3. Ni(III) Mott–Hubbard Compounds

For a long time, MX-chain compounds have been extensively studied since Wolffram reported Cl-bridged Pt complex in 1900. To date, more than 300 compounds were reported by combining metal ions (M = Ni, Pd, Pt), bridging halogens (X = Cl, Br, I) in plane ligands (L = ethylenediamine(en), 1R,2R-diaminocyclohexane(chxn), etc.), and counteranions (ClO ₄ ⁻ , BF ₄ ⁻ , X⁻, etc.). However, the number of Ni compounds is very few compared to Pd or Pt compounds. The first Ni complex was reported by Yamashita et al. in 1981. Although there had been much controversy about the electronic state of these compounds, it has been clarified that this complex is in averaged valence state by means of X-ray crystal structure analysis [1]. In the Ni complexes, many attractive physical properties have been reported such as gigantic third-order nonlinear optical susceptibility [2], spin-Peierls transition [3], etc. In this chapter, we introduce structure, physical properties, and recent progresses in the Ni complexes.