Variable character of O - O and M - O bonding in side-on (η²) 1:1 metal complexes of O₂

The structures and the O - O and M - O bonding characters of a series of reported side-on (η²) 1:1 metal complexes of O₂ are analyzed by using density functional theory calculations. Comparison of the calculated and experimental systems with respect to O - O bond distance, O - O stretching frequency, and O - O and M - O bond orders provides new insights into subtle influences relevant to O₂ activation processes in biology and catalysis. The degree of charge transfer from the generally electronrich metals to the dioxygen fragment is found to be variable, such that there are species well described as superoxides, others well described as peroxides, and several cases having intermediate character. Increased charge transfer to dioxygen takes place via overlap of the metal d_xy orbital with the in-plane π* orbital of O₂ and results in increased M - O bond orders and decreased O - O bond orders. Comparison of theory and experiment over the full range of compounds studied suggests that reevaluation of the O - O bond lengths determined from certain x-ray crystal structures is warranted; in one instance, an x-ray crystal structure redetermination was performed at low temperature, confirming the theoretical prediction. Librational motion of the coordinated 02 is identified as a basis for significant underestimation of the O - O distance at high temperature.