Tuning of Electronic Structures of Quasi-One-Dimensional Halogen-Bridged Ni–Pd Mixed-Metal Complexes, [Ni1−xPdx(chxn)2X]X2 (X = Cl, Br) with Strong Electron Correlation

This article describes the tuning of the electronic structures in a series of quasi-one-dimensional halogen-bridged Ni–Pd mixed-metal complexes, [Ni1−xPdx(chxn)2X]X2 (chxn = 1R,2R-diaminocyclohexane; X, Y = Cl or Br). In these compounds, planar [M(chxn)2] units are bridged by halogen ions (X), forming linear chain M–X–M structures. Therefore, their electronic structures are 1D and are composed of hybridization between the dz2 orbitals of the metal ions and pz orbitals of the bridging halogens. In this system, electron-correlation on the Ni sites and electron–phonon interaction on the Pd sites should compete with each other. Their electronic structures have been investigated by means of the single-crystal reflectance, ESR and IR spectroscopies, electrical conductivities, and X-ray diffuse scattering. Moreover, their local electronic structures have been directly observed by scanning tunneling microscopy (STM) using single-crystals at room temperature and under ambient pressure. As a result, while the oxidation state of the Ni sites is +3 in all mixing ratios in these compounds due to strong electron-correlation, the conversion from the PdIII Mott–Hubbard states to the PdII–PdIV mixed-valence states is observed around x ≈ 0.90 for Br and x ≈ 0.85 for Cl. Interestingly, the spin-soliton in Ni0.05Pd0.95(chxn)2Br3 was directly observed by STM for the first time.