Pd(III) Mott–Hubbard Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 38

Fig. 4.1 ORTEP drawings of [Pd(en) 2 Br](C 5 -Y) 2 H 2 Oat(a) 259 K and (b) 162 K. Water

molecules and hydrogen atoms are omitted for clarity. White : Pd, Brown : Br, Blue :N, Yellow :S,
Red :O, Gray :C

respectively. Therefore, this indicates that [Pd(en) 2 Br](C 5 -Y) 2 H 2 Oisina

3D-ordered CDW state at this tem-

perature. At 162 K, on the other hand, the twofold periodicity disappeared, and all
of the Pd ions became equivalent. The Pd-Br bond lengths were 2.613(1) and 2.604
(1) ˚ , which are intermediate between Pd(II)-Br and Pd(IV)-Br bond lengths,
suggesting that the complex is in an MH state [-Pd(III)-Br-Pd(III)-Br-Pd(III)-]
at 162 K.
The temperature dependence of the average neighboring Pd

ð Pd(II) Br Pd(IV) Br Pd(II) Þ

Pd distance along

the chain is shown in Fig. 4.2 . The average Pd Pd distance in [Pd(en) 2 Br]

H 2 O at 293 K was 5.31 ˚ . This distance is longer than that in [Pd
(chxn) 2 Br]Br 2 , which has the shortest Pd

(C 5 -Y) 2

Pd distance so far reported and is in
a CDW state [ 2 ]. Upon cooling, this distance decreased by approximately 2 % and
became 5.21 ˚ at 110 K, while that of [Pd(chxn) 2 Br]Br 2 showed almost no
temperature dependence. The large temperature dependence in the present complex
is considered to be due to suppression of the thermal motion of the alkyl chains. In
addition, we found a discontinuity in the Pd

Pd distance with a small hysteresis at
205 K, indicating the first-order phase transition occurs at this temperature.