129I moessbauer spectroscopic study of a metallic MMX chain system

Article abstract of DOI:10.1021/ic9007558

One-dimensional iodide-bridged mixed-valence binuclear platinum complexes (the so-called "MMX chains") and their Pt(III) dimer precursors were investigated with ¹²⁹I Moessbauer spectroscopy. Spectra consisting of two sets of octuplets were observed at low temperatures for a neutral MMX chain complex, Pt₂(dtp)₄l (dtp = C ₂H₅CS₂^-), with various charge-ordering states at the Pt dimers, indicating that the charge-ordering state is in an alternate-charge-polarization phase (ACP: · · ·[Pt²⁺-Pt³⁺]-l^0.4--[Pt ³⁺-Pt²⁺]- · · · -1^0.3-- · · ·), which is consistent with a previous low-temperature X-ray diffraction study. The estimated valence states of the bridging iodines of [(C₂H₅)₂NH₂]₄[Pt ₂(pop)₄l] (pop = H₂P₂O ₅^2-), with a charge-polarization phase (CP: · · · [Pf-Pt³⁺]-I⁰-^4-· · · [Pt²⁺-Pt³⁺]-I^0.4--· · ·), and [H₃N(CH₂)₆NH ₃]₂[Pt₂(pop)₄l], with a charge-density-wave phase (CDW: · · · [Pt ²⁺-Pt²⁺] · · ·I^0.3- -[Pt^3+-Pt³⁺]-I^30.3- · · ·), suggest that the covalent bond interaction is dominant in the CDW phase, whereas the Coulomb interaction is dominant in the CP phase. The estimated absolute quadrupole coupling constant (QCC) values for negatively charged MMX chain complexes with pop ligands are larger than those for neutral MMX chain complexes with CH₃CS₂^- (dta) ligands, implying that the Madelung potential formed by the more-negative pop ligands and countercations effectively contributes to the physical properties of the pop system. The three Pt(III) dimer complexes Pt₂(dta)₄l ₂, Pt₂(dtp)₄l₂, and K ₄[Pt₂(pop)₄l₂] showed almost the same isomer shifts, indicating that the valence state of the iodide ion (1 ^0.5-) depends negligibly on the terminal ligand. The QCC value observed for K₄[Pt₂(pop)₄l₂] was larger than those for Pt₂(dta)₄l₂ and Pt ₂(dtp)₄l₂, originating from the anisotropic arrangement of the iodide anions, which form layers lying on the ab plane in the crystal.

Full text of DOI:10.1021/ic9007558

8044 Inorg. Chem. 2009, 48, 8044–8049
DOI: 10.1021/ic9007558
::
129I Mossbauer Spectroscopic Study of a Metallic MMX Chain System
Atsushi Kobayashi,*,†, Shinji Kitao,‡ Makoto Seto,‡ Ryuichi Ikeda,† and Hiroshi Kitagawa*,†,§,^
†Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki 6-10-1, Higashi-ku,
Fukuoka 812-8581, Japan, ‡Research Reactor Institute, Kyoto University, Kumatori, Sennan, Osaka 590-0494,
Japan, and §JST-CREST, Sanbancho 5, Chiyoda-ku, Tokyo 102-0075, Japan. Present address: Division of
Chemistry, Faculty of Science, Hokkaido University, North-10, West-8, Kita-ku, Sapporo 060-0810, Japan.
^Present address: Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa
Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan
Received April 20, 2009
One-dimensional iodide-bridged mixed-valence binuclear platinum complexes (the so-called “MMX chains”) and their Pt(III)
::
dimer precursors were investigated with 129I Mossbauer spectroscopy. Spectra consisting of two sets of octuplets were
observed at low temperatures for a neutral MMX chain complex, Pt2(dtp)4I (dtp = C2H5CS2-), with various charge-ordering
states at the Pt dimers, indicating that the charge-ordering state is in an alternate-charge-polarization phase
(ACP:
[Pt2+-Pt3+]-I0.4--[Pt3+-Pt2+] I0.3- ), which is consistent with a previous low-temperature X-ray
3 3 3
3 3 3
3 3 3
diffraction study. The estimated valence states of the bridging iodines of [(C2H5)2NH2]4[Pt2(pop)4I] (pop = H2P2O52-), with
a charge-polarization phase (CP:
[Pt2+-Pt3+]-I0.4- [Pt2+-Pt3+]-I0.4- ), and [H3N(CH2)6NH3]2[Pt2(pop)4I],
3 3 3
3 3 3 3 3 3
with a charge-density-wave phase (CDW:
[Pt2+-Pt2+] 0.3--[Pt3+-Pt3+]-I0.3- ), suggest that the covalent
I
3 3 3
3 3 3
3 3 3
bond interaction is dominant in the CDW phase, whereas the Coulomb interaction is dominant in the CP phase. The
estimated absolute quadrupole coupling constant (QCC) values for negatively charged MMX chain complexes with pop
ligands are larger than those for neutral MMX chain complexes with CH3CS2- (dta) ligands, implying that the Madelung
potential formed by the more-negative pop ligands and countercations effectively contributes to the physical properties of the
pop system. The three Pt(III) dimer complexes Pt2(dta)4I2, Pt2(dtp)4I2, and K4[Pt2(pop)4I2] showed almost the same isomer
shifts, indicating that the valence state of the iodide ion (I0.5-) depends negligibly on the terminal ligand. The QCC value
observed for K4[Pt2(pop)4I2] was larger than those for Pt2(dta)4I2 and Pt2(dtp)4I2, originating from the anisotropic
arrangement of the iodide anions, which form layers lying on the ab plane in the crystal.
