text: Sigma non-innocence is a special form of non-innocence, an oxidation characteristic in metal complexes. It is mainly discussed in coordination complexes of late transition metals in their high formal oxidation states. Complexes exhibiting sigma non-innocence differ from classical Werner coordination complexes in that their bonding and antibonding orbitals have an inverted distribution of metal and ligand character (c.f. inverted ligand field). The oxidation of the ligand and a lowered charge at the metal center renders the assignment of the oxidation state non-trivial.