Chemical Bonding and Electronic Structure of the Early Transition Metal Borides: ScB, TiB, VB, YB, ZrB, NbB, LaB, HfB, TaB, and WB

The predissociation thresholds of the early transition metal boride diatomics (MB, M = Sc, Ti, V, Y, Zr, Nb, La, Hf, Ta, W) have been measured using resonant two-photon ionization (R2PI) spectroscopy, allowing for a precise assignment of the bond dissociation energy (BDE). No previous experimental measurements of the BDE exist in the literature for these species. Owing to the high density of electronic states arising from the ground and low-lying separated atom limits in these open d-subshell species, a congested spectrum of vibronic transitions is observed as the energy of the ground separated atom limit is approached. Nonadiabatic and spin–orbit interactions among these states, however, provide a pathway for rapid predissociation as soon as the ground separated atom limit is reached, leading to a sharp decrease in signal to background levels when this limit is reached. Accordingly, the BDEs of the early transition metal borides have been assigned as D 0(ScB) 1.72(6) eV, D 0(TiB) 1.956(16) eV, D 0(VB) 2.150(16) eV, D 0(YB) 2.057­(3) eV, D 0(ZrB) 2.573(5) eV, D 0(NbB) 2.989(12) eV, D 0(LaB) 2.086(18) eV, D 0(HfB) 2.593(3) eV, D 0(TaB) 2.700(3) eV, and D 0(WB) 2.730(4) eV. Additional insight into the chemical bonding and electronic structures of these species has been achieved by quantum chemical calculations.