Introduction
are one of the most attractive materials in 1-D electronic
systems because of their characteristic electronic states and
unique physical properties, such astheir metallic conductivity
and pressure- or light-induced phase transitions, spin-Peierls
transitions, and so on.1,2 The degrees of freedom of the
charge polarization in the metal dimer units play an impor-
tant role in the physical properties of MMX chain systems.
Several charge-ordering states have been observed in the
MMX chain and are listed as follows:
One-dimensional (1-D) halogen-bridged mixed-valence
binuclear metal complexes, the so-called “MMX chains”,
*To whom correspondence should be addressed. Tel.: +81-11-706-3818
(A.K.), +81-75-753-4035 (H.K.). Fax: +81-11-706-3447 (A.K.), +81-75-
753-4035 (H.K.). E-mail: akoba@sci.hokudai.ac.jp (A.K.), kitagawa@
kuchem.kyoto-u.ac.jp (H.K.).
(1) (a) Matsuzaki, H.; Matsuoka, T.; Kishida, H.; Takizawa, K.; Miya-
saka, H.; Sugiura, K.; Yamashita, M.; Okamoto, H. Phys. Rev. Lett. 2003,
90, 046401. (b) Matsuzaki, H.; Kishida, H.; Okamoto, H.; Takizawa, K.;
Matsunaga, S.; Takaishi, S.; Miyasaka, H.; Sugiura, K.; Yamashita, M. Angew.
Chem., Int. Ed. 2005, 44, 3240–3243.
(2) (a) Kitagawa, H.; Onodera, N.; Sonoyama, T.; Yamamoto, M.;
Fukawa, T.; Mitani, T.; Seto, M.; Maeda, Y. J. Am. Chem. Soc. 1999,
121, 10068–10080. (b) Mitsumi, M.; Murase, T.; Kishida, H.; Yoshinari, T.;
Ozawa, Y.; Toriumi, K.; Sonoyama, T.; Kitagawa, H.; Mitani, T. J. Am. Chem.
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S.; Nakazawa, Y.; Sato, A.; Mitsumi, M.; Yamashita, T.; Toriumi, K.; Sorai, M. J.
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H. Eur. J. Inorg. Chem. 2006, 3567–3570.
(1) Averaged valence (AV) state:
-[M2.5+-M2.5+]-X-[M2.5+-M2.5+]-X-
(2) Charge-polarization (CP) state:
[M2+-M3+]-X [M2+-M3+]-X
(3) Charge-density-wave (CDW) state:
3 3 3
3 3 3
3 3 3
3 3 3
[M2+-M2+
]
X-[M3+-M3+]-X
3 3 3
3 3 3
(4) Alternate-charge-polarization (ACP) state:
[M2+-M3+]-Xa-[M3+-M2+
]
Xb
3 3 3
3 3 3
3 3 3
So far, two types of MMX chain systems have been
reported, classified by their terminal ligands. One is the pop
system, represented as A4[Pt2(pop)4X] nH2O (A = Li+,
3
Na+, K+, NH4+, etc.; pop = H2P2O52-; X = Cl-, Br-,
I-), which consists of anionic MMX chains and countercations,
r
pubs.acs.org/IC
Published on Web 07/16/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 16, 2009 8045
as shown in Figure 1a.1,3 The other is the dta system,
represented as M2(RCS2)4I (M = Pt, Ni; R = alkyl group),
which consists of neutral MMX chains without counterions,
as shown in Figure 1b.2 Extensive work on both the pop and
dta systems has revealed that the pop system exhibits semi-
conducting or insulating behavior whereas the dta system is
highly conductive, and some platinum complexes exhibit
metallic conduction and metal-insulator transitions.1-4
The CP or CDW state of the pop system is the ground state,
and some types undergo photoinduced or pressure-induced
phase transitions between the CP and CDW phases. In
contrast, in the dta system, the AV state is often observed
in the metallic phase. At low temperatures, a metal-insulator
transition occurs because of the strong electron-lattice
interactions.2 The ground state of the dta system is consid-
ered to be in the ACP state.4e-g These data clearly indicate
that the electronic states in the dta and pop systems differ
markedly, although their Pt dimer units are in the isoelec-
tronic d7-d8 configuration. The origin of these differences in
their electronic states is still under discussion. Except for
some theoretical studies,5,6 there have been few systematic
studies of the two MMX chain systems. To gain insight into
the origin of the differences between the dta and pop systems,
some quantitative measurements must be made to investigate
Figure 1. Crystal structures of (a) [NH3(CH2)6NH3]2[Pt2(pop)4I] and
(b) Pt2(dtp)4I.
the electronic states of the MMX chain complexes.
