THE N2F+ CATION - AN UNUSUAL ION CONTAINING THE SHORTEST PRESENTLY KNOWN NITROGEN FLUORINE BOND

The N2F+AsF6- salt was prepared in high yield from trans-N2F2 by thermal trans-cis isomerization in the presence of AsF5 at 70-degrees-C. A displacement reaction between N2F+AsF6- and FNO yields exclusively cis-N2F2. The Lewis acids BF3 and PF5 do not form a stable adduct with cis-N2F2 at temperatures as low as -78-degrees-C and do not catalyze the N2F2 trans-cis isomerization. A semiempirical molecular orbital model is used to explain the puzzling differences in the reaction chemistry of cis- and trans-N2F2. The crystal structure of N2F+AsF6- (monoclinic, C2/m, a = 9.184 (5) angstrom, b = 5.882 (2) angstrom, c = 5.160 (2) angstrom, beta = 90.47 (4)-degrees, Z = 2) was determined. Alternate space groups (Cm and C2) can be rejected on the basis of the observed vibrational spectra. Since in C2/m the N2F+ cations are disordered, only the sum of the N-F and N-N bond distances could be determined from the X-ray data. Local density functional calculations were carried out for N2F+ and the well-known isoelectronic FCN molecule. The results from these calculations allowed the sum of the N2F+ bond lengths to be partitioned into the individual bond distances. The resulting N-F bond length of 1.217 angstrom is by far the shortest presently known N-F bond, while the N-N bond length of 1.099 angstrom is comparable to the shortest presently known N-N bond length of 1.0976 (2) angstrom in N2. The surprising shortness of both bonds is attributed to the high s-character (sp hybrid) of the sigma-bond orbitals on nitrogen and the formal positive charge on the cation. Thus, the shortening of the N-F bond on going from sp3-hybridized NF4+ (1.30 angstrom) to sp-hybridized N2F+ (1.22 angstrom) parallels those found for the C-H and C-F bonds in the CH4, CH2 = CH2, CH = CH and CF4, CF2 = CF2, FC = N series, respectively. The oxidative power of N2F+ has also been studied. The N2F+ cation oxidized Xe and ClF to XeF+ and ClF2+, respectively, but did not oxidize ClF5, BrF5, IF5, XeF4, NF3, or O2.