Understanding the preference for the coplanarity of alkenyl and carbonyl ligands in η1-alkenyl transition-metal complexes:: A simple molecular orbital approach and ab initio calculations

The preference for coplanarity of alkenyl and carbonyl ligands in yl alkenyl transition-metal complexes can be understood in terms of a simple molecular orbital model in which the nonbonding metal t(2g) orbitals interact with the pi* orbitals of these ligands. Such back-bonding interactions are most favorable when the alkenyl and carbonyl ligands are coplanar, as all three t(2g) orbitals are utilized by the ligands. Optimized geometries obtained from ab initio calculations for a variety of ruthenium alkenyl complexes show the preference for alkenyl-carbonyl 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 complexes with three or more carbonyl ligands this energy decreases due to competition by carbonyl ligands for favorable back-bonding interactions. This competition effectively rules out any preferential alkenyl-carbonyl arrangement, and instead steric interactions dominate. The role of the pi-donor chloride ligand in stabilizing the planar alkenyl-carbonyl arrangement was also investigated.