Iron and manganese oxo complexes, oxo wall and beyond

High-valent metal-oxo species with multiply-bonded M-O groups have been proposed as key intermediates in many biological and abiological catalytic oxidation reactions. These intermediates are implicated as active oxidants in alkane hydroxylation, olefin epoxidation and other oxidation reactions. For example, [Fe^ivO(porphyrinato^•-)]⁺ cofactors bearing π-radical porphyrinato^•- ligands oxidize organic substrates in cytochrome P450 enzymes, which are common to many life forms. Likewise, high-valent Mn-oxo species are active for H₂O oxidation in photosystem II. The chemistry of these native reactive species has inspired chemists to prepare highly oxidized transition-metal complexes as functional mimics. Although many synthetic Fe-O and Mn-O complexes now exist, the analogous oxo complexes of the late transition metals (groups 9-11) are rare. Indeed, late-transition-metal-oxo complexes of tetragonal (fourfold) symmetry should be electronically unstable, a rule commonly referred to as the 'oxo wall'. A few late metal-oxos have been prepared by targeting other symmetries or unusual spin states. These complexes have been studied using spectroscopic and theoretical methods. This Review describes mononuclear non-haem Fe-O and Mn-O species, the nature of the oxo wall and recent advances in the preparation of oxo complexes of Co, Ni and Cu beyond the oxo wall.