Chemistry Reference
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Fig. 10.3 Crystal structure of K
2
(NC
3
N)[Pt
2
(pop)
4
I]
4H
2
O(
red
,O;
orange
,P;
purple
,I;
yellow
,
Pt) and schematic chain structure in ACP, CDW and ACP + CDW state. The countercations,
lattice water, and hydrogen atoms are omitted for clarity
The ACP-like distortion is corresponding to spin-Peierls distortion, which has the
stable structure of 1D metal at low temperature. Very few compounds have been
reported to have spin-Peierls distortion even at room temperature [
34
,
35
].
Although ACP state has been reported in dta-type MMX chains [
36
-
45
], the
complexes containing NC
3
N
2+
, Me-NC
3
N
2+
or Cl-NC
3
N
2+
are the first
comopounds with ACP-like distortion in pop-type MMX chains. However, the
structure is not the simple ACP state because Pt-I distances are shorter in one
[Pt
2
(pop)
4
] unit (
d
(Pt-I)), but are longer in the other [Pt
2
(pop)
4
] unit
,
which is the characteristic of CDW state and is consistent with the polarized Raman
spectra.
Figure
10.3
shows the chain structure of K
2
(NC
3
N)[Pt
2
(pop)
4
I]
ðdð
Pt
I
ÞÞ
4H
2
O at 100 K,
for example [
46
]. Two different
d
(Pt-I-Pt) (5.6305(8)
˚
, 5.9786(9)
˚
) indicate a
ACP-like twofold periodicity along the chain. One [Pt
2
(pop)
4
] unit is coordinated
by neighboring I
ions with short Pt-I distances (
d
(Pt-I)
2.641(3)
˚
, 2.828
¼
(6)
˚
), whereas the other is coordinated by I
-
ions with a longPt
I bond distances
˚
, 3.151(6)
˚
), which is the characteristic of the CDW state.
This new electronic state of the MMX chains containing NC
3
N
2+
, Me-NC
3
N
2+
or Cl-NC
3
N
2+
is named ACP + CDW state [
46
,
47
]. Schematic representations of
the structure in ACP, CDW and ACP + CDW state are shown in Fig.
10.3
. Except
for K
2
(NC
3
N)[Pt
2
(pop)
4
I]
ðdð
Pt
I
Þ¼
2
:
990
ð
3
Þ
4H
2
O, the positions of the bridging iodide ions are three-
dimensionally ordered, which is very rare and has been reported only recently [
48
].
The
d
(Pt-I-Pt),
d
(Pt-I) and
dð
Pt
I
Þ
of MMX chains with binary countercations
are summarized in Table
10.1
.
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