UNDERSTANDING THE PREFERENCE FOR THE COPLANARITY OF ALKENYL AND CARBONYL LIGANDS IN ETA(1)-ALKENYL TRANSITION-METAL COMPLEXES - A SIMPLE MOLECULAR-ORBITAL APPROACH AND AB-INITIO CALCULATIONS

The preference for coplanarity of alkenyl and carbonyl ligands in yl a lkenyl transition-metal complexes can be understood in terms of a simp le molecular orbital model in which the nonbonding metal t(2g) orbital s interact with the pi orbitals of these ligands. Such back-bonding i nteractions are most favorable when the alkenyl and carbonyl ligands a re coplanar, as all three t(2g) orbitals are utilized by the ligands. Optimized geometries obtained from ab initio calculations for a variet y of ruthenium alkenyl complexes show the preference for alkenyl-carbo nyl planarity when only one or two carbonyl ligands are present in the complex. In these complexes the energy required to rotate the alkenyl ligand is calculated to be approximately 7 kcal/mol, while in complex es with three or more carbonyl ligands this energy decreases due to co mpetition by carbonyl ligands for favorable back-bonding interactions. This competition effectively rules out any preferential alkenyl-carbo nyl arrangement, and instead steric interactions dominate. The role of the pi-donor chloride ligand in stabilizing the planar alkenyl-carbon yl arrangement was also investigated.