Comparison of Binuclear Phospholyl Chromium Carbonyl Derivatives with their Cyclopentadienyl Analogues: Role of the Phosphorus Atom in Ligand-Metal Bonding

Abstract The structures and energetics of the binuclear phospholyl chromium carbonyl derivatives (C 4 H 4 P) 2 Cr 2 (CO) n ( n  = 6, 5, 4, 3) have been investigated by density functional theory. The lowest energy (C 4 H 4 P) 2 Cr 2 (CO) n ( n  = 6, 4, 3) structures are completely analogous to their Cp 2 Cr 2 (CO) n analogues with two terminal pentahapto phospholyl rings. However, for the pentacarbonyl (C 4 H 4 P) 2 Cr 2 (CO) 5 the lowest energy structure has a bridging seven-electron donor η 5 ,η 1 -C 4 H 4 P ligand using the phosphorus lone pair in addition to the π-system of the phospholyl ligand as well as a Cr-Cr single bond. Thus, except for the pentacarbonyl the phosphorus lone pair of the phospholyl ligand is seen to play a minor role in the energetically preferred structures of the binuclear phospholyl chromium carbonyl derivatives.