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.