Mixed tetrahaloborate ions. Detection and study by nuclear magnetic resonance

Article abstract of DOI:10.1021/ic50111a005

BF₄^-, BCl₄^-, BBr₄^-, and BI₄^- undergo halogen exchange in methylene chloride solution to form the mixed tetrahaloborate ions, for which ¹⁹F and ¹¹B nmr data are presented. A previous report of very rapid halogen exchange in the BF₄^--BCl₄^- system is incorrect. Only in the BF₄^--BI₄^- system is halogen redistribution so rapid that separate nmr signals are not observed for the mixed-halogen anions. Exchange with methylene halide solvents gives rise to ternary-halogen species such as BF₂ClBr^-. Additional methods of preparation of mixed tetrahaloborate anions are investigated. The nmr parameters of the mixed tetrahaloborate anions show trends which resemble those of the mixed boron trihalides. The trends can be interpreted in terms of the presence of boron-fluorine π bonding. The results of CNDO/2 calculations provide an alternative interpretation of the trends in chemical shifts. The ¹¹B and F¹⁹ chemical shifts of the tetrahaloborate anions are found to fit Malinowski's criteria of pairwise additivity. .