Cation-Radical Salt counterions, structure

Fig. 4. Structure of the cation-radical salt of N,N-di-pheny1-p-pheny-lene diamine with I -counterions (black dots) hydrogen bonds are indicated by dotted lines cf. ref. 22.

Several crystal structure determinations of [AuX2l salts with different cations have been carried out for [AuC12],1823,1904,2854,3065-3068 [AuBr2],3065,3069,3070 or [AuF]-.3065 3066 3071-3074 Many other structural determinations have been reported in which [AuX2]- salts act as counterions of conducting or superconducting ion radical salts as bis-ethylenedithiotetrathiafulvalene (ET) and related organic donors. 

There is a wealth of crystal lattice types of interest in this review. We can differentiate between solid CT complexes and ion-radical salts. The former are the solid-state equivalents of the Mulliken solution CT complexes, and are the "two-chain" crystals. The latter are the so-called "one-chain" compounds, or ion-radical salts, where the organic cation (anion) crystallizes with inorganic anions (cations) as counterions. The crystal structure types have been reviewed by Herbstein [61], Tanaka [62], Soos [63-65] and many others. TaWes 1 and 2 update a classification introduced by Soos [63] and modified later by Wiygul et al. [66]. A few examples are shown diagrammatically in Fig. 4. The IS lattices are the ion-radical salts the IM lattices are the CT crystals, the crystal equivalents of the solution CT complexes. The 2S lattices are the first organic metals found (TTF-TCNQ). 

A typical structural feature of the crystalline radical-cation salts is the formation of stacks. Thereby, the hydrocarbon units are packed in columns leaving space for channels in which the closed-shell counterions are placed. The planar hydrocarbons are oriented perpendicularly to the stacking axes with interplanar distances between 0.32-0.33 nm which are significantly smaller than the van-der-Waals radii of the carbon centers and allow for strong w-orbital overlap. It is also characteristic that the molecules are alternately rotated by 180°, as e.g. in the fluoranthene salt [17,18]. Closed inspection shows also that a particularly strong interaction arises between r-centers which in the corresponding radical cation possess a high local spin density. 

As noted before, radical cation salts are always associated with an inert counterion. It plays a crucial role in the properties [331]. Indeed, the structural organizations and their instabilities [335] depend on the size and the shape, the electrical charge and, eventually, the magnetism of the counterions present. These salts are usually binary or even ternary compounds when either a neutral organic solvent or water are involved. They are defined by the general formula... 

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