Observation of π-Backbonding in a Boronyl-Coordinated Transition Metal Complex TaBO.

The boronyl anion (BO-) is isoelectronic to CO and CN-, but its metal complexes are rare. Here we report the observation of a boronyl complex with an early transition metal (TaBO-) produced by a laser vaporization supersonic cluster source. Its electronic structure and chemical bonding are investigated by photoelectron imaging and theoretical calculations. Vibrationally resolved photoelectron spectra of TaBO- are obtained for several low-lying electronic states of neutral TaBO and the electron affinity of TaBO is measured to be 1.729(2) eV. Theoretical calculations show that the ground states of TaBO- (4Δ) and TaBO (5Δ) are both linear with high spin multiplicities. Natural atomic orbital analyses reveal that π-backbonding is present in both TaBO- and TaBO. This study presents the first observation of π-backbonding to a boronyl ligand and suggests that early transition metals may be good candidates to form boronyl complexes.