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orbitals, analogous to the Ni MX compounds [
8
]. The details of the properties of the
pop-family of A
4
[Pt
2
(pop)
4
X]
n
H
2
O have been described in this topic by Iguchi,
Takaishi, and Yamashita.
[M
2
(MeCS
2
)
4
I]
1
(M
Pt (1), Ni (7)) was first prepared and characterized by
Bellitto et al. in 1983 and 1985, respectively [
28
,
37
]. Yamashita et al. first reported in
1992 that the single crystal electrical conductivity of [Pt
2
(MeCS
2
)
4
I]
1
(1)around
room temperature is nearly temperature independent [
29
]. The transport and spectro-
scopic studies of 1 by Kitagawa et al. revealed that this compound shows a
high-electrical conductivity of 13 S cm
1
at room temperature and exhibits metallic
behavior above 300 K [
31
]. In contrast, [Ni
2
(MeCS
2
)
4
I]
1
(7) has an AV state at RT,
but exhibits a sharp peak near 0.6 eV in the reflectance spectrum for light polarized
parallel to the 1D chain, which is attributed to a Mott-Hubbard gap due to a relatively
large on-site Coulomb repulsion energy
U
of the nickel atoms [
29
]. Accordingly, this
compound has been regarded as a Mott-Hubbard semiconductor.
To reveal the essence of these MMX compounds as a 1D electronic system, two
series of Pt and Ni MMX-chain compounds, [Pt
2
(RCS
2
)
4
I]
1
(R
¼
¼
Et (2),
n
-Pr (3),
n
-Bu (4),
n
-Pen (5), and
n
-Hex(6)) [
32
-
36
] and [Ni
2
(RCS
2
)
4
I]
1
(R
Et (8),
n
-Pr (9),
and
n
-Bu (10)) [
38
] have been prepared by our group, and the correlations between
their crystal structures and properties have been studied in detail. In the platinum
complexes, it was found that the valence fluctuation originating from the Pt
2+
and Pt
3+
mixed-valence state plays an essential role in the appearance of the
metallic or dynamic electronic state. On the other hand, the nickel complexes do not
exhibit such a valence fluctuation but behave as a Class III-A averaged-valence
state of -Ni
2.5+
-Ni
2.5+
-I
- due to strong electron-electron correlation of the
Ni atoms. Theoretical studies of the MMX-chain compounds to reveal the elec-
tronic structure have also been described [
39
-
43
]. The elongation of the alkyl
chains is expected to cause not only increasing one-dimensionality (reducing trans-
versal effects) but also increasing motional degrees of freedom in the system.
Furthermore, the interplay between electronic degrees of freedom and molecular
dynamics has actually caused the diverse and intriguing structural phase tran-
sitions accompanying electronic and/or magnetic transition never observed for
[M
2
(MeCS
2
)
4
I]
1
(M
¼
Pt (1) and Ni (7)).
¼
9.2 Platinum Complexes
9.2.1 Syntheses
Dithiocarboxylic acids are prepared in moderate yields by the reaction between
carbon disulfide and the corresponding alkylmagnesium halide in a mixed solvent
of tetrahydrofuran and diethyl ether [
44
]. A precursor complex [Pt
2
II
(MeCS
2
)
4
]is
prepared by the reaction of dithioacetic acid with K
2
[PtCl
4
] in toluene under reflux,
but the yield is at most 40 % because of the low reactivity of K
2
[PtCl
4
] with
dithioacetic acid in toluene [
45
]. However, the yield can be improved to about
90 % by using platinum(II) chloride instead of K
2
[PtCl
4
]. The precursor complexes
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