The single-crystal structures have been determined for the Cd(CN), host clathrates Cd( CN),-xBu",O-yH,O (x, y z 0.5) 1, Cd(CN),~0.5Bui,0 2a, Cd(CN),~0.5(Pr'CH,CH,),O 2b, Cd(CN),-Pr'CI 20.Cd(CN),-CHCI,CH,CI 3a. Cd(CN),-Pr'Br 3b and Cd(CN),-Pr'CN 4, prepared in order to mimic the polymorphism of SiO, by Cd(CN),. The hexagonal PGJmmc host lattice of 1 is isostructural with the high-temperature form of tridymite, accommodating the dibutyl ether guest in the channel cavity extending along the c axis and the watei molecule is hydrogen bonded to the ether in the cage neighbouring the channel. The cubic Fd3m host lattice of 2 in the high-temperature cristobalite structure provides two neighbouring tetrahedral cayities for the respective alkyl ether guests in 2a and 2b; the structure of 20 is the same. The cubic Fd3m lattice is transformed into the tetragonal P4,2,2 one in 3 and 4, similar to the deformation from high-to low-temperature cristobalite, the lattice of 4 ( Z = 8) being more distorted than those of 3a and 3b (Z = 4). The polymorphic behaviour of these Cd(CN), lattices is discussed in terms of the geometry and function of the guest molecules.The Cd(CN),-G clathrates that we have reported previously have a three-dimensional host lattice of space group F&rn isomorphous to the high-temperature form of cristobalite (Hcristobalite). Although the Cd(CN), host provides a cavity approximating to a regular tetrahedron for each guest G, the accommodated guests exhibit not only tetrahedral or pseudotetrahedral symmetry, e.g. CMe,-,Cl, ( n = 0 4 ) , but also molecular symmetries considerably lower than Td, e.g.

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Charge-transfer salts of branched-alkyl biferrocenes, (1',1' ''-R2-1,1' '-biferrocene)[Ni(mnt)2] (1a, R = isopropyl; 2a, R = dineopentyl) and (1',1' ''-R2-1,1' '-biferrocene)2[Co(mnt)2]2 (1b, R = isopropyl; 2b, R = dineopentyl), were prepared. Their valence states were investigated using X-ray crystallography and Mössbauer spectroscopy. Complexes 1a and 1b show segregated-stack crystal structures that contain columns of acceptors, whereas structures of 2a and 2b, which contain bulky donors, are rather discrete. All of the complexes contain mixed-valent biferrocenium monocations. A two-step valence transition was found in complex 1a. The crystal contains two crystallographically independent cations: one undergoes valence localization below room temperature; the other undergoes valence localization below ca. 130 K. The former transition is derived from asymmetry of the crystal environment around the cation, whereas the latter one is caused by symmetry lowering coupled with a spin-Peierls transition (T(C) = 133.2 K) associated with the dimerization of the acceptors. This compound was found to exhibit a dielectric response based on valence tautomerization. Other complexes (1b, 2a, and 2b) show a valence-trapped state. In all complexes, charge localization was found to occur through local electrostatic interactions between the donor's cationic moiety and the acceptor's electronegative moieties.

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