Chemistry Reference
In-Depth Information
9.1
Introduction
Mixed valency has played a pivotal role in the appearance of many interesting
properties such as electrical, magnetic, and optical or their combination [
1
]. Such a
mixed valency was first realized in inorganic compound and, in particular, the
mixed-valence platinum complexes. One-dimensional (1D) halogen-bridged metal
complexes, MX-chain compounds, have received significant attention for several
decades because they display a variety of electronic states originating from the
competition between electron-lattice interaction and strong electron-electron cor-
relation [
2
-
6
]. MX chain compounds with M
¼
Pt and Pd are usually in a Class II
X
-, and show a shift of the halogen
atoms from the midpoint between two metal atoms due to Peierls instability.
These Class II compounds exhibit characteristic physical properties such as strong
intervalence charge-transfer (IVCT) absorption [
7
,
8
], luminescence with a large
Stokes shift [
9
], and resonance Raman scattering with high-overtone progression
[
10
,
11
] originating from the strong electron-lattice interactions. On the other hand,
nickel compounds tend to be in a Class III-A averaged-valence state of -X
-Ni
3
þ
-X
-Ni
3+
-X
- and the bridging halogen atoms are located at the midpoint between
two Ni atoms due to strong electron-electron correlation of the Ni atoms [
12
].
These Ni compounds are characterized as Mott-Hubbard semiconductors with
a spin-density-wave (SDW) ground state [
13
], and exhibit very interesting solid-
state properties such as gigantic third-order nonlinear optical susceptibility (
X
-M
2+
-X
‐
M
4+
mixed-valence state,
‐
‐
(3)
)[
14
]
w
and an
S ¼
1/2 1D Heisenberg antiferromagnetic (AF) chain with a strong AF
interaction (
J
/
k
B
¼
3,600 K) between the Ni
3+
ions [
8
]. Using magnetic susceptibility
and nuclear quadrupole resonance (NQR) data, Takaishi et al. recently showed
that
1
R
,2
R
-diaminocyclohexane)
undergoes a spin-Peierls transition in the temperature range of 40-130 K [
15
].
The Ni compounds have also attracted attention as a 1Dmodel candidate for high-
T
c
copper oxide superconductors because of the similarity in the electronic band
structure to La
2
CuO
4
which is the precursor compound to the series La
2
x
Sr
x
CuO
4
[
16
].
The details of the properties of these MX-chain compounds have been described in
this topic.
The evolution from MX-chain compounds to 1D halogen-bridged mixed-
valence dinuclear chain compounds, MMX-chain compounds, has recently
received increased attention, as they display a variety of electronic states and subtle
balance of solid-state properties originating from the charge-spin-lattice coupling
and the fluctuation of these degrees of freedom. MMX-chain compounds reported
so far belong to two families, i.e., the pop-family of A
4
[Pt
2
(pop)
4
X]
{[NiBr(1
R
,2
R
-chxn)
2
]Br
2
}
1
(1
R
,2
R
-chxn
¼
n
H
2
O (pop
¼
P
2
O
5
H
2
2
;A
¼
¼
Li, K, Cs, various alkyl ammonium; X
Cl, Br, I) [
17
-
27
] and the
dithioacetato, MeCS
2
)[M
2
(RCS
2
)
4
I]
1
(M
¼
¼
¼
dta-family (dta
Pt, R
Me (1),
Et (2),
n
-Pr (3),
n
-Bu (4),
n
-Pen (5),
n
-Hex (6); M
¼
¼
Me (7), Et (8),
n
-Pr (9),
n
-Bu (10)) [
28
-
38
]. These compounds exist in a mixed-valence state
composed of M
2+
Ni, R
(d
8
,
S ¼
0) and M
3+
(d
7
,
S ¼
1/2) with a formal oxidation
number of
þ
2.5. Therefore, the MMX-chain compound is a 1D d-p electronic
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