Observation of an electron-precise metal boryne complex: [BiBH]

Metal-boron triple bonds are rare due to the electron deficiency of boron. This study uncovers a simple electron-precise metal boryne complex, [BiBH]^-, which is produced within an ion trap through chemical reactions of the open-shell BiB^- anion with H₂. Photoelectron imaging is used to investigate the electronic structure and chemical bonding of the BiBH^- complex. The B atom in the linear closed-shell BiBH^- is found to undergo sp hybridization, forming a B-H single bond and a BiB triple bond. Photoelectron imaging reveals three detachment transitions from the BiBH^- (¹Σ⁺) anion to the neutral BiBH, including the ground state (²Π_3/2) and two excited states (²Σ⁺ and ²Π_1/2). Strong vibronic coupling is observed between the ²Π_3/2 and ²Σ⁺ states, evidenced by the appearance of bending vibrations and their unique photoelectron angular distributions. The BiBH^- complex not only stands as the simplest metal boryne complex, but also serves as an ideal molecular system to investigate both spin-orbit and vibronic couplings.