Abstract
The celebrated Fe(II)(O2) vs. Fe(III)(O2-) debate over the formal representation of the FeO2 moiety in Hb 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)(O2-) formulation is both chemically reasonable and most useful. In conjunction with a qualitative molecular orbital overlap picture, the differing magnetic states of end-bound MO2 complexes and their geometrical features can be rationalized or predicted.