PB and AsB triple bonds in the linear PB₂O⁻ and AsB₂O⁻ species

Due to its electron deficiency, boron triple bonds are relatively scarce. We use high-resolution photoelectron imaging to investigate the structures and bonding of the EB₂O^- (E = P, As) type of clusters, which are found to have [E[triple bond, length as m-dash]B-B[triple bond, length as m-dash]O]^- closed-shell linear structures with E[triple bond, length as m-dash]B triple bonds. The B atoms in the linear EB₂O^- species undergo sp hybridization, while the E atoms also undergo sp hybridization to form a σ bond with the sp orbital of B along with two π bonds formed by the p _<i>x</i> and p _<i>y</i> orbitals. The high-resolution photoelectron imaging data reveal detachment transitions from the EB₂O^- (¹Σ⁺) anions to the EB₂O (²Π) neutrals. The electron affinities of PB₂O and AsB₂O are measured to be 3.592(1) eV and 3.432(1) eV, respectively; the vibrational frequencies for the E-B, B-B, and B-O stretching modes are measured for both species. The spin-orbit splitting of the ²Π state to ²Π_3/2 and ²Π_1/2 is measured to be 153 cm^-1 and 758 cm^-1 for PB₂O and AsB₂O, respectively.