Early Transition Metals Strengthen the B₂ Bond in MB₂ Complexes

The bond dissociation energies of early transition metal diborides (M-B₂, M = Sc, Ti, V, Y, Mo) have been measured by observation of the sharp onset of predissociation in a highly congested spectrum. Density functional and CCSD(T) ab initio calculations, extrapolated to the complete basis set limit, have been used to examine the electronic structure of these species. The computations demonstrate the formation of bonding orbitals between the metal d orbitals and the 1π_u bonding orbitals of B₂, leading to the transfer of metallic electron density into the bonding 1π_u orbitals, strengthening both the M-B and B-B bonds in the molecule. This runs counter to most metal-ligand π interactions, where electron density is generally transferred into π antibonding orbitals of the ligand.