Computational Analysis of Transition Metal-Terminal Boride Complexes.

A computational analysis of model transition-metal terminal boride [MB(PNP)] 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 d?-boron p? bonding. Comparison of metal-boride, -borylene, and-boryl bond lengths confirms the presence of metal-boron ? bonds, albeit the modest shortening (?3%) of the metal-boron bond suggests that the ?-bonding is very weak in terminal borides. Calculated free energies of H addition to the boride complexes to yield the corresponding boryl complexes indicate that metal-boride ?-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 ? but two very polarized metal-boron ? bonds. The high polarization of the boron-to-metal ? bonds indicates that the terminal boride is an acceptor or Z-type ligand.