On the bonding of FeO2 in hemoglobin and related dioxygen complexes

The celebrated Fe(II)0(2) versus Fe(III) (02-) debate over the formal representation of the Fe02 moiety in hemoglobin can be resolved by consideration of the utility of each formalism. In the context of rationalizing the gross structural and electronic features of end-bound dioxygen, particularly in light of a new closely related chromium complex, the M(III)-(02-) formulation is both chemically reasonable and most useful. In conjunction with a qualitative molecular orbital overlap picture, the differing magnetic states of enc-bound M02 complexes and other geometrical features can be rationalized or predicted.