Bond dissociation energies of diatomic transition metal selenides: ScSe, YSe, RuSe, OsSe, CoSe, RhSe, IrSe, and PtSe.

The diatomic transition metal selenides, MSe (M = Sc, Y, Ru, Os, Co, Rh, Ir, and Pt), were studied by resonant two-photon ionization spectroscopy near their respective bond dissociation energies. As these molecules exhibit high densities of vibronic states near their dissociation limits, the spectra typically appear quasicontinuously at these energies. Spin-orbit and nonadiabatic couplings among the multitudes of potential curves allow predissociation to occur on a rapid timescale when the molecule is excited to states lying above the ground separated atom limit. This dissociation process occurs so rapidly that the molecules are dissociated before they can be ionized by the absorption of a second photon. This results in an abrupt drop in the ion signal that is assigned as the 0 K bond dissociation energy for the molecule, giving bond dissociation energies of 4.152(3) eV (ScSe), 4.723(3) eV (YSe), 3.482(3) eV (RuSe), 3.613(3) eV (OsSe), 2.971(6) eV (CoSe), 3.039(9) eV (RhSe), 3.591(3) eV (IrSe), and 3.790(31) eV (PtSe). The enthalpies of formation, ΔH° (g), for each diatomic metal selenide were calculated using thermochemical cycles, yielding ΔH° (g) values of 210.9(4.5) kJ mol (ScSe), 203.5(4.5) kJ mol (YSe), 549.2(4.5) kJ mol (RuSe), 675.9(6.5) kJ mol (OsSe), 373.9(2.6) kJ mol (CoSe), 497.4(2.7) kJ mol (RhSe), 557.4(6.5) kJ mol (IrSe), and 433.7(3.6) kJ mol (PtSe). Utilizing a thermochemical cycle, the ionization energy for ScSe is estimated to be about 7.07 eV. The bonding trends of the transition metal selenides are discussed.