Boron-lead multiple bonds in the PbB₂O^- and PbB₃O₂^- clusters

Despite its electron deficiency, boron can form multiple bonds with a variety of elements. However, multiple bonds between boron and main-group metal elements are relatively rare. Here we report the observation of boron-lead multiple bonds in PbB₂O^- and PbB₃O₂^-, which are produced and characterized in a cluster beam. PbB₂O^- is found to have an open-shell linear structure, in which the bond order of B☱Pb is 2.5, while the closed-shell [Pb≡B-B≡O]^2- contains a B≡Pb triple bond. PbB₃O₂^- is shown to have a Y-shaped structure with a terminal B = Pb double bond coordinated by two boronyl ligands. Comparison between [Pb≡B-B≡O]^2-/[Pb=B(B≡O)₂]^- and the isoelectronic [Pb≡B-C≡O]^-/[Pb=B(C≡O)₂]⁺ carbonyl counterparts further reveals transition-metal-like behaviors for the central B atoms. Additional theoretical studies show that Ge and Sn can form similar boron species as Pb, suggesting the possibilities to synthesize new compounds containing multiple boron bonds with heavy group-14 elements.