Optical and magnetic properties of the halogen-bridged metal complexes modified by hydrogen bondings: {M(1R,2R-cyclohexanediamine)₂Br}Br₂ (M=Pt, Pd, and Ni)

Polarized reflection and ESR measurements have been carried out on single crystals of newly synthesized halogen-bridged one-dimensional (1D) metal complexes {M(chxn)₂Br}Br₂ (M=Pt, Pd, Ni; chxn=1R,2R-cyclohexanediamine), which have tight hydrogen bonds between ligands (chxn) and counter anions (Br^-) and construct a two-dimensional (2D) hydrogen-bond network. Both the polarized reflection spectra and the temperature-dependent ESR signals indicate that the electronic state of M=Ni is essentially different from that of M=Pt (or Pd). From the analysis of these results, it can be concluded that the complex for M=Ni is in a monovalent state, where a Mott insulator is formed in a (-Ni³⁺-Br^--) regular chain, in contrast to the mixed-valent state (-M²⁺-Br^--M⁴⁺-Br^--) for M=Pt and Pd. Thermally excited paramagnetic spins observed for the Pd complex can be explained by the soliton-kink model under the influence of the 2D hydrogen-bond network.