Due to its electron deficiency, boron triple bonds are relatively scarce. We use high-resolution photoelectron imaging to investigate the structures and bonding of EB2O− (E = P, As) type of clusters, which are found to have [E≡B−B≡O]− closed-shell linear structures with E≡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 s bond with the sp orbital of B along with two p bonds formed by the pxand py orbitals. The high-resolution photoelectron imaging data reveal detachment transitions from the EB2O− (1S+) anions to the EB2O (2P) 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 systems. The spin-orbit splitting of the 2P state to 2P3/2 and 2P1/2 is measured to be 153 cm-1 and 758 cm-1 for PB2O and AsB2O, respectively.
