V alence localization in [ M ( triphos ) ( 3,5-di-tert-butyl-catecholate) ] + ions (M = Co, Rh or Ir) probed by resonance Raman spectroscopy

The nature of the electronic transitions responsible for the intense visible absorption band of the pentacoordinate [ M (triphos) (DBCat) ] + ions, M = Co, Rh or lr, triphos = MeC(CH2PPh2)3, DBCat = 3,5-di-tert-butyl-catecholate, has been investigated by resonance Raman (rR) spectroscopy. ]'he extent of the valence delocalization within the [ M (Dl/Cat) ] + fragment, related to the DBCat ~ M ¢r-donation, has been estimated from the relative intensities of the high-wavenumber Raman peaks attributed to the intra-catecholate ~,(C-O), 8(C-H) and u(CC) vibrations and the Iow-wavenumber peaks attributed to vibrations coupled with the z,(M--O) symmetric stretch and/or deformation vibrations of the I¢1(O-C-C-O) chelate ring. While the rR spectra of the Co complex show only the low-wavenumber peaks, both the highand Iow-wavenumber peaks are apparent in the spectra of the Rh and lr species; the ratio oftbe intensities oftbe high- to the low-wavenamber peaks being greatest for the lr species. The rR spectroscopic patterns ird~cate that the valence delocalization increases substantially in the order ir < Rh << Co, the Co-catecholate bonding being completely delocalized. The metal dependence of the spectroscopic, electrochemical and chemical properties of [ M (triphos) (DBCat) ] + is closely related t¢, the extent of the valence delecalization in each ion.