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Fig. 9.40 Relationship between relative arrangement of 1D chains
in the crystal of
[M
2
(MeCS
2
)
4
I]
1
(M
Ni (7), Pt(1)) and possible twofold periodic valence ordering to minimize
the Coulomb repulsion [
38
]. The dashed lines represent the interchain S
¼
S contacts, which are
relatively close to the van der Waals contact distance between sulfur atoms (3.60
˚
)
becomes more pronounced with the elongation of the alkyl chain of the dithiocar-
boxylato ligands.
9.4 Conclusions
Unlike the MX-chain compounds, an unpaired electron of the MMX-chain
compounds is possible to behave as an itinerant electron, since the influence of
U
becomes smaller by sharing of an unpaired electron through a metal-metal
bond. Actually, the first observation of metallic conductivity was made for
[Pt
2
(MeCS
2
)
4
I]
1
(1) above 300 K. The analyses of the diffuse scattering observed
in the metallic state of [Pt
2
(EtCS
2
)
4
I]
1
(2) has revealed that the metallic state has
appeared by the valence fluctuation accompanying the dynamic valence-ordering
state of the CDW type. The fact indicates that the electron-lattice interaction has
also governed the electronic states of the Pt MMX compounds, in analogy with the
Pt MX-chain compounds. The observed dynamic electronic state is considered to
appear through the electron-lattice interaction by the thermal energy being com-
parable to the stretching vibration energy of Pt-I bonds, because the electronic
states of the Pt MMX compounds [Pt
2
(RCS
2
)
4
I]
1
(R
¼
Et (2),
n
-Bu (4), and
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