::
Mossbauer spectroscopy has been widely used to investigate
the valence states of measurable atoms (such as 57Fe, 119Sn,
::
129I, and 197Au).7 The Mossbauer parameters, isomer shift
(IS) and the quadrupole coupling constant (QCC), allow us
to discuss the electronic states of measured atoms quantita-
::
tively. Therefore, we have used 129I Mossbauer spectroscopy
to probe the differences between the electronic states of the
two MMX chain systems. We also measured as controls the
spectra of Pt(III) dimer complexes, which are the precursors
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Onodera, N.; Mitani, T.; Toriumi, K.; Yamashita, M. Synth. Met. 1997, 86, 1931–
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Toriumi, K.; Iso, Y.; Kitagawa, H.; Mitani, T. Angew. Chem., Int. Ed. 2002, 41,
2767–2771. (f) Kitagawa, H.; Sonoyama, T.; Mitani, T.; Seto, M.; Maeda, Y.
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of MMX chain complexes.
::
In this paper, we report the 129I Mossbauer spectra of these
two types of MMX chain systems, and we discuss the
electronic states of the iodide-bridged MMX chain com-
plexes quantitatively.
Experimental Section
Synthesis. The starting materials, tetrakis(dithioacetato)-
diplatinum(II), Pt2(dta)4 (dta = CH3CS2-);8 tetrakis(dithiopro-
pionate)diplatinum(II), Pt2(dtp)4 (dtp = CH3CH2CS2-);9 and
potassium tetrakis(pyrophosphito)diplatinate(II), K4[Pt2(pop)4],10
were prepared according to the published procedures.
::
Sample Preparation for 129I Mossbauer Spectroscopy. The
samples for iodine Mossbauer spectroscopy were synthesized
::
using the radioisotope 129I at the Kyoto University Reactor
(KUR) with the following chemical reactions.
For Pt2(dta)4I2 (1), Pt2(dtp)4I2 (2), and Pt2(dtp)4I (3):
2Na129I þ Na2SO3 þ 2H2O2 þ H2SO4 f 2Na2SO4
þ 3H2O þ 129I2
ðIÞ
Pt2ðdtaÞ4
þ
129I2 f Pt2ðdtaÞ 129I2 ð1Þ
ðIIÞ
4
::
(7) (a) Parish, R. V. Mossbauer Spectroscopy Applied to Inorganic
::
Chemistry. In Mossbauer Spectroscopy with Iodine Isotopes; Long, G. J.,
Ed.; Plenum: New York, 1984; Vol. 2, p 391 and references therein. (b) Ruby, S.
::
(5) (a) Yamamoto, S. J. Phys. Chem. Solids 2002, 63, 1489–1493. (b)
Yamamoto, S. J. Phys. Soc. Jpn. 2001, 70, 1198–1201. (c) Yonemitsu, K.;
Miyashita, N. Phys. Rev. B 2003, 68, 075113. (d) Yamamoto, S.; Ichioka, M. J.
Phys. Soc. Jpn. 2002, 71, 189–196. (e) Ohara, J.; Yamamoto, S. Phys. Rev. B
2004, 70, 115112. (f) Ohara, J.; Yamamoto, S. J. Phys. Soc. Jpn. 2005, 74, 250–
253. (g) Ohara, J.; Yamamoto, S. J. Phys. Chem. Solids 2005, 66, 1571–1574.
(6) (a) Kuwabara, M.; Yonemitsu, K. J. Mater. Chem. 2001, 11, 2163–
2175. (b) Kuwabara, M.; Yonemitsu, K. Physica B 2000, 284-288, 1545–1546.
L.; Shenoy, G. K. In Mossbauer Isomer Shifts; Hendy, G. K., Wagner, F. E., Eds;
North-Holland: Amsterdam, 1978; Chapter 9b.
(8) Bellitto, C.; Flamini, A.; Piovesana, O.; Zanazzi, P. F. Inorg. Chem.
1980, 19, 3632.
(9) Mitsumi, M.; Yoshinari, T.; Ozawa, Y.; Toriumi, K. Mol. Cryst. Liq.
Cryst. 2000, 342, 127.
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Chem. 1985, 24, 4662–4665.

8046 Inorganic Chemistry, Vol. 48, No. 16, 2009
Kobayashi et al.
