Aluminum trichloride crystal structure

There seems to be even less structural similarity for many other metal halides as the crystalline systems are compared with the molecules in the vapor phase. Aluminum trichloride, e.g., crystallizes in a hexagonal layer structure. Upon melting, and then, upon evaporation at relatively low temperatures, dimeric molecules are formed. At higher temperatures they dissociate into monomers (Figure 9-58) [107], The coordination number decreases from 6 to 4 and then to 3 in this process. However, at closer scrutiny, even the dimeric aluminum trichloride molecules can be derived from the crystal structure. Figure 9-59 shows another representation of crystalline aluminum trichloride which facilitates the identification of the dimeric units. A further example is chromium dichloride illustrated in Figure 9-60. The small oligomers in its vapor have structures [108] that are closely related to the solid structure [109], Correlation between the molecular composition of the vapor and their source crystal has been established for some metal halides [110],... 

Regitz and co-workers have reported a spirocyclotrimerization of a stable phosphaalkyne with aluminum trichloride in a 3 1 molar ratio (Scheme 3) <92AG(E)1055>. The reaction gives a betaine with the incorporation of the Lewis acid and its structure, bearing a spiranic diphosphirene, was confirmed by x-ray crystal structure analysis (cf (4e), Table 1). 

There seems to be even less structural similarity for many other metal halides when the crystalline systems are compared with the molecules in the vapor phase. Aluminum trichloride, for example, crystallizes in a hexagonal layer structure. Upon melting and then evaporation at relatively low temperatures, dimeric molecules are formed. At higher temperatures, they dissociate into monomers (Figure 9-60) [9-60]. The coordination number decreases from six to four and then to three in this process. 

Under closer scrutiny, even the dimeric aluminum trichloride molecules can be derived from the crystal structure. Figure 9-61 shows another representation of crystalline aluminum trichloride which facilitates the identification of the dimeric units. Correlation between the molecular composition of the vapor and the source crystal has been established for some metal halides [9-62]. 

Figure 9-61. The crystal structure of aluminum trichloride, after Ref. f9-61]. The dimeric unit with a four-niembered ring is discernible. Copyright (1993) John Wiley Sons. Used by permission.

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