VALENCE LOCALIZATION IN S)(3,5-DI-TERT-BUTYL-CATECHOLATE)](-SPECTROSCOPY() IONS (M=CO, RH OR IR) PROBED BY RESONANCE RAMAN)

The nature of the electronic transitions responsible for the intense v isible absorption band of the pentacoordinate [M(triphos)(DBCat)](+) i ons, M = Co, Rh or Ir, triphos = MeC(CH(2)PPh(2))(3), DBCat = 3,5-di-t ert-butyl-catecholate, has been investigated by resonance Raman (rR) s pectroscopy. The extent of the valence delocalization within the [M(DB Cat)](+) fragment, related to the DBCat --> M pi-donation, has been es timated from the relative intensities of the high-wavenumber Raman pea ks attributed to the intra-catecholate nu(C-O), delta(C-H) and nu(C-C) vibrations and the low-wavenumber peaks attributed to vibrations coup led with the nu(M-O) symmetric stretch and/or deformation vibrations o f the M(O-C-C-O)) chelate ring. While the rR spectra of the Co complex show only the low wavenumber peaks, both the high- and low-wavenumber peaks are apparent in the spectra of the Rh and Ir species; the ratio of the intensities of the high- to the low-wavenumber peaks being gre atest for the Ir species. The rR spectroscopic patterns indicate that the valence delocalization increases substantially in the order Ir < R h << Co, the Co-catecholate bonding being completely delocalized. The metal dependence of the spectroscopic, electrochemical and chemical pr operties of [M(triphos)(DBCat)](+) is closely related to the extent of the valence delocalization in each ion.