::
Table 1. Mossbauer Parameters for MMX Chain Complexes and Their Precursors
complex
terminal ligand
T (K)
ISa (mm s-1
)
QCC (MHz)
Up
Fc
area
b
hp
-
1
2
CH3CS2
C2H5CS2-
16
16
11
11
80
80
16
15
15
3.66(7)
3.71(7)
3.94(8)
3.99(8)
3.80(7)
3.95(8)
3.64(7)
3.80(7)
3.96(8)
-1172(35)
-1177(35)
-1213(36)
- 1432(43)
-1238(37)
- 1413(42)
-1530(46)
-1567(47)
-1680(50)
0.47(4)
0.51(4)
0.59(5)
0.69(5)
0.56(4)
0.67(5)
0.46(4)
0.57(4)
0.67(5)
0.51(2)
0.51(2)
0.53(2)
0.62(2)
0.54(2)
0.62(2)
0.67(2)
0.68(2)
0.73(2)
-0.53(4)
-0.49(4)
-0.41(5)
- 0.31(5)
-0.44(4)
- 0.33(5)
-0.54(4)
-0.43(4)
-0.33(5)
-
3-Ia
3-Ib
3-Ia
3-Ib
4
C2H5CS2
0.85(1)
1.00
0.79(2)
1.00
-
C2H5CS2
2-
H2P2O52-
5
6
H2P2O52-
H2P2O5
a Referenced to Mg3TeO6. b Calculated from eq 2 with hs set to zero. c Calculated with the equation F = hp - 1.
number of p holes (hp), the p-electron imbalance (Up), the charge
on the iodine (F), and the relative integral intensity (area) are
given in Table 1. The relationship between the Mossbauer
parameters and the valence shell configuration of the iodine
atom has been discussed many times.7 IS is given by
Pt2ðdtpÞ4
þ
129I2 f Pt2ðdtpÞ 129I2 ð2Þ
ðIIIÞ
ðIVÞ
4
::
Pt2ðdtpÞ þ Pt2ðdtpÞ 129I2 f 2Pt2ðdtpÞ 129I ð3Þ
4
4
4
Reaction I is the extraction process for 129I2 from aqueous
solution. The sample preparation processes II, III, and IV were
performed according to the procedures published previously.2b
For K4[Pt2(pop)4I2] (4), [(C2H5)2NH2]4[Pt2(pop)4I] (5), and
[NH3(CH2)6NH3]2[Pt2(pop)4I] (6):
IS ¼ CΔÆR2æΔFð0Þ
ð1Þ
where C is a constant containing the nuclear parameters (>0),
ΔÆR2æ is the change in the square of the nuclear radius between
the excited and ground states, which has been estimated to
be +19.5 ꢁ 103 fm2 for 129I, and ΔF(0) is the difference between
the contact densities of the source and the absorber. Therefore,
for 129I, a positive IS indicates an increase in the electron density
at the nucleus, F(0). In the case of the iodide ion, which has a
5s25p6 configuration, the IS value is given by the following
empirical equation:7
2Na129I þ PbðCH3COOÞ2 f 2CH3COONa þ Pb129I2 ðVÞ
Pb129I2 þ Na2CO3 f 2Na129I þ PbCO3
Na129I þ 127I2 f Na129I127I2
ðVIÞ
ðVIIÞ
IS ¼ -9:2hs þ 1:5hp - 0:54
ð2Þ
Na129I127I2 þ K4½Pt2ðpopÞ4ꢀ f
NaI þ K4½Pt2ðpopÞ4I2ꢀ ð4Þ
where the ISvalues (mms-1) are referencedto ZnTe andhs and hp
are the hole numbers of the s and p shells, respectively. Therefore,
the oxidation state of the iodine atom can be estimated from this
equation. The iodine atom has a larger IS value, in the order I- <
I0 < I+, because of the reduction in the occupation of the p
orbitals in this order, because the valence and core s orbitals are
screened from the nucleus. Therefore, the IS value reflects the
number of both s and p electrons. When the s electron of iodine
does not contribute to the Pt-I bond, we can assume that the hs
value is zero. Conversely, the QCC is a direct measure of the
electric field gradient (EFG) at the iodine nucleus:7
ðVIIIÞ
K4½Pt2ðpopÞ4I2ꢀ þ K4½Pt2ðpopÞ4ꢀ
þ ex:ððC2H5Þ2NH2Þ2SO4 f ½ðC2H5Þ2NH2ꢀ4½Pt2ðpopÞ4Iꢀ ð5Þ
ðIXÞ
K4½Pt2ðpopÞ4I2ꢀ þ K4½Pt2ðpopÞ4ꢀ þ ex:ðH3NðCH2Þ6NH3ÞSO4
f ½NH3ðCH2Þ6NH3ꢀ2½Pt2ðpopÞ4Iꢀ ð6Þ
ðXÞ
QCC ¼ e2qQ
ð3Þ
Reactions V, VI, and VII are purification processes for 129I-.
