Evidences for a localized chromophore in the Ti(IV)/squarate complex: a resonance Raman investigation

The present work continues a previous investigation on the interaction of oxocarbonic anions with highly charged metal ions. When squarate or croconate solutions are added to Ti(IV) acidic solutions, a rather strong and broad band shows up that is not present in the optical spectrum of the reagents. Excitation of the Raman spectrum of the resulting coloured solutions close to their absorption maxima leads to the enhancement of several modes of the oxocarbon. In particular the totally symmetric nu(C=O) mode of the Ti(IV)/squarate or croconate complexes shows an outstanding enhancement, a completely different behaviour of that observed in the Raman spectrum of the free oxocarbons, where the nu(CO) modes are barely observed. Such result indicates a rather strong Ti(IV)-oxocarbon interaction via the CO moieties with the formation of a strongly allowed LMCT (ligand to metal charge transfer) state. In addition the rather selective enhancement of the nu(CO) modes strongly suggests that the chromophore involved in the LMCT state is highly localized in the Ti(IV)-CO moieties of the complexes and that the enhancement mechanism involved is very likely the one described by Albrecht's A term, i.e. Franck-Condon one state mechanism.