The Reactions of Cr(CO)6, Fe(CO)5, and Ni(CO)4 with O2 Yield Viable Oxo-Metal Carbonyls

Transition metal complexes with terminal oxo and dioxygen ligands exist in metal oxidation reactions, and many are key intermediates in various catalytic and biological processes. The prototypical oxo-metal [(OC)(5)CrO, (OC)(4)FeO, and (OC)(3)NiO] and dioxygen-metal carbonyls [(OC)(5)CrOO, (OC)(4)FeOO, and (OC)(3)NiOO] are studied theoretically. All three oxo-metal carbonyls were found to have triplet ground states, with metal-oxo bond dissociation energies of 77 (CrO), 74 (FeO), and 51 (NiO) kcal/mol. Natural bond orbital and quantum theory of atoms in molecules analyses predict metal-oxo bond orders around 1.3. Their featured (MO, M=metal) vibrational frequencies all reflect very low IR intensities, suggesting Raman spectroscopy for experimental identification. The metal interactions with O-2 are much weaker [dissociation energies 13 (CrOO), 21 (FeOO), and 4 (NiOO) kcal/mol] for the dioxygen-metal carbonyls. The classic parent compounds Cr(CO)(6), Fe(CO)(5), and Ni(CO)(4) all exhibit thermodynamic instability in the presence of O-2, driven to displacement of CO to form CO2. The latter reactions are exothermic by 47 [Cr(CO)(6)], 46 [Fe(CO)(5)], and 35 [Ni(CO)(4)] kcal/mol. However, the barrier heights for the three reactions are very large, 51 (Cr), 39 (Fe), and 40 (Ni) kcal/mol. Thus, the parent metal carbonyls should be kinetically stable in the presence of oxygen. (c) 2014 Wiley Periodicals, Inc.