The sample preparation processes VIII, IX, and X were per-
where e is the charge on the proton (e > 0), Q is the nuclear
quadrupole moment (negative for both the excited and ground
states of 129I), and EFG is often designated eq (principal [z]
component of EFG). The concentrationofthe negativecharge on
the xy plane makes EFG positive (QCC is negative because eQ is
negative for 129I), and that on the z axis makes it negative (QCC is
positive). Most of the iodide (I-), iodine (I0), and iodine cation
(I+) complexes have negative QCC values, because as
the electronic configurations range from the closed-shell iodide
ion, 5s25p6, to the neutral iodine atom I, 5s25p5, to the iodine
cation (I+), 5s25p4, the electron density is steadily lost from the
5pz orbital along the series. Therefore, an IS-QCC linear corre-
lation is well-known in this series. The following relationship is
also well-known for nuclear quadrupole resonance data:7
formed according to the published procedures.3
::
129I Mossbauer Spectroscopy. A 129mTe source was obtained
by the neutron irradiation of enriched Mg3128TeO6 in the
nuclear reaction 128Te[n,γ]129mTe at KUR. The half-life of
::
129mTe is 33 days. Mossbauer spectroscopic measurement of a
27.7 keV γ-ray transition in 129I was carried out while both the
source and the absorber were cooled, using a constant-accelera-
tion spectrometer with a NaI(Tl) scintillation counter and a
Daikin CryoKelvin closed-cycle refrigerator system using he-
lium gas as the working medium.
::
Analysis of 129I Mossbauer Spectra. The observed spectra
were analyzed with a computer program that included folding
and least-squares fitting with Lorentzian lines. The half-life of
the excited state is rather long (16.8 ns), producing well-resolved
spectra with small natural line widths. The nuclear spins of the
excited and ground states are 5/2 and 7/2, respectively. There-
fore, the quadrupole-split spectra consisted of a minimum of
eight lines. For all six measured complexes, IS, QCC, the
Up ¼ hz - ðhx þ hyÞ=2
ð4Þ
ð5Þ
Up ¼ -e2qQ127
h
-1=2293 MHz

Article
Inorganic Chemistry, Vol. 48, No. 16, 2009 8047
::
Figure 2. 129I Mossbauer spectra of the MMX chain precursors 1, 2,
and 4.
Figure 3. Crystal structure of complex4 (ref 11). Cations (Pt and K) and
anions (I and pop ligand) are shown in gray and black, respectively.
where Up is the difference between the populations of iodine 5pz
orbitals and the 5px and 5py orbitals, and is sometimes referred to
asthe p-electronimbalance, andhn (n = x, y, z) isthe holenumber
of the pn orbitals. The QCC value purely reflects the number of p
electrons. The s electron has no effect on the QCC value because
the electron density is spherically symmetrical. When an I atom
has no π-bonding character, the assumption that hx = hy = 0 is
valid because the px and py electrons only contribute to the π
bond.
::
analysis of the 129I Mossbauer spectrum in the Experi-
mental Section).7 The estimated hp and Up values for
complexes 1 and 2 were almost the same (ca. 0.5),
indicating that these assumptions are valid. However, as
described above, the QCC value for 4 was larger by ca.
400 MHz than that for 1 or 2, despite their similar IS
values. The larger QCC value observed for 4 is probably
Results and Discussion
::
4-
attributable to the alternate stacking of the Pt2(pop)4I2
129I Mossbauer Spectra of the MMX Chain Precursors.
units and the potassium cations along the c axis.11 As
shown in Figure 3, the iodide anions lie on the same ab
planes, whereas they are located between the Pt and K
cations along the c axis. This arrangement of cations and
anions around the iodide ions (i.e., negative charges on
the ab plane and positive charges along the c axis)
apparently increases the EFG at the iodide nucleus. As
a result, a larger QCC was observed for 4, even though the
electron density at the nucleus of iodine and the EFG
derived from the p-electron density imbalance in 4 are
almost the same as those in 1 and 2. The crystal structure
of neutral 2 does not show such regular stacking of
cations and anions.2b From their IS values, the oxidation
states (F) of the iodide ions were estimated to be -0.5. The
larger QCC value of 4 is attributed to the Coulomb
potential from the counter cations and the more-nega-
::
The 129I Mossbauer spectra for the precursors of the
MMX chain complexes Pt2(dta)4I2 (1), Pt2(dtp)4I2 (2),
and K4[Pt2(pop)4I2] (4) at 16 K are shown in Figure 2. The
Mossbauer parameters obtained are summarized in Ta-
ble 1. The three Pt(III) dimer complexes have the same
::
isoelectronic configuration (d7-d7) and lantern-type Pt3+
-
2
(L) I2 molecular structure.4a,2b,11 The differences among
them are the terminal ligands L and their charges. Com-
plexes 1 and 2 are composed of neutral molecules with
4
-
-
CH3CS2 and C2H5CS2 ligands, respectively, whereas
complex 4 is composed of K+ and anionic molecules
2-
[Pt2(pop)4I2]4- with a more negatively charged H2P2O5
ligand.
