Ionization energies and cationic bond dissociation energies of RuB, RhB, OsB, IrB, and PtB

Two-photon ionization thresholds of RuB, RhB, OsB, IrB, and PtB have been measured using resonant two-photon ionization spectroscopy in a jet-cooled molecular beam and have been used to derive the adiabatic ionization energies of these molecules. From the measured two-photon ionization thresholds, IE(RuB) = 7.879(9) eV, IE(RhB) = 8.234(10) eV, IE(OsB) = 7.955(9) eV, IE(IrB) = 8.301(15) eV, and IE(PtB) = 8.524(10) eV have been assigned. By employing a thermochemical cycle, cationic bond dissociation energies of these molecules have also been derived, giving D₀(Ru⁺-B) = 4.297(9) eV, D₀(Rh⁺-B) = 4.477(10) eV, D₀(Os-B⁺) = 4.721(9) eV, D₀(Ir-B⁺) = 4.925(18) eV, and D₀(Pt-B⁺) = 5.009(10) eV. The electronic structures of the resulting cationic transition metal monoborides (MB⁺) have been elucidated using quantum chemical calculations. Periodic trends of the MB⁺ molecules and comparisons to their neutral counterparts are discussed. The possibility of quadruple chemical bonds in all of these cationic transition metal monoborides is also discussed.