Equilibria in Halogen Redistribution

Random or statistical redistribution of substituents X and Y, present in equal amounts about a central atom E in a system E(X, Y) , gives the following amounts of species (127)  

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). 

Redistribution of these halogens is near-random about a number of other central atoms as well (97, 119). 

However, equilibria may be far from random when fluorine is involved. Table I shows the effects of the donor on disproportionation constants Kl and Kz (127) for redistribution of fluorine and chlorine  

Practically all deviations from random redistribution are in the direction of less than the statistical amounts of mixed-halogen species. Deviations are especially pronounced with the sulfur-donor adducts, with uncomplexed BFnCl3 being intermediate between the oxygen-donor and sulfur-donor systems. Constant K1 is consistently larger 

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