The best fit for each spectrum was obtained with one
octuplet, indicating that only one chemically independent
iodide site exists in each complex, which corresponds to
each crystal structure.2b,11 The spectra of 1 and 2 were
almost the same, indicating that the dta and dtp ligands
have the same σ-donation effect via Pt ions on the valence
state of iodine. In contrast, the quadrupole splitting in 4
was significantly larger than that in 1 or 2. Nevertheless,
the values for IS for these three complexes are very close
(3.62-3.71 mm s-1).
The valence state of iodine can be estimated from both
the IS and QCC values. If the s and pπ orbitals of iodine
do not contribute to the Pt-I bond, the number of p holes
(hp) estimated from IS and the p-electron imbalance (Up)
estimated from QCC should agree (see the section on the
tively charged pop ligand.
::
129I Mossbauer Spectra of MMX Chain Complexes. The
::
129I Mossbauer spectra of the MMX chain complexes
Pt2(dtp)4I (3), [(C2H5)2NH2]4[Pt2(pop)4I] (5), and
[NH3(CH2)6NH3]2[Pt2(pop)4I] (6) are shown in Figure 4.
::
The Mossbauer parameters obtained for them are sum-
marized in Table 1. In previous works,1a,4e MMX chain
complexes 3, 5, and 6 were found to have different 1-D
charge orderings in their ground states: an ACP state for 3
(ACP:
state for 5 (CP:
[Pt2þ-Pt3þ]-I-[Pt3þ-Pt2þ
]
I
), a CP
3 3 3
3 3 3 3 3 3
[Pt2þ-Pt3þ]-I [Pt2þ-Pt3þ]-I ),
3 3 3
3 3 3 3 3 3
and
a
CDW state for
6
(CDW: -
[Pt2þ
3 3 3
Pt2þ
]
I-[Pt3þ-Pt3þ]-I ). It is clear that the ob-
3 3 3
3 3 3
served spectra for the MMX chain complexes are quite
different. The spectrum of 3 consists of two sets of
octuplets, whereas the spectra of 5 and 6 consist of one
octuplet. The IS and QCC values for 6 are larger by about
(11) Alexander, K. A.; Bryan, S. A.; Fronczek, F. R.; Fultz, W. C.;
Rheingold, A. L.; Roundhill, D. M.; Stein, P.; Watkins, S. F. Inorg. Chem.
1985, 24, 2803–2808.

8048 Inorganic Chemistry, Vol. 48, No. 16, 2009
Kobayashi et al.
more tightly an atom is bound in a crystal, the larger its
recoil-free fraction becomes.12 The fact that the relative
intensity of the Ia site increased with decreasing tempera-
ture is consistent with the procession of the ACP-type
distortion, because the Ia site becomes more rigid at lower
temperatures because of this distortion. Therefore, the
increase in the relative intensity of the Ia site with ACP-
type distortion also supports our assignment discussed
above. From the QCC values of 3, the chain structure and
oxidation states of I are considered to be as follows:
[Pt2þ-Pt3þ]-Ia0.41--[Pt3þ-Pt2þ
]
Ib
The
3 3 3
3 3 3 0.31- 3 3 3
valence state of Pt expressed as a -Pt2þ-Pt3þ- repre-
sents a formal oxidation number. In the isoelectronic
complex Pt2(dta)4I (7), almost the same results for the
Mossbauer parameters were observed. The fact that the
::
0.41-
more-negative Ia
is located between the more-posi-
tive Pt3þ’s suggests that the Coulomb interaction plays an
important role in the ACP electronic state of the dta
system.
It is noteworthy that the intensity ratio of the two
components should be 1.0, because of the Ia/Ib composi-
tional ratio in stoichiometric Pt2(dtp)4I. In the ACP
phase, we assume that the recoil-free fraction of
Ia between the Pt3þ sites is larger than that of Ib between
the Pt2þ sites, because the Pt3þ-Ia distance is shorter than
::
Figure 4. 129I Mossbauer spectra of MMX chain complexes 3 (at 11 K),
5, and 6 (at 15 K).
0.2 mm s-1 and 120 MHz, respectively, than those for 5.
These results should reflect the characters of the ACP,
CP, and CDW states.
the Pt2þ Ib distance. The results were the reverse at
3 3 3
both 11 and 80 K, which might be attributable to
the competition between the ACP- and CDW-type dis-
tortions or the incomplete 3-D ordering of the ACP
phase. It is well-known that Peierls- or spin-Peierls-type
lattice distortions cannot be ordered three dimensionally,
even at 0 K, in a pure 1-D electronic system. In fact,
complex 3 has a nearest-neighbor interchain distance
According to the X-ray crystal structural analysis by
Mitsumi et al.,4e complex 3 is in the ACP phase at 48 K,
where two chemically independent iodine sites are situ-
ated between the Pt3þ sites and between the Pt2þ sites.
