The new HPF5- salt, N(CH3)(4)HPF5, was prepared and the infrared and Raman spectra of N(CH3)(4)HPF5 and CsHPF5 were recorded. The spectra were assigned with the help of ab initio molecular orbital and local density functional calculations, and a normal coordinate analysis was carried out. For comparison, the unknown isoelectronic molecule HSF5 was also calculated by the same methods. The internal stretching force constants of HPF5- are compared to those of closely related phosphorus and sulfur fluorides and hydrides and confirm the existence of a cis-effect in these hydrogen-substituted main group hexafluorides. The observed substitution effects are explained in terms of a hypervalent bonding scheme and result in a preferential weakening of the four equatorial cis-bonds.