PENTAAZA- AND PENTAPHOSPHACYCLOPENTADIENIDE ANIONS AND THEIR LITHIUM AND SODIUM DERIVATIVES - STRUCTURES AND STABILITIES

Both the pentazole, N5- (4), and pentaphosphole, P5- (5), anions favor planar D5h geometries, in contrast to hexazine (N6) and to hexaphosph abenzene (P6), which have D2 twist-boat structures. While the pentazol e anion 4 is thermodynamically unstable relative to N3- + N2, a barrie r of 19.4 kcal/mol at MP4SDTQ/6-31+G//MP2(full)/6-31G* + DELTAZPE(HF/ 6-31G) inhibits the dissociation. In contrast to 4, 5 is stable both toward P3- + P2 dissociation (DELTAE = 71.6 kcal/mol) and toward 2(5) --> 2P3- + P4(Td) disproportionation (DELTAE = 116.4 kcal/mol). The li thium salt, N5Li, favors the planar C2v structure 15, in contrast to t he cyclopentadienyllithium, (CH)5Li (27), and to the sodium P5Na deriv ative (12), both of which prefer C5v symmetry. While 15 is 33.5 kcal/m ol unstable relative to dissociation into N3Li (C(infinity v)) and N2 (MP4SDTQ/6-31+G//MP2/6-31G* + DELTAZPE(MP2/6-31G*), pyramidal 12 is h ighly stable toward disproportionation into P3Na (either C3v triplet 3 4 or C2v singlet 35) and P4 (T(d)). Comparison of the calculated IR sp ectra for 12 and for 35 with the experimental spectrum for 12 shows cl early that 35 may be present.