Preparation, Detection, and Properties of Mixed Boron Trihalide Adducts

Preparation, Detection, and Properties of Mixed Boron Trihalide Adducts 

The following methods have been used to prepare the mixed boron trihalide adducts D BX Y3 for in situ study by spectroscopic methods. 

If the donor-acceptor bond is weak, two different boron trihalide adducts of the same donor will exchange halogen to form the mixed-halogen adducts. The weaker the donor-acceptor bond, the more rapidly the mixed adducts form, indicating that free boron trihalide formed by dissociation is an active species in halogen exchange. 

Reaction of a donor molecule with a previously equilibrated mixture of free boron trihalides gives an initial adduct mixture corresponding to the equilibrium mixture of the free boron trihalides. This method should be suitable for all mixtures except BF3/BI3 (as noted previously). However, equilibria in the adducts can be quite different from the corresponding equilibria in the free boron trihalides. If halogen redistribution is fast and if the mixed adducts are discriminated against, then this method does not succeed (28). 

Halide ion Y in the form of soluble tetraalkylammonium salts can in some cases displace halide ion X forming a mixed-halogen adduct. However, the method has met with variable success. It is successful with tetrahaloborate ions (80) and with Me3N BI3 (9). A similar reaction with insoluble silver halides or pseudohalides has also given mixed adducts (43). 

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