly but in contrast with the product from carbon monoxide has a closo-tetrahedral metal core. The hydrido-ligands occupy edge-bridging sites on the cluster framework, with two bridging Os–Pt vectors [cis to the P(cyclo-C₆H₁₁)₃ ligand on platinum] and two bridging Os–Os vectors. Thus the M₄ tetrahedron has D_2d symmetry with two short unbridged edges and four longer bridged edges. Diazomethane readily adds to [Os₃Pt(µ-H)₂(CO)₁₀{P(cyclo-C₆H₁₁)₃}] to give the complex [Os₃Pt(µ-H)₂(µ-CH₂)-(CO)₁₀{P(cyclo-C₆H₁₁)₃}] which exists as an equilibrium mixture of two isomeric forms (orange-yellow and red crystals). Both species have a closo distorted tetrahedral geometry for the metal atoms, with the methylene group bridging an Os–Os edge cisoid to the P(cyclo-C₆H₁₁)₃ ligand on the platinum in the orange-yellow form and transoid to the phosphine group in the red isomer. The relative positions of the hydrido-ligands also differ. In the orange-yellow form they bridge the remaining two Os–Os edges, while although not located directly in the red isomer, being assigned on potential-energy grounds, they bridge an Os–Os edge and the Os–Pt edge cis to the P(cyclo-C₆H₁₁)₃ ligand. N.m.r. studies reveal that the two isomers maintain these solid-state structures in solution. The mechanism of formation of the two isomers is discussed in the context of the orange-yellow form being the kinetically controlled product, which after several days in solution establishes a 1 : 4 equilibrium with the red form.