EXCITED-STATES OF METAL-METAL BONDED DIIMINE COMPLEXES VARY FROM EXTREMELY LONG-LIVED TO VERY REACTIVE WITH FORMATION OF RADICALS OR ZWITTERIONS

The metal-metal bonded complexes Re(MLn)(CO)(3)(alpha-diimine) [MLn = Mn(CO)(5), Re(CO)(5), SnPh3, etc.] have a lowest (3)sigma pi excited state, in which sigma represents the Re-M bonding orbital and pi the lowest empty orbital of the alpha-diimine. This excited state, which c an be occupied via the Re-->alpha-diimine MLCT states, is normally rea ctive, giving rise to homolysis of the Re-M sigma bond. Only the compl ex Re(SnPh3)(CO)(3)(bpy) was found to be photostable and much longer l ived in its (3)sigma pi State than related complexes having a lowest (MLCT)-M-3 state of comparable energy. This was ascribed to the streng th of the Re-Sn bond and to the weak distortion of this complex in its (3)sigma pi state with respect to the ground state. In the complexes Ru(E)(E')(CO)(2)(alpha-diimine) the axial ligands E and E' can vary s trongly and so do their excited state properties. Of special interest are the compounds in which both E and E' are SnP3 ligands, strongly bo nded to Ru by a high-lying sigma orbital. Because of the strength of t hese bonds and the strong mixing of the sigma(Sn-Ru-Sn) orbital with t he n(alpha-diimine) orbital, the structural effects of the sigma-->pi transition are small. This is reflected in very small shifts of the GO-stretching vibrations with respect to the ground state and in a ver y long lifetime of the (3)sigma pi state (tau = 264 mu s for Ru(SnPh3 )(2)(CO)(2)(iPr-DAB) in a 2-MeTHF glass at 77 K). Similarly, irradiati on of a cluster Os-3(CO)(10)(alpha-diimine) in an apolar solvent gives rise to homolysis of an Os-Os bond with formation of a biradical, whi ch was detected with EPR and time-resolved absorption spectroscopy (ta u = 5-110 ns). In a coordinating solvent such as CH3CN, the homolysis reaction is accompanied by the coordination of a solvent molecule, whi ch induces an intramolecular electron transfer. A zwitterion is then f ormed, which has a lifetime of seconds in CH3CN. The biradical can als o be transformed into such a zwitterion if a Lewis base is added to th e apolar solvent. In this article the mechanistic details of these rea ctions are presented and discussed. (C) 1998 Elsevier Science S.A.