Understanding the Preference for the Coplanarity of Alkenyl and Carbonyl Ligands in η¹-Alkenyl Transition-Metal Complexes:  A Simple Molecular Orbital Approach and <i>ab Initio</i> Calculations

Abstract

The preference for coplanarity of alkenyl and carbonyl ligands in η 1 -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 π* 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 π-donor chloride ligand in stabilizing the planar alkenyl−carbonyl arrangement was also investigated.