General Introduction - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 1

Chapter 1
General Introduction

S. Takaishi and M. Yamashita

1.1 Quasi-one-Dimensional Halogen-Bridged Metal Complexes

For a long time, quasi-one-dimensional (Q1D) halogen-bridged metal complexes
have attracted much attention of chemists and physicists because of significant
physical properties based on their Q1D electronic structures such as intense and
dichroic charge transfer (CT) bands [ 1 - 3 ], progressive overtones in the resonance
Raman spectra [ 4 - 7 ], midgap absorptions attributable to solitons and polarons
[ 8 - 10 ], gigantic third-order nonlinear optical susceptibilities [ 11 , 12 ], spin-Peierls
transitions [ 13 ], thermochromism in the organic media [ 14 ], insertion of 1D chains
into artificial peptides [ 15 ], current oscillation phenomena [ 16 ], and field effect
carrier doping [ 17 ].
Q1D halogen-bridged metal complexes have chain structure of group 10 metals
(M Cl, Br, I), in which d z 2 orbital of

M and p z orbital of X overlap forming 1D electronic system (Fig. 1.1 ). They are
classified into two types. One has the infinite chain structure of alternating stacking
of M and X, such as -M-X-M-X-, so-called MX chains. The other has infinite
chain structure of alternating stacking of M-M dimer unit of paddle-wheel structure
and X, such as -M-M-X-M-M-X-, so-called MMX chains.

¼ Ni, Pd, Pt) bridged by halide ions (X ¼

1.2 History of MX-Chains

History of MX-chains began at the end of the nineteenth century since Wolffram
reported Cl-bridged Pt complex [Pt(etn) 4 Cl]Cl 2 ·2H 2 O (etn

aminoethane,
Fig. 1.2 )[ 18 ]. This complex is called Wolffram's red salt because of its lustrous

¼