Tetrafluoroborate and Hexafluorophosphate Ions are not Interchangeable: A Density Functional Theory Comparison of Hydrogen Bonding

Abstract Due to their similarities in charge and composition, the tetrafluoroborate (BF 4 − ) and hexafluorophosphate (PF 6 − ) ions are often used interchangeably in chemical studies. As a result, the ability of BF 4 − , but not PF 6 − , to inhibit the hydrolysis of alkyl thiosulfates in non‐aqueous solution was surprising. This divergence in reactivity may reflect differences in the ability of these ions to form strong hydrogen bonds to hydronium ion, which catalyzes the hydrolysis, and prompted the current computational studies. Calculations using density functional theory (DFT) gave atomic partial charges, occupation numbers, and energies relevant to hydrogen bonding of these anions. Topological analysis using the Atoms‐in‐Molecule (AIM) method elucidated the nature of the interactions, and natural‐bonding‐orbital (NBO) analysis provided insight on the strength of hydrogen bonding within the different tetrafluoroborate and hexafluorophosphate complexes. The calculated second‐order perturbation energies (E(2)) revealed that BF 4 − and H 3 O + formed stronger hydrogen bonds than did PF 6 − and H 3 O + , suggesting that clusters formed by the former pair are more stable.