Chemistry Reference
In-Depth Information
Table 2.1 The M-M distance
L
, the M-X distance
l
1
and
l
2
(
l
1
< l
2
), the bridging halogen
displacement
/
L
), the CT-exciton energy
E
CT
, and
the photoluminescence energy
E
lm
for various MX-chain compounds
d
(
¼
(
l
2
l
1
)/2), the distortion parameter
d
(
¼
2
d
E
CT
(eV)
E
Im
(eV)
L
(
˚
)
I
1
(
˚
)
I
2
(
˚
)
(
˚
)
d
d
1.
[Pt(chxn)
2
][Pt(chxn)
2
Cl
2
](ClO
4
)
4
5.730
2.314
3.416
0.511
0.190
3.20
1.49
2.
[Pt(en)
2
][Pt(en)
2
Cl
2
](ClO
4
)
4
5.403
2.318
3.095
0.3885 0.144
2.73
1.17
3.
[Pt(chxn)
2
][Pt(chxn)
2
Cl
2
]Cl
4
5.158
2.324
2.834
0.255
0.099
1.99
0.90
4.
[Pt(en)
2
][Pt(en)
2
Br
2
](ClO
4
)
4
-I
5.493
2.487
3.006
0.2595 0.094
1.98
0.78
5.
[Pt(en)
2
][Pt(en)
2
Br
2
](ClO
4
)
4
-II
5.695
2.487
3.208
0.3605 0.1266 2.40
1.11
6.
[Pt(chxn)
2
][Pt(chxn)
2
Br
2
]Br
4
5.372
2.490
2.882
0.196
0.073
1.40
0.68
7.
[Pt(en)
2
][Pt(en)
2
I
2
](ClO
4
)
4
5.820
2.726
3.093
0.1835 0.063
1.38
0.60
8.
[Pt(chxn)
2
][Pt(chxn)
2
I
2
]I
4
5.673
2.708
2.965
0.2564 0.0453 0.94
-
9.
[Pd(en)
2
][Pt(en)
2
Cl
2
](ClO
4
)
4
5.357
2.324
3.003
0.3545 0.130
2.05
0.86
10.
[Pd(en)
2
][Pd(en)
2
Br
2
](ClO
4
)
4
5.407
2.911
2.496
0.2075 0.075
1.13
0.40
11.
[Pd(chxn)
2
][Pd(chxn)
2
Br
2
]Br
4
5.296
2.523
2.773
0.125
0.047
0.75
-
12.
[Pd(en)
2
][Pt(en)
2
Cl
2
](ClO
4
)
4
5.415
2.315
3.100
0.3925 0.145
3.22
1.65
13.
[Pd(en)
2
][Pt(en)
2
Br
2
](ClO
4
)
4
5.502
2.467
3.035
0.284
0.103
2.59
1.54
14.
[Pd(en)
2
][Pt(en)
2
I
2
](ClO
4
)
4
5.866
2.678
3.188
0.255
0.087
2.28
1.12
15.
[Ni(chxn)
2
Cl
2
]Cl
2
4.894
2.447
0
0
1.83
-
16.
[Ni(chxn)
2
Br
2
]Br
2
5.160
2.580
0
0
1.28
-
Parameter values are taken from [
5
] and references therein
Typical MX-chain compounds have ClO
4
for the counteranion (Y). The data of
those compounds are shown by open marks in Fig.
2.3
. The data of the compounds
with halogen ions for Y are plotted by solid marks in the same figure. By changing
the counteranion from ClO
4
to halogen ions, the H-bonds between the amino
groups of the ligands and the counteranions are strengthened. As a result, the M-M
distance
L
is considerably decreased. Such a feature was ascertained by the infrared
(IR) and X-ray measurements [
28
,
32
,
56
]. As shown in Fig.
2.3
, the lengths
L
in the
compounds with Y
¼
halogen (solid marks) are relatively smaller than those of the
compounds with Y
¼
ClO
4
(open marks).
The position of the bridging halogen (X) ion between the neighboring two M
ions, or the M-X distance
l
1
and
l
2
, depends strongly on the choice of X. As seen in
Fig.
2.3
, the shorter M-X distance
l
1
is almost constant in Pt or Pd compounds
having the same X ions, even when
L
is considerably changed. The constant values
of
l
1
(
2.33
˚
for X
2.50
˚
for X
2.73
˚
for X
I) are
close to the sum of the ionic radii of the bridging halogen ions (Cl
: 1.81
˚
,Br
:
1.96
˚
and I
: 2.16
˚
[
57
,
58
]) and the metal (Pd
4
þ
: 0.62
˚
and Pt
4
þ
: 0.63
˚
[
59
]).
Starting from the M
3
þ
-X
regular chain structure, the deviation of
l
1
from the
dotted line (
L
/2) corresponds to the amplitude of the halogen displacements
¼
Cl and
¼
Br, 2.68
¼
.
Therefore, the decrease of
L
induces the decrease of the longer M-X distance
l
2
, and
then induces the decrease of
d
. Thus, we can consider that the decrease of
L
suppresses effectively the e-l interaction. At the same time, the decrease of
L
leads to the increase of transfer energy (
t
) between the neighboring M ions, making
the electronic state more delocalized and also decreasing
d
d
, that is, the CDW
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