NITROGEN PENTAFLUORIDE - COVALENT NF5 VERSUS IONIC NF4+F- AND STUDIES ON THE INSTABILITY OF THE LATTER

Recent ab initio calculations suggesting that covalent NF5 and even NF6- are vibrationally stable and might be experimentally accessible prompted us to critically evaluate the presently available theoretical and experimental data for NF5 and to carry out experiments on the stability of ionic NF4+F-. It is shown that covalent NF5 of D3h symmetry and crystalline NF4+F- should be of comparable energy but that covalent NF5 should suffer from severe ligand-crowding effects that would make its synthesis experimentally very difficult. On the other hand, crystalline NF4+F- should be readily accessible from the well-known solvated NF4+ and F- ions. The recent discoveries of a convenient synthesis of truly anhydrous N(CH3)4F as a source of soluble "naked" fluoride ions and of solvents which possess sufficient kinetic stability toward strong oxidizers allowed us to carry out experiments on the thermal stability of NF4+F-. It is shown that at temperatures as low as -142-degrees-C crystalline NF4+F- is unstable toward decomposition to NF3 and F2, a process which is calculated to be exothermic by about 32 kcal mol-1.