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This review is an extension of the talk given by the author at the Annual Chemical Congress, York, 1975. Both the talk and the review are based on an article published by the author in Endeavour (1975,34, 123) and he thanks the Editor for permission to use the material. D. L. Stockman, in 'Current Problems in Electrophotography', ed.

An X-ray crystallographic investigation of the complex between the diquat dication and dibenzo-30-crown-I 0, rDiquat.DB30CI 01 [PF,],, reveals gross structural features for host-guest complexes (I) and (11) in which weak [C-H . * -01 hydrogen bonding and charge transfer are the major components of host-guest binding;these same components are also responsible for the formation of stable and ordered 1 : 1 solution complexes between dibenzo-3n-crown-n (n = 9-1 2) and diquat bis(hexaf1uorophosphate) (2).

The crystal structure of [1,1′‐bis(p‐fluorophenyl)‐4,4′‐bipyridinium]2+(7,7,8,8‐tetracyano‐p‐quinodimethanide) 22−,(FΦBP) (TCNQ)2, its electrical conductivity and the properties of 51 related simple salts are reported. (FΦBP) (TCNQ)2s triclinic, space group PI, with a = 7.112(7), b = = 13.702(11), c = 12.425(9) Å, α = 115.6(1)°, β = 85.6(1)°, γ = 105.3(1)°, U = 1053 Å3Dm = 1.21 Mg m−3, Z = 1, Dc 1.19 Mg m−3. Within the stoichiometric unit the TCNQ's overlap in an exocyclic bond to quinonoid ring manner with a mean intermolecular spacing of 3.14 A. In the direction of stacking the TCNQ diads are separated by the FΦBP dications and consequently the electrical conductivity, Órt = 6 × 10−9 S cm−1, of (FΦBP) (TCNQ)2 is low compared with values obtained for the congeneric pyridinium and bipyridinium TCNQ simple salts. These fall in the range 10−10 to 60 S cm−1 and eight salts have Órt <1 S cm−1.