Bond dissociation energies of FeB, CoB, NiB, RuB, RhB, OsB, IrB, and PtB.

Abstract

The bond dissociation energies (BDEs) of the diatomic late transition metal borides (MB, M = Fe, Co, Ni, Ru, Rh, Os, Ir, and Pt) have been assigned from the measurement of a predissociation threshold using resonant two-photon ionization (R2PI) spectroscopy. The open d-shell configurations of the transition metal constituents in the molecules studied here lead to large ML degeneracies, resulting in a dense manifold of states near the ground separated atom limit. This high density of states causes prompt predissociation to occur as soon as the ground separated atom limit is exceeded, allowing a precise assignment of the BDE of the molecule. The measured predissociation thresholds give BDEs of D0(FeB) = 2.43(2) eV, D0(CoB) = 2.954(3) eV, D0(NiB) = 3.431(4) eV, D0(RuB) = 4.815(3) eV, D0(RhB) = 5.252(3) eV, D0(OsB) = 4.378(3) eV, D0(IrB) = 4.928(10) eV, and D0(PtB) = 5.235(3) eV. The gaseous enthalpies of formation at 0 K for these molecules have been derived using a thermochemical cycle that relates atomic enthalpies of formation and the BDE of the molecule, giving ΔfH0K°(g) (FeB) = 733.6(12.2) kJ mol-1, ΔfH0K°(g) (CoB) = 695.1(12.2) kJ mol-1, ΔfH0K°(g) (NiB) = 652.1(14.7) kJ mol-1, ΔfH0K°(g) (RuB) = 740.2(12.7) kJ mol-1, ΔfH0K°(g) (RhB) = 600.1(12.7) kJ mol-1, ΔfH0K°(g) (OsB) = 921.7(13.6) kJ mol-1, ΔfH0K°(g) (IrB) = 748.0(13.6) kJ mol-1, and ΔfH0K°(g) (PtB) = 613.9(12.2) kJ mol-1. This work reports the first experimental measurements of the BDEs of FeB, CoB, NiB, and OsB. Periodic trends are discussed.