Ni(III) Mott–Hubbard Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 30

Chapter 3
Ni(III) Mott-Hubbard Compounds

S. Takaishi and M. Yamashita

3.1 Introduction

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 ¼ Ni, Pd, Pt), bridging

halogens (X ethylenediamine(en), 1 R ,2 R -

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 intro-
duce structure, physical properties, and recent progresses in the Ni complexes.

¼ Cl, Br, I) in plane ligands (L ¼

3.2 Crystal Structure

Figure 3.1 shows a crystal structure of the Ni III complex [Ni(chxn) 2 Br]Br 2 [ 1 ].
Ni(chxn) 2 moieties are bridged by Br ions, forming a -Ni-Br-Ni-Br- linear chain
along b -axis. The crystal structure of this compound is isomorphic to the
corresponding Pd and Pt compounds [M(chxn) 2 X]X 2 (M

Cl,
Br, and I). There is significant difference in the position of the bridging halide ions

¼ Pd and Pt; X ¼