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.