Prototypical metal–oxo bonds: the reactions of Cr(PF3)6, Fe(PF3)5, and Ni(PF3)4 with oxygen

Abstract

Terminally bound oxo complexes of late transition metals have been difficult to synthesize or isolate. Their scarcity is in part due to the electronic repulsion between oxo ligands and metal elements with highly occupied d orbitals. This study attempts to provide extra stability to such metal–oxo species. The prototypical metal–oxo trifluorophosphane complexes [(PF3)5CrO, (PF3)4FeO, and (PF3)3NiO] are studied here. The results are compared with the corresponding carbonyl counterparts [(CO)5CrO, (CO)4FeO, and (CO)3NiO]. Predicted bond dissociation energies are 90 (Cr–O), 83 (Fe–O), and 59 (Ni–O) kcal/mol, higher than those of their carbonyl counterparts by around 10 kcal/mol. Consistent with bonding considerations and population analyses, the metal–oxo bonds are strengthened from the carbonyl to the trifluorophosphine complexes. Although the improvement is modest, it proves clearly that strong electron-withdrawing frameworks help in stabilizing metal–oxo complexes. This general idea may be utilized in further studies to seek or even design elusive terminal metal–oxo species and to keep pushing the limits of this area. In addition, improved viabilities are also found for the dioxygen–metal complexes [(PF3)5CrO2, (PF3)4FeO2, and (PF3)3NiO2], compared with their carbonyl counterparts.