Pentagonal Planar AX ₅ Species:  Synthesis and Characterization of the Iodine(III) Pentafluoride Dianion, IF ₅ ^2-

Abstract

The new IF 5 2 - dianion, which is only the second known example of a pentagonal planar AX 5 species, was prepared as its N(CH 3 ) 4 + salt from N(CH 3 ) 4 IF 4 and N(CH 3 ) 4 F in CH 3 CN solution. Its structure was established by infrared and Raman spectroscopy, ab initio calculations, and a comparison to isoelectronic XeF 5 - . Furthermore, vibrational spectroscopy and X-ray powder diffraction data show that the previously reported composition “Cs 3 IF 6 ” is actually a mixture of Cs 2 IF 5 and CsF. Ab inito calculations also show that the most probable geometries for the free IF 6 3 - ion are vibrationally unstable and undergo spontaneous F - ion loss with formation of either pentagonal planar IF 5 2 - or square planar IF 4 - . The synthesis and some properties of the new N(CH 3 ) 4 IF 4 salt and a revised normal coordinate analysis of XeF 5 - are also presented.