Tetrahaloborate adducts

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). 

In boron trihalide adducts and tetrahaloborate ions, halogen redistribution equilibria are reasonably close to this ideal random case when chlorine, bromine, and iodine are involved (27, 28, 80, 100, 112), as are equilibria in the uncomplexed heavier boron trihalides (111). 

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