Pentagonal Planar AX₅Species:  Synthesis and Characterization of the Iodine(III) Pentafluoride Dianion, IF₅^2-

The new IF52- dianion, which is only the second known example of a pentagonal planar AX(5) species, was prepared as its N(CH3)(4)(+) salt from N(CH3)(4)LF4 and N(CH3)(4)F in CH3CN solution. Its structure was established by infrared and Raman spectroscopy, ab initio calculations, and a comparison to isoelectronic XeF5-. Furthermore, vibrational spectroscopy and X-ray powder diffraction data show that the previously reported composition "Cs3IF6" is actually a mixture of Cs2IF5 and CsF. Ab inito calculations also show that the most probable geometries for the free LF63- ion are vibrationally unstable and undergo spontaneous F- ion loss with formation of either pentagonal planar IF52- or square planar IF4-. The synthesis and some properties of the new N(CH3)(4)IF4 salt and a revised normal coordinate analysis of XeF5- are also presented.