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freedom persists down to 2 K, similar to 1 [ 32 ]. This quite remarkable difference in
the magnetic behaviors of 2 and 4 is attributed to the degree of lattice distortion in
the 1D MMX chains. As described in the Crystal Structures section, X-ray crystal
structure analyses revealed that differences between Pt 3+ -I and Pt 2+ -I distances
of 4 (ca. 0.07 ˚ ) are remarkably larger than that for 2 (0.02-0.03 ˚ ). As these
distance differences correspond to the lattice distortion of a 1D MMX chain, it is
reasonable to consider that the alternation of an exchange interaction in 4 increases
with an increase in lattice distortion, resulting in the spin-singlet state of the low
temperature ACP state.
[Pt 2 (n-PenCS 2 ) 4 I] 1 (5)
The temperature dependence of
w M of [Pt 2 ( n -
PenCS 2 ) 4 I] 1 (5) is shown in Fig. 9.30 [ 35 ]. The magnetic susceptibility in the RT
and HT phases is of the order of 1-2.5
the magnetic susceptibility
10 5 emu mol 1 , which is of the same order
of magnitude as those observed in 2 and 4. The low susceptibility is consistent with
Pauli paramagnetism as one may expect for a conductor. The most striking feature is an
abrupt decrease in the
w M to the spin-singlet state with the first-order RT-LT phase
transition, analogous to compound 4. In the case of 4, the spin-singlet state appears with
the first-order RT-LT phase transition from the AV state to the ACP state around 205 K
[ 33 ]. As described in the X-ray diffraction, the compound 5 in the RT phase exhibits the
diffuse scattering corresponding to twofold repetition length of a -Pt-Pt-I- period, in
addition to the main Bragg spots, which changes to the Bragg spots in the LT phase.
Therefore, the compound 5 is considered to take the spin-singlet state with the first-
order phase transition from the valence-ordered state close to AV state in the RT phase
to the ACP or CDW state in the LT phase. Taking into account the valence-ordered
state of 2 and 4 in the LT phase [ 33 ], the most probable ground state of 3 and 5 would
betheACPstateof-Pt 2+ -Pt 3+ -I -Pt 3+ -Pt 2+ -I -. The temperature dependence of
w M
in the first heating process below RT coincides with that in the first cooling process but
exhibits a slight increase at the monotropic phase transition to the HT phase in the
temperature range of 324-335 K. It is confirmed from the result of the second cooling
process that the HT phase does not return to the RT phase in agreement with the results
with the heat capacity measurement [ 52 ], EPR measurement [ 78 ], and transport
properties [ 35 ].
9.3 Nickel Complexes
9.3.1 Syntheses
Unlike the diplatinum(III) complex [Pt 2 (RCS 2 ) 4 I 2 ], it is difficult to isolate the dinickel
(III) complex [Ni 2 (RCS 2 ) 4 I 2 ] and, consequently, [Ni 2 (RCS 2 ) 4 I] 1 (R
Me (7), Et (8),
n -Pr (9), and n -Bu (10)) are directly prepared by the reaction of [Ni 2 (RCS 2 ) 4 ] with
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