POP-Type MMX-Chain Compounds with Binary Countercations and Vapochromism - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 211

Fig. 10.7 Schematic representation of the chain structure of MMX chains with unitary ( left ) and
binary ( right ) countercations

The typical MMX chains with unitary countercation, whose countercation links
only intra-chain [Pt 2 (pop) 4 ] units, are in “alternate” packing (Fig. 10.7 left) [ 54 ]. On
the other hand, in all MMX chains with binary countercations, A + links not only
intra-chain but also inter-chain [Pt 2 (pop) 4 ] units by A þ

O coordination bonds,

and B 2+ links inter-chain [Pt 2 (pop) 4 ] units by N þ O hydrogen bonds. These

hydrogen bonds and coordination bond networks constrain each [Pt 2 (pop) 4 ] unit in
same ab plane, that is in “synchronized” packing (Fig. 10.7 right). The example of
hydrogen bond and coordination bond networks in K 2 (NC 5 N)[Pt 2 (pop) 4 I]

H

4H 2 Ois
shown in Fig. 10.8 . Because of the “synchronized” packing, the layer-like structure
appeared perpendicular to c -axis (chain axis). The larger anion and cation,
[Pt 2 (pop) 4 ] 3 unit and B 2+ , exist in the same ab plane. Formal charge in this ab
plane is 1 per formula, therefore, the layer is densely packed anionic layer. On the

other hand, loosely packed cationic layer consists of smaller anion and cation, I
and two A + , per formula. The vacancy in loosely packed cationic layer is filled with
H 2 O molecules, which coordinate to A + ions. These H 2 O molecules and lattice
fixed by hydrogen bond and coordination bond networks play an important role in
dehydration-rehydration of the MMX chains discussed later.
The other important structural feature is the short d (Pt-I-Pt). The reason of the
short d (Pt-I-Pt) can be explained as follows:
The hydrogen bonds between pop ligands and -NH 3 + of countercations link intra-
chain [Pt 2 (pop) 4 ] units in typical MMX chains with unitary countercation. On the
other hand, coordination bonds between pop ligands and A + ions link intra-chain (and
also inter-chain) [Pt 2 (pop) 4 ] units in MMX chains with binary countercations.
Although the distances between nitrogen atom of -NH 3 + and O(pop) are almost
comparable to those between A +

and O(pop) ( d (N-O) d (A-O)) [ 31 ], the

minimum values of coordination bond angle ( (O-A-O)) are smaller than those

∠

of hydrogen bond angles ( (O-N-O)), resulting in the shorter d (Pt-I-Pt) in MMX

chains with binary countercations. This difference originates from the isotropic

∠