METAL-CARBONYL BOND STRENGTHS IN FE(CO)N(-) AND NI(CO)N(-)

Energy-resolved collision-induced dissociation of Fe(CO)n- (n = 1-4) and Ni(CO)n- (n = 2, 3) is used to determine the metal-carbonyl bond energies (kcal/mol) D[(CO)3Fe--CO] = 41.7 +/- 2.5, D[(CO)2Fe--CO] = 42.4 +/- 3.5, D[(CO)Fe--CO] = 35.7 +/- 3.5, D[Fe--CO] = 33.7 +/- 3.5, D[(CO)2Ni--CO] = 38.5 +/- 2.3, and D[(CO)Ni--CO] = 43.4 +/- 5.8. The sum of the iron-carbonyl bond strengths is within error limits of the value derived from previous experimental results. Combining the nickel data with the known energy for loss of all CO ligands from Ni(CO)3- gives D[Ni--CO] = 32.4 +/- 5.8 kcal/mol. The bond energies in the anions can be used with literature electron affinities to give bond energies (kcal/mol) for the neutral metal carbonyls, D[(CO)3Fe-CO] = 27.9 +/- 8.8, D[(CO)2Fe-CO] = 29.1 +/- 5.8, D[(CO)Fe-CO] = 36.7 +/- 3.5, D[Fe-CO] = 8.1 +/- 3.5, D[(CO)2Ni-CO] = 28.3 +/- 2.3, D[(CO)Ni-CO] = 47.1 +/- 5.8, and D[Ni-CO] = 40.5 +/- 5.8. Estimates of the ionization potentials of neutral Ni(CO)n and Fe(CO)n fragments are also derived. These results are compared to previous experimental and theoretical estimates of the M-CO bond strengths. Consideration of the electronic structures and electron binding energies of the M(CO)n- ions suggests that the dissociations occur adiabatically, with little or no effects of electron detachment on the measured dissociation thresholds.