Metal-dioxygen and metal-dinitrogen complexes: where are the electrons?

Transition-metal complexes of O-2 and N-2 play an important role in the environment, chemical industry, and metalloenzymes. This Perspective compares and contrasts the binding modes, reduction levels, and electronic influences on the nature of the bound O-2 or N-2, group in these complexes. The charge distribution between the metal and the diatomic ligand is variable, and different models for describing the adducts have evolved. In some cases, single resonance structures (e.g. M-superoxide = M-O-2(-)) are accurate descriptions of the adducts. Recent studies have shown that the magnetic coupling in certain N-2(2-) complexes differs between resonance forms, and can be used to distinguish experimentally between resonance structures. On the other hand, many O-2 and N-2 complexes cannot be described well with a simple valence-bond model. Defining the situations where ambiguities occur is a fertile area for continued study.