Tetrafluoroborate

Tetrafluoroborate is the anion BF₄⁻. This tetrahedral species is isoelectronic with tetrafluoromethane, CF₄ and tetrafluoroammonium NF₄⁺, and is valence isoelectronic with many stable and important species including the closely related anion perchlorate, ClO₄⁻. It arises by the reaction of fluoride salts with the Lewis acid BF₃, treatment of tetrafluoroboric acid with base, or by treatment of boric acid with hydrofluoric acid.

The popularization of BF₄⁻ has led to decreased use of ClO₄⁻ in the laboratory. With organic compounds, especially amine derivatives, ClO₄⁻ forms potentially explosive derivatives. One disadvantage to BF₄⁻ is its slight sensitivity to hydrolysis, whereas ClO₄⁻ does not suffer from this problem. Safety considerations, however, overshadow this inconvenience.

The utility of BF₄⁻ arises because its salts are often more soluble in organic solvents than the related nitrate or halide salts. Furthermore, BF₄⁻ is less nucleophilic and basic than nitrates and halides. Thus, when using salts of BF₄⁻, one can usually assume that the cation is the reactive agent and this tetrahedral anion is inert. BF₄⁻ owes its inertness to two factors: (i) it is symmetrical so that the negative charge is distributed equally over several (four) atoms, and (ii) it is composed of highly electronegative fluorine atoms, which diminish the basicity of the anion. Related to BF₄⁻ is hexafluorophosphate, PF₆⁻, which is even more stable toward hydrolysis and whose salts tend to be more lipophilic.

Illustrative of a fluoroborate salt is [Ni(CH₃CH₂OH)₆](BF₄)₂, a kinetically labile octahedral complex, which is used as a source of Ni²⁺.

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