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)5Cr-O, (OC)4Fe-O, and (OC)3 Ni-O] and dioxygen-metal carbonyls [(OC)5Cr-OO, (OC)4Fe-OO, and (OC)3Ni-OO] are studied theoretically. All three oxo-metal carbonyls were found to have triplet ground states, with metal-oxo bond dissociation energies of 77 (Cr-O), 74 (Fe-O), and 51 (Ni-O) kcal/mol. Natural bond orbital and quantum theory of atoms in molecules analyses predict metal-oxo bond orders around 1.3. Their featured ν(MO, M = metal) vibrational frequencies all reflect very low IR intensities, suggesting Raman spectroscopy for experimental identification. The metal interactions with O2 are much weaker [dissociation energies 13 (Cr-OO), 21 (Fe-OO), and 4 (Ni-OO) 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 O2 , 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.