Crystal Structures and Properties of MMX-Chain Compounds Based on Dithiocarboxylato-Bridged Dinuclear Complexes - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 184

unpaired electrons on the adjacent Ni 3+ sites are expected to take a singlet state due
to the strong antiferromagnetic (AF) coupling through a bridging iodine atom.
However, as will be described in the Magnetic Properties section, the magnetic
data of 9 can be fitted as an S ¼

1/2 1D AF Heisenberg model. Therefore, the
compound 9 at RT phase can be assigned to the valence-ordered state close to the
AV state. Adjacent Ni 2 (CS 2 ) 4 units of Ni1-Ni2 and Ni3-Ni3 0 are twisted by ca .11
from the eclipsed arrangement. The n -propyl groups of the dithiobutanato ligands in
the Ni1-Ni2 unit have the anti form, whereas those of the Ni3-Ni3 0 unit take the
gauche form. Therefore, the origin of threefold periodic structure cannot be
attributed to the valence ordering of the nickel atoms, but to both the twist of the
adjacent dinickel units and the difference in the conformation of the ligands.

LT Phase of [Ni 2 ( n -PrCS 2 ) 4 I] 1 (9)

The space group of 9 changed from C 2/ m in the RT phase to C 2 in the LT phase
with the first-order phase transition [ 38 ]. Crystal structure of 9 in the LT phase at
140 K is shown in Fig. 9.33 . The opposite twist of NiS 4 planes between the adjacent
dinickel units in the 1D chain and the difference in conformation of the ligands
observed at RT phase have disappeared in the LT phase, and the periodicity of
the crystal lattice in the 1D chain direction changes from threefold of a -Ni-Ni-I-
period in the RT phase to onefold in the LT phase. The structure consists of three
crystallographically independent dinickel units; two of them lie on the crystallo-
graphic twofold axis parallel to the b axis. The Ni-Ni distances are Ni1-Ni2

¼ 2.5404

2.5388(8) ˚ . The Ni-I distances in each

(11), Ni3-Ni4 ¼ 2.5362(11), and Ni5-Ni6 ¼

2.8994(8) ˚ ,Ni3-I2

2.9235(8) and Ni2-I1 0 ¼ dinickel unit are Ni1-I1 ¼ ¼ 2.9187

2.9152(7) ˚ ,
respectively. Taking into account the distinct difference of Ni-I distances, the valence-
ordered state of 9 at LT phase should be assigned to a charge-polarization (CP) state of
-Ni (2.5 d )+ -Ni (2.5+ d )+ -I -Ni (2.5 d )+ -Ni (2.5+ d )+ -Ni (2.5 d )+ -I -(

2.9084(9) ˚ ,Ni5-I3 (9) and Ni4-I2 0 ¼ 2.9108(6) and Ni6-I3 0 ¼ ¼

d 0.5) close to the

AV state, similarly to 8 in RT phase.

9.3.3 X-Ray Photoelectron Spectra

The Ni 2p 3/2 and 2p 1/2 core level spectra for the mixed-valence compounds 8-10 are
shown in Fig. 9.34 [ 38 ]. Binding energies of the Ni 2p 3/2 and 2p 1/2 core level are
summarized in Table 9.5 , together with those of the Ni 2p 3/2 core level for
the compound 7 [ 82 ]. The Ni 2p 3/2 and 2p 1/2 peaks of 8-10 were broad compared
to those of the corresponding Ni 2+ -Ni 2+ compounds, [Ni 2 (RCS 2 ) 4 ](R

Et, n -Pr, n -
Bu), and could be resolved into Ni 2+ 2p 3/2,1/2 and Ni 3+ 2p 3/2,1/2 doublets. The results
have revealed that 8-10 exist in the mixed-valence state composed of Ni 2+ and Ni 3+
on the time scale of XPS spectroscopy (ca . 10 17 s). Although the peak area ratio of
Ni 2+ 2p 3/2,1/2 and Ni 3+ 2p 3/2,1/2 doublets of 8-10 should be equal because of the

¼