Tetrafluorophosphite, PF4-, anion

Article abstract of DOI:10.1021/ja00086a018

N(CH₃)₄⁺PF₄^-, the first example of a PF₄^- salt, has been prepared from N(CH₃)₄F and PF₃ using either CH₃CN, CHF₃, or excess PF₃ as a solvent. The salt is a white, crystalline solid which is thermally stable up to 150 °C where it decomposes to N(CH₃)₃, CH₃F, and PF₃. The structure of the PF₄^- anion was studied by variable temperature ³¹P and ¹⁹F NMR spectroscopy, infrared and Raman spectroscopy, SCF, MP2, local and nonlocal density functional calculations, a normal coordinate analysis, and single-crystal X-ray diffraction. The anion possesses a pseudo trigonal bipyramidal structure with two longer axial bonds and an equatorial plane containing two shorter equatorial bonds and a sterically active free valence electron pair. In solution, it undergoes an intramolecular exchange process by the way of a Berry pseudorotation mechanism. The vibrational frequencies observed for PF₄^- in solid N(CH₃)₄PF₄ are in excellent agreement with those calculated for free gaseous PF₄^-. The X-ray diffraction study tetragonal, space group P4?2₁m, a = 8.465(3) A?, c = 5.674(2) A?, Z = 2, R = 0.0723 for 268 observed [F ≥ σ(F)] reflections suffers from a 3-fold disorder with unequal occupancy factors for the equatorial fluorine atoms of PF₄^- but confirms its pseudo trigonal bipyramidal structure and the axial P-F bond length calculated for the free ion.