Determination of the bond dissociation energies of FeX and NiX (X = C, S, Se).

The bond dissociation energies of FeC, NiC, FeS, NiS, FeSe, and NiSe have been measured by the observation of a predissociation threshold in their resonant two-photon ionization spectra. Because the lowest separated atom limits generate a vast number of potential energy curves, it is thought that the molecules dissociate as soon as the ground separated atom limit is exceeded in energy. From the observed thresholds, dissociation energies have been measured as D0 (FeC) = 3.961(19), D0 (NiC) = 4.167(3), D0 (FeS) = 3.240(3), D0 (NiS) = 3.651(3), D0 (FeSe) = 2.739(6), and D0 (NiSe) = 3.218(3) eV. Through the use of thermochemical cycles, these values have been combined with other precisely known values to improve the accuracy of other quantities, providing: D0 (Fe+ -C) = 4.270(19) eV, D0 (Ni+ -C) = 3.435(3) eV, IE(FeS) = 8.06(4) eV, IE(NiS) = 8.82(4) eV, and D0 (Fe-S- ) = 2.92(10) or 2.89(10) eV, depending on the reference employed for EA(FeS- ). Comparisons to previous values are noted, when available. The periodic trends observed are discussed in terms of a molecular orbital diagram for these species. Finally, these results have also been used to calculate 0 K enthalpies of formation of the gaseous MX molecules.