B2(BO)22- -: Diboronyl diborene:: A linear molecule with a triple boron-boron bond

We have produced and investigated an unique boron oxide cluster, B4O2, using photoelectron spectroscopy and ab initio calculations. Relatively simple and highly vibrationally resolved PIES spectra were obtained at two photon energies (355 and 193 nm). The electron affinity of neutral B4O2 was measured to be 3.160 +/- 0.015 eV. Two excited states were observed for B4O2 at excitation energies of 0.48 and 0.83 eV above the ground state. Three vibrational modes were resolved in the 355 nm spectrum for the ground state of B4O2 with frequencies of 350 +/- 40, 1530 +/- 30, and 2040 +/- 30 cm(-1). Ab initio calculations showed that neutral B4O2 (D-infinity h, (3)Sigma(-)(g)) and anionic B4O2- (D-infinity h, (2)Pi(u)) both possess highly stable linear structures (O B-B = B-B O), which can be viewed as a B-2 dimer bonded to two terminal boronyl groups. The lowest nonlinear structures are at least 1.5 eV higher in energy. The calculated electron detachment energies from the linear B4O2- and the vibrational frequencies agree well with the experimental results. The three observed vibrational modes are due to the B-B, B = B, and B O symmetric stretching vibrations, respectively, in the linear B-2(BO)(2). Chemical bonding analyses revealed that the HOMO of B-2(BO)(2), which is half-filled, is a bonding 7 orbital in the central B-2 unit. Thus, adding two electrons to B-2(BO)(2) leads to a B B triple bond in [O B-B B-B O](2-). Possibilities for stabilizing B-2(BO)(2)(2-) in the form of B-2(BO)(2)Li-2 are considered computationally and compared with other valent isoelectronic, triple bonded species, B2H2-Li-2; B2H22-, and C2H2. The high stability of B-2(BO)(2)(2-) suggests that it may exist as a viable building block in the condensed phase.