Oxidative additions take place by a reductive cleavage of a σ bond: M n + + X 2 → M n + 2 (X - ) 2 . At intermediate stages, X 2 is assumed to be coordinated as a ligand: M n + -X 2 . Complexes of this type are characterized by M n + → σ * (X 2 ) MLCT transitions which represent the spectroscopic counterpart of the most important step of an oxidative addition. This new type of MLCT transition was detected in the electronic spectra of complexes such as Re I 2 Br 2 (CO) 6 (Se 2 Ph 2 ) and Cr I V (O 2 ) 2 (NH 3 ) 3 . In contrast to d → π * MLCT states, d → σ * MLCT states are not luminescent but reactive. Cr I V → σ * (O 2 - 2 ) MLCT excitation of Cr I V (O 2 ) 2 (NH 3 ) 3 induces an intramolecular oxidative addition [Cr I V (O 2 - 2 ) → Cr V I (O 2 - ) 2 ] which finally leads to the formation of CrO 2 - 4 as stable photoproduct.