Electron anions

The Hiickel rule predicts aromaticity for the six-7c-electron cation derived from cycloheptatriene by hydride abstraction and antiaromaticity for the planar eight-rc-electron anion that would be formed by deprotonation. The cation is indeed very stable, with a P Cr+ of -1-4.7. ° Salts containing the cation can be isolated as a product of a variety of preparative procedures. On the other hand, the pK of cycloheptatriene has been estimated at 36. ° This value is similar to those of normal 1,4-dienes and does not indicate strong destabilization. Thus, the seven-membered eight-rc-electron anion is probably nonplanar. This would be similar to the situation in the nonplanar eight-rc-electron hydrocarbon, cyclooctatetraene. 

The triatomic cations X3+ are nonlinear and thus isostructural with other 20-electron species such as XY2+ (p. 839) and SCI2 (p. 689). The contrast in bond lengths and angles between I3+ (Fig. 17.15) and the linear 22-electron anion I3 (p. 836) is notable, as is its similarity with the isolectronic Tc3 anion (p. 764). Likewise, Br3Asp6 is isomorphous with I3ASF6 and the non-linear cation has Br Br 227.0 pm and an angle of 102.5° > (cf. Br3", Table 17.15). The structures of the penta-atomic cations Brs+ (2) and I5+ (3) have been determined by X-ray analysis of their AsFe salts and shown to have centrosymmetric Cjh symmetry like the... 

Although five equivalent resonance structures can be drawn for all three species, Huckel s rule predicts that only the six-ir-electron anion should be aromatic. The four-77-electron cyciopentadienyl carbocation and the five-7r-electron cyciopentadienyl radical are predicted to be unstable and antiaromatic. 

Similar arguments can be used to predict the relative stabilities of the cyclo-heptatrienyl cation, radical, and anion. Removal of a hydrogen from cyclohepta-triene can generate the six-77-electron cation, the seven-77-electron radical, 01 the eight-77-elec iron anion (Figure 15.6). All three species again have numerous resonance forms, but HiickeTs rule predicts that only the six-7r-electron cyclohep-tatrienyl cation should be aromatic. The seven-77-electron cycloheptatrienyl radical and the eight-77-electron anion are antiaromatic. 

F- analyt-gauss variation-iteration on a ten electron anionic system 27,28... 

An interesting feature is the ability to form a 67t-electron anion that is isoelectronic with the cyclopentadienyl ion. The cation that is produced by hydride substraction contains a 47t-electron system. 

N- Methylbenzodihydroazocines (83), (84), and (85) have been studied from the standpoint of possible stabilization of the IOtt- electron anions available by deprotonation (71JA4016). The kinetic acidities, determined by deuterium exchange rates, indicate that (83) and (84) are converted to the anions about 80 and 30 times faster, respectively, than the corresponding 1,2- and 1,4-dihydroquinoline models the enhancement in (85) was slight. By this criterion the benzazocinyl anion (86) was judged to have some degree of aromatic character. 

The 8ir-electron anions derived from 1,3,5-oxadiazines (30) (75AG(E)581) are unstable and readily ring contract to give imidazolones (141 Scheme 9). The related anions from 1,3,5-thiadiazines (100 Scheme 1) also ring contract to give imidazoles in high yield... 

Electron-anion antiport has been realized, for instance with redox active carriers such as ferrocene derivatives [6.51a] or alkylviologens [6.44-6.46,6.51b] via ferricinium or reduced viologen species, respectively. The latter have been used extensively in light-driven systems and in studies on solar energy conversion [6.44-6.46],... 

Whereas in ligand bridged dinuclear complexes, removal or addition of two electrons makes or breaks one metal-metal bond (15) this does not seem to be the case for clusters, presumably because of their delocalized bonding. At least for one case, however, two-electron reduction can induce a significant change in cluster shape (18,42) the 84-electron cluster Os6(CO),g with framework 1 is easily reduced to the 86-electron anion Os6(CO) g with framework 2, in accordance with skeletal electron counting rules. 

S. Matsuishi, Y. Toda, M. Miyakawa, K. Hayashi, T. Kamiya, M. Hirano, I. Tanaka and H. Hosono, High-density electron anions in a nanoporous single crystal [Ca24Al28064]4+(4e-). Science 301, 626-9 (2003). 

Finally, it must be noted that in a few cases radical anions have also been observed to be generated from even electron anions One example concerns the CID loss of a methyl radical from the (M — H) ion of methoxyaceto-nitrile (Dawson and Nibbering, 1980) as shown in (80). The capto-dative character (Viehe et al., 1979 Crans et al., 1980) of the generated radical... 

All the mononuclear species effect V(Co)6 in the above table satisfy the 18-electron rule. The bi and tri-nuclear species do also if (i) the two electrons in a metal-metal bond are counted as contributing to the valence shells of both metal atoms concerned, and (ii) a bridging CO contributes one electron to each metal. Monomeric Mn and Co carbonyls would have an odd number of electrons and dimerise in consequence. V(CO)6 is exceptional as a stable radical with 17 valence-shell electrons, presumably because it is satirically impossible for it to dimerise without losing one CO ligand. It does, however, readily form the 18-electron anion [V(CO)6]-. 

Although it is certainly true that in general 17-electron organometallic complexes are more reactive than nonradical analogues, there are a few exceptions to this pattern. For example, the 17-electron [CpMo(PMe3)2I2] is more kinetically stable to iodide substitution than is the 16-electron cation (vide infra).19 Similarly, the 17-electron [CpCr(NO)(PPh3)I] is more inert to iodide substitution than is the 18-electron anionic complex.20 A study of complexes of the form [CpCr(NO)L2]0,+ led to the conclusion that the 17-electron cation is preferred when ligand L is a c-donor such... 

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