Of the four kinds of charge-ordering phases, only the
ACP phase has two chemically independent iodine sites.
Therefore, the observed spectrum for 3 is consistent with
the X-ray crystal structure study. These two chemically
independent iodine sites are denoted Ia and Ib in Figure 4
and Table 1. Because the QCC value of the Ia site in 3
is closer to that in the Pt(III) dimer complex 1 or 2, the
˚
about 1.4 A longer than that of 7, which has a shorter
alkyl-chain ligand. It is possible that the ACP-type lattice
distortion was still not completely ordered in three di-
mensions at 11 K.
Conversely, the observed spectra for the complexes of
the pop system, 5 and 6, consisted of one octuplet,
indicating that only one iodine site exists in these com-
plexes. The obtained spectra for 5 and 6 around 50 K were
the same as the spectra at 16 K. Above 80 K, owing to a
very low recoil-free fraction, the spectra became very
weak and finally meaningless. On the basis of the mag-
netic susceptibility and Raman spectra for these com-
plexes reported by Matsuzaki et al.,1a the charge-ordering
states of complexes 5 and 6 are considered to be CP and
CDW, respectively. Because these phases contain only
one iodine site, located between the Pt3þ and Pt2þ ions,
the observed temperature-independent spectra consisting
of one octuplet are consistent with previous reports. It is
noteworthy that the valence states of iodine in 5 and 6 are
significantly different. The oxidation state F of iodine in
the CP complex 5 is estimated from the IS values to be
-0.43(4), which is lower than that for the CDW complex
6 (-0.33(5)). X-ray structural studies of complexes 5 and
two sites Ia and Ib can be assigned as follows:
]
[Pt2þ
-
-
3 3 3
Pt3þ]-Ia-[Pt3þ-Pt2þ
Ib [Pt2þ-Pt3þ]-Ia-[Pt3þ
3 3 3 3 3 3
Pt2þ
]
Ib This assignment is consistent with our pre-
3 3 3 3 3 3
vious work.2a
Complex 3 exhibits high electrical conductivity (∼5 S
cm-1) and a metal-insulator transition at 205 K. Its
electronic state changes with the temperature: the AV
state with a CDW fluctuation above 205 K, the CP state at
160-205 K, and the ACP state below 160 K.2a-e,4e
Especially below 110 K, the 2-fold diffuse scatterings at
k = n þ 0.5 (k corresponds to the 1-D chain axis; n is an
integer) gradually varied to superlattice Bragg spots,2b
which indicates the gradual distortion of the ACP-type
[Pt2þ-Pt3þ]-Ia-[Pt3þ-Pt2þ
]
Ib
with decreas-
3 3 3
3 3 3 3 3 3
ing temperature and the 3-D ordering of the ACP state at
48 K. This distortion is similar to a spin-Peierls-like
distortion. Comparing the relative integral intensities
(areas) of Ia and Ib at 11 K with those at 80 K, the area
of Ia (0.85(1)) at 11 K was larger than that (0.79(2)) at
80 K. It is well-known that the area intensity relates to the
recoil-free fraction at the nucleus. Roughly speaking, the
1a
˚
6 revealed that the Pt-I bond distance is 2.718 A in 5,
which is slightly shorter than that in the Pt(III) dimer
11
˚
complex 4 (2.746 A), whereas that in 6 is markedly
longer by about 0.2 A (2.922 A).1a From the viewpoint of
ligand field theory, the short Pt-I bond should destabi-
lize the 5dz2 orbital of Pt3þ and stabilize the 5pz orbital of
I-. In contrast, the overlap integral between these orbitals
˚
˚
(12) Viegers, T. P. A.; Trooster, J. M.; Brouten, P.; Rit, T. P. J. Chem.
Soc., Dalton Trans. 1977, 2074.

Article
Inorganic Chemistry, Vol. 48, No. 16, 2009 8049
(complexes 1, 2, 3, and 7). The QCC value reflects the
electric field gradient at the iodine nucleus, which includes
the contribution not only from the 5p electron imbalance
but also from the charge on the ions surrounding the
iodine. Although a few data points in the IS-QCC
correlation diagram give conclusive information, the fact
that larger absolute QCC values were observed for the
pop system implies that the positive charge on the coun-
tercation and the more-negatively charged terminal li-
gand significantly affect the electronic state of the 1-D
MMX chain. In other words, the differences in the
electronic states of the highly conductive dta system and
the insulating pop system originate from the difference in
the Coulomb interaction in the 1-D chains, which are
indirectly affected by the terminal ligands or counterions.
