Computational Analysis of Transition Metal-Terminal Boride Complexes

A computational analysis of model transition-metal terminal boride [MB­(PNPR)] complexes is reported. A combination of density functional theory methods, natural bond orbital analysis, and multiconfiguration self-consistent field calculations were employed to investigate the structure and bonding of terminal boride complexes, in particular, the extent of metal dπ–boron pπ bonding. Comparison of metal–boride, −borylene, and–boryl bond lengths confirms the presence of metal–boron π bonds, albeit the modest shortening (∼3%) of the metal–boron bond suggests that the π-bonding is very weak in terminal borides. Calculated free energies of H2 addition to the boride complexes to yield the corresponding boryl complexes indicate that metal–boride π-bond strengths are 22 kcal/mol or less as compared to 44 kcal/mol for an analogous nitride complex. It is concluded that, for the boride complexes studied, covering a range of different 4d and 5d metals, that the metal–boride bond consists of a reasonably covalent σ but two very polarized metal–boron π bonds. The high polarization of the boron-to-metal π bonds indicates that the terminal boride is an acceptor or Z-type ligand.