Covalent metal-alkyl or metal-metal bonds in organometallic compounds that contain an electron-accepting ligand in their coordination sphere are often prone to photochemical homolytic dissociation.'Metal-metal bonded complexes L,M-Re(C0)3(a-diimine)' (L,M = (CO)SMn, (CO)SRe, Ph3Sn, (C0)4Co) and (OC)sMnRu(Me)(C0)2(a-diimine)2 and metalalkyl complexes R~(X)(iPr)(C0)2(a-diimine)~ (X = halide), M(R)(CO)da-diimir~e)~-~ (M = Mn, Re), Ir(R)(CO)(PAr3)2-(mnt),' Pt(Me)d(a-diimir~e),~ and ZnR~(a-diimine)~ are representative examples.Bond homolysis from a an* excited state is usually a s ~u m e d I ~~~~-' ~ to be the primary photochemical step involved.This excited state originates in excitation of an electron from the to-be-split u bond to the n* orbital of the acceptor ligand, usually a-diimine.Except for Pt(Me)4(adiimine) and ZnRZ(a-diimine) c o m p l e x e ~~~' ~ the un* transition was not observed spectroscopically and the an* state may only be populated nonradiatively from optically excited MLCT states.'Unusually long-lived emission from (OQ-
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Rossenaar et al. (1995) studied this question.