Chemistry Reference
In-Depth Information
Fig. 1.3 Phase diagram
of MX-chains
have studied optical properties of this complex in the 1950s and shown that the
origin of the color is due to the intervalence charge transfer (IVCT) transition
from Pt
2+
to Pt
4+
species [
20
,
21
]. Kida et al. developed derivatives of the
complexes in the 1960s [
22
]. These compounds were spotlighted as model
compounds of 1D Peierls-Hubbard theory in the 1980s [
23
-
25
], and their physical
properties had been extensively studied. In this period, many derivatives had been
also developed by changing metal
¼
alkylamine, ethylenediamine (en), 1
R
,2
R
-diaminocyclohexane (chxn) etc.), and
bridging halides (Cl, Br, I).
Yamashita et al. reported the first Ni-based complex [Ni(chxn)
2
Br]Br
2
in 1981
[
26
]. At that time, this complex was thought to be a Ni
2+
-Ni
4+
mixed-valence state,
but it has been clarified that this complex is in averaged-valence state by means of
X-ray crystal structure analysis [
27
]. In the Ni complexes, many attractive physical
properties have been reported such as gigantic third-order nonlinear optical suscep-
tibility [
11
,
12
], spin-Peierls transition [
13
], etc.
ions (M
¼
Pt, Pd),
in-plane ligand (L
1.3 Electronic State of MX-Chains
The electronic states of MX chains are interpreted by extended Peierls-Hubbard
model considering electron-lattice interaction (
S
), transfer integral between neigh-
bor orbital (
T
), on-site coulomb repulsion (
U
), and intersite coulomb repulsion (
V
)
(Fig.
1.3
)[
23
-
25
]. Actually, the energy of their electronic states is often explained
by the competition between
S
and
U
for clarity.
When M
Ni,
U
is the dominant factor because of the small 3
d
z
2
orbital,
resulting in a uniform oxidation state of Ni
3+
(
d
7
). This electronic state is called
average valence (AV) or Mott-Hubbard (MH) state. When M
¼
Pd and Pt, on the
other hand,
S
is larger than
U
because of the larger 4
d
z
2
or 5
d
z
2
orbital, resulting
M
2+
(
d
8
) and M
4+
(
d
6
) mixed-valence state. This electronic state is called charge-
¼
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