Metal halides in the fused and vapour states

A complete picture of the structural chemistry of a compound would require a knowledge of its structure in the solid, liquid, and gaseous states. The amount of information obtainable about the structure of a liquid halide is very limited, and few X-ray studies have been made. (Examples include Snl4, InClg, and Cdl2. ) We are therefore obliged to make direct comparisons of crystal and vapour. Strictly, these comparisons relate only to the process of sublimation, and if the compound is polymorphic the relevant crystal structure is that of the polymorph stable at the temperature of sublimation. 

The structures of many halide molecules have been studied in the vapour state. 

MXs trigonal bipyramidal Nb, Ta, As, and Sb pentahalides (Tables 9.25 and 20.3) MXg octahedral MoFg, WClg, TeFg, and Table 9.25. 

Estimates of the bond angles include CaF2 (140°), SrF2 (108°), and BaF2 (100°). 

The relation between structure in the crystalline and vapour states is simplest in the case of a molecular crystal which vaporizes to molecules of the same kind as those in the crystal, a process which merely involves the separation of molecules against the van der Waals forces with very little change in the internal structure of the molecule. The molecule may be mononuclear (Snl4, WClg) or, rarely, polymeric (Al2Brg). In all other cases there is breakdown of more extensive metal-halogen systems, either directly to MX molecules, or in some instances to polymeric species which dissociate to monomers at higher temperatures. The vaporization is accompanied by a reduction in the coordination number of the metal. 

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