Electronic structure of Os3(CO)8(C2Ph2)2: deformation of a trimetal framework by acetylene ligands

The electronic structure of Os3(CO)8(C2Ph2)2 is discussed. Each acetylene ligand is bonded to the triosmium framework via a μ3-η2-mode, the C-C bond vectors being parallel to a common Os-Os edge. Thus, each alkyne can be viewed as forming a π-bond to one osmium atom, and two σ-bonds to the other two osmium atoms. This simple bonding description is confirmed by the results of a Fenske-Hall molecular orbital analysis. Compared to Os3(CO)12, Os3(CO)8(C2Ph2)2 exhibits a significantly deformed metal carbonyl framework. The first deformation is a noticeable shortening of two of the metal-metal edges. This feature is shown to be due to a combination of direct metal-metal bonding and indirect metal-metal interaction via the acetylene ligands. The second framework deformation is a change in carbonyl orientation along the series Os3(CO)12−2x(C2Ph2)x (x = 0, 1, 2). This is shown to be essential for maximum metal-ligand interaction.