Anions 18 -Annulene, structure

State, as shown in Fig. 10.8 [65]. The "trefoil bonding state, previously proposed and predicted to exist in aromatic hydrocarbon annulene molecules, is finally encountered, albeit in an extended intermetallic network [65]. This also highlights the important role of interactions between incompletely filled lone pairs in the stabilization of low-dimensional anion structures. 

Reductive Generation of Anionic Species 1101 Tab. 2 Structure formula of annulenes and cyclic conjugated systems (1-17)... 

Does the deprotonation lead to a nonafulvenolate -type structure 171 M+ (an enolate contact ion pair 122 123,124)) or to an acyl-substituted [9]annulene anion 171 M+, possibly a solvent separated ion pair 1271... 

The cation possesses a more favourable geometry than 1,6-methano-[10]annulene, being able to incorporate the 10 -system into a more planar structure. It is evidently more stable than the tropylium ion, as it can be formed from its neutral hydrocarbon precursor 57 by a hydride-transfer reaction with tropylium tetrafluoroborate 136>. The anion, on the other hand, involves substantial steric strain and is not as planar as either the cation or the neutral bridged [lOJannulene. Whereas formation of the cation from 57 was seen to be most favourable, the anion precursor 58 is much less acidic than cyclopentadiene, although in the presence of... 

A homoaromatic structure also best explains the n.m.r. characteristics of the anion 75 (see Table 15), which was derived from 1,e-methanols 10] annulene 219>. This 10r9C system has peripheral protons which absorb from t 3.19 to 5.28 (or ca. t 2.1 to 4.2 on correction for charge density), and bridge protons which are about 2.5 ppm more shielded than the corresponding protons in 65, the compound formed on protonation of the anion. 

The structure of the bis-THF adduct of Nd2(CgHQ)353 shows a bis([8]annulene)neodymate(III) anion where the local symmetry of the anion is C,. The carbon atoms of the two [8]annulene rings (A and B) are almost eclipsed, but the rings are not parallel planar and these planes intersect at an angle of 7.28°. The complex anion is an ion pair with a bis-THF solvated [8]annuleneneodymate(III) cation. The metal-ring carbon distances shown in Figure 7 illustrate the effect of the presence of two trivalent atoms in this molecule. 

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