IR STUDY OF THE H-BOND COUPLED WITH THE MIXED-VALENCE STATE OF HALOGEN-BRIDGED METAL-COMPLEXES

IR spectroscopic measurements were made on the halogen bridged metal complexes of {M(chxn)2Br}Br2 (M = Pt, Pd and Ni) which have interchain N-H...Br bonds strongly coupled with the electronic state of chains. The splitting of the N-H stretching modes for Pt and Pd complexes show a good correlation with the amplitude of the CDW in the chain, i.e. with the displacement of bridging halogen ion. No splitting of the N-H band was observed for the Ni complex, which is an evidence of a transformation from a mixed valence (CDW) state to a mono-valence (the Mott-Hubbard) state by a replacement of M = Pt (or Pd) by Ni. Such IR studies were applied for the estimation of the CDW amplitude under hydrostatic pressure. The results indicated that more detailed information can be derived by the IR study than the X-ray analysis.