Chemistry Reference
In-Depth Information
density-wave (CDW) state. The bridging halide ion is close to M
4+
ion, inducing the
twofold periodicity.
1.4 MX-Chains as a Mixed-Valence Complex
Mixed-valence complexes have been extensively studied. Such a system is inter-
esting not only in solid state chemistry but also in solution chemistry. A binuclear
Ru
2+
-Ru
3+
mixed valence complex, Creutz-Taube ion: [(NH
3
)
5
Ru
2+
(pz)Ru
3+
(NH
3
)
5
]
5+
(pz
pyrazine) [
28
,
29
], and its analogues have been enormously studied from the
viewpoint of photoinduced electron transfer [
30
-
32
]. The MX-chains have been also
attractive targets as 1D extended Creutz-Taube like complexes.
Robin and Day have classified the mixed-valence system into three categories
[
33
]. This classification is often used in mixed-valence systems. Here we briefly
introduce this classification.
Class I: This comprises compounds in which the different oxidation states are
associated with very different environments. The energy required to transfer an
electron between the two is large. Thus, there is essentially no interaction between
the different oxidation states, and no special properties associated with the mixed
valency.
Class II: These compounds also have different environments for the different
oxidation states, but the sites are now sufficiently similar that electron transfer
requires only a small energy. These compounds are semiconductors and have
optical absorptions resulting from the kind of intervalence transition in visible
near infrared regions.
Class III: These compounds have all atoms in an identical, fractional oxidation
state with electrons delocalized between them. This category can be divided into
two subclasses: In Class IIIA the electron delocalization occurs within a finite
cluster; Class IIIB is where the electrons are delocalized throughout the solid.
When the MX-chains are classified according to Robin-Day's classification,
Pt and Pd complexes are classified into class II, Ni complexes are classified into
class IIIA category. It should be noted that we describe the electronic structure of the
CDW complexes as
¼
M
2
þ
M
4
þ
X
X
X
in this article for clarity. More
M
3
r
M
3
þ
r
strictly, however, this state should be noted as
X
X
X
,
where
is the degree of valence disproportionation or CDW amplitude (0
1).
r
< r <
1.5 MMX Chains
Q1D dinuclear halogen-bridged complexes, those are abbreviated as MMX-chains,
have also got attracted for several decades. They have larger degrees of freedom in
the electrons in the MMX chains compared to those in the MX chains, and that
cause a larger variety of electronic states. On the basis of theoretical calculations
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