Conclusion
::
Figure 5. IS-QCC correlation diagram for complexes 1-6 and Pt2-
(dta)4I (7).2a Solid and dashed lines represent guides for the pop and dta
systems, respectively.
129I Mossbauer spectroscopy was applied to two different
kinds of MMX chain systems and Pt(III) dimers. The almost
identical IS values for the dimers Pt2(dta)4I2 (1), Pt2(dtp)4I2
(2), and K4[Pt2(pop)4I2] (4) indicate that the valence state of
iodine (I0.5-) does not depend on the terminal ligand. The
QCC value for 4 was larger than those for 1 and 2, which is
attributed to the anisotropic arrangement of the iodide ions,
should be larger for the shorter Pt-I bond. Thus, if the
Coulomb interaction is more dominant than the covalent
bond interaction, the more-reduced iodine (i.e., the iodide
ion) connects to the Pt3þ ion. The fact that iodine is more
reduced in the CP state of 5 (I0.43-) than in the CDW state
of 6 (I0.33-) suggests that the Coulomb interaction plays a
more important role in the CP phase. In contrast, the
covalent bond interaction is more dominant in the CDW
phase.
Effect of the Terminal Ligand of the MMX Chain
System. As described in the Introduction, all MMX chain
complexes can be classified into two systems, the pop and
dta systems, according to their terminal ligands. These
two systems exhibit significantly different physical prop-
erties, although they have the same electronic configura-
tions (d7-d8) in the Pt dimer unit. In this section, the
difference in the electronic states of the dta and pop
systems is discussed. Figure 5 shows an IS-QCC correla-
tion diagram for complexes 1-6 and Pt2(dta)4I (7), which
is the first metallic MMX chain complex.2a It is well-
known that a linear correlation exists between the IS and
QCC values for the series of the iodide anion (I-), iodine
(I0), and the iodine cation (Iþ).7 Iodide ions in the MMX
chain systems were also found to follow this linear IS-
QCC correlation. However, when analyzed in more de-
tail, the dta and pop systems seem to follow different IS-
QCC linear correlations (see the guides shown as dashed
and solid lines in Figure 5). The IS values depend only on
the coordination environment of iodine and depend
negligibly on the terminal ligand. The IS values for the
which form a layer lying on the ab plane in the crystal. The
::
Mossbauer spectra observed for the three MMX chain
complexes Pt2(dtp)4I (3), [(C2H5)2NH2]4[Pt2(pop)4I] (5),
and [H3N(CH2)6NH3]2[Pt2(pop)4I] (6) reflect the characters
of their charge-ordering phases: ACP (ACP:
[Pt2þ
-
-
3 3 3
Pt3þ]-Ia0.4--[Pt3þ-Pt2þ
]
Ib
) for 3; CP (CP:
0.3-
3 3 3
3 3 3
3 3 3
[Pt2þ-Pt3þ]-Ia
[Pt2þ-Pt3þ]-I0.4- ) for 5; and
0.4-
3 3 3
3 3 3
CDW phase (CDW:
[Pt2þ-Pt2þ]-I0.3--[Pt3þ-Pt3þ]-
3 3 3
::
I0.3- ) for 6. The Mossbauer parameters for 3, especially
3 3 3
the fact that the more-negative Ia0.41- is located between the
more-positive Pt3þ’s, suggest that the Coulomb interaction
plays an important role in the ACP electronic state of the dta
system. Conversely, the oxidation states estimated for iodine
in 5 (I0.4-) and 6 (I0.3-) suggest that the Coulomb interaction
predominates in the CP phase and that the covalent bond
interaction predominates in the CDW phase. The fact that
larger absolute QCC values were observed for the pop system
relative to those for the dta system, despite their similar IS
values, implies that the Madelung potential given by the
more-negative terminal pop ligand and the counter cations is
responsiblefor thedifferences inthephysicalproperties of the
dta and pop systems.
Acknowledgment. This work was partly supported by
Grant-in-Aid for Scientific Research for Priority Areas
(Chemistry of Coordination Space) no. 16074212 and the
Joint Project for Chemical Synthesis Core Research In-
stitutions from the Ministry of Education, Culture,
Sports, Science and Technology, Japan, and by Grants-
in-Aid for Scientific Research (B) no. 14340215 and no.
16350033 from the Japan Society for the Promotion of
Science, and by Kurata Memorial Hitachi Science.
terminal iodine in the discrete Pt(III) dimer (I-Pt3þ
-
Pt3þ-I) are around 3.7 mm s-1, and those for the brid-
ging iodine in the MMX chain complexes are about 0.1-
0.4 mm s-1 higher. Conversely, the QCC values seem to
depend on the terminal ligands. The QCC values for the
pop system (complexes 4, 5, and 6) are markedly larger,
by about 300 MHz, than those for the dta system