REINVESTIGATION OF THE VISIBLE ABSORPTION-BANDS OF THE 2,2'-BIPYRIMIDINE COMPLEXES W(CO)4(BPYM) AND (MU-BPYM)[M(CO)4]2 (M = MO, W) WITH RESONANCE RAMAN-SPECTROSCOPY - THE EMISSION-SPECTRUM OF (MU-BPYM)[MO(CO)4]2

The character of the two lowest energy transitions of W(CO)4(bpym) and (mu-bpym)[M(CO)4]2 (M = Mo, W) were established with resonance Raman spectroscopy. According to these spectra the two bands belong to MLCT transitions to different pi orbitals of the bpym ligand. Contrary to expectations it is not the first (lowest energy) but the second and mo re intense electronic transition which, according to the resonance Ram an spectra, is directed to the lowest lying pi orbital (b2u*, LUMO) o f these complexes. This interpretation explains the different band int ensities and the untypically low g values of the ESR signals of corres ponding anion radicals. Excitation of (mu-bpym)[Mo(CO)4]2 in CH2Cl2 at 400 nm produced a weak emission with an onset at 700 nm. According to the excitation spectrum, this emission originates from the lowest MLC T-excited state of the complex.