Theoretical study of electronic structures and chemical bonding in MOB^- and MBO^- (M = Sc-Zn) molecules

The boronyl anion (BO^-) is a more polarized, isoelectronic analogue of CO, rendering it a promising ligand for expanding "carbonyl-like" coordination chemistry into realms of heightened reactivity and electronic diversity. Intrinsically, BO^- differs fundamentally from CO in that its frontier molecular orbitals are shifted upward, making BO^- a stronger σ donor yet a weaker π acceptor. Herein, we present a comprehensive theoretical investigation into the isomerism of monoboronyl complexes M(BO)^- and M(OB)^- across the 3d series (M = Sc-Zn), using DFT, CCSD(T) calculations and EDA-NOCV analyses. Geometrically, both isomers adopt a predominantly linear configuration. The B-bound isomer is favored for nearly the entire 3d series, with the exception of Sc, Ti and Mn, for which the O-bound isomer becomes the electronic ground state. EDA-NOCV calculations show that electrostatic attraction constitutes the dominant attraction (≈58-72%), with orbital interactions being governed primarily by the σ-type interaction. The subtle Sc/Ti/Mn preference for O-binding is attributed to a competitive balance between Pauli repulsion and attractive interaction. Extending our study to 4d(Pd) and 5d(Pt) congeners further amplifies the preference for B-coordination and highlights the increasing significance of π-type orbital interactions for the heavier metals. This work not only expands the isoelectronic-ligand concept beyond CO but also offers a robust theoretical foundation for developing new boronyl-based coordination motifs and functional materials.