A NEW-TYPE OF MLCT TRANSITION RELEVANT TO OXIDATIVE ADDITIONS - D-]SIGMA-ASTERISK EXCITATION

Oxidative additions take place by a reductive cleavage of a sigma bond : Mn+ + X-2--> Mn+2(X-)(2). At intermediate stages, X-2 is assumed to be coordinated as a ligand: Mn+-X-2. Complexes of this type are charac terized by Mn+ --> sigma( X-2) MLCT transitions which represent the s pectroscopic counterpart of the most important step of an oxidative ad dition. This new type of MLCT transition was detected in the electroni c spectra of complexes such as (Re2Br2)-Br-I(CO)(6)(Se2Ph2) and Cr-IV( O-2)(2)(NH3)(3). In contrast to d-->pi MLCT states, d-->sigma* MLCT s tates are not luminescent but reactive. Cr-IV-->sigma(O-2(2-)) MLCT e xcitation of Cr-IV(O-2)(2)(NH3)(3) induces an intramolecular oxidative addition [Cr-IV(O-2(2-)) --> Cr-VI(O2-)(2)] which finally leads to th e formation of CrO42- as stable photoproduct. (C) 1998 Elsevier Scienc e S.A.