Cationic structures radical anions

The relation between the structure of carbocation, carbanion, radical cation, and radical anion can be best illustrated by considering the reaction of anthracene with different reagents (Scheme 2.45). 

Abstract Inorganic polysulfide anions and the related radical anions S play an important role in the redox reactions of elemental sulfur and therefore also in the geobio chemical sulfur cycle. This chapter describes the preparation of the solid polysulfides with up to eight sulfur atoms and univalent cations, as well as their solid state structures, vibrational spectra and their behavior in aqueous and non-aqueous solutions. In addition, the highly colored and reactive radical anions S with n = 2, 3, and 6 are discussed, some of which exist in equilibrium with the corresponding diamagnetic dianions. 

Nowadays, ultramarine-type pigments are produced synthetically. Inside the zeolite structure the highly reactive sulfur radical anions are well protected which explains the stability of the blue color over thousands of years in air. However, the species responsible for the blue color should not be confused with the sulfur radical cations responsible for the blue color of sulfur solutions in fuming sulfuric acid (oleum) and similar oxidizing mixtures... 

E.s.r. showed that, X. ray irradiation of tetraalkyldiphosphine diphosphides gave phosphoranyl radicals with t.b.p. structures (39).114 A structure has been assigned to phosphiny1hydrazy1s (40). The dimethy1 ami no radical was particularly persistent.115 The e.s.r. parameters of the electrogenerated pyrazine radical cations (41) have been recorded.116 The spectra of a stable furanyl phosphate radical adduct117 and a phenalene radical anion which involves injection of spin density into half an attached cyclophosphazene ring,11 are reported. 

It has been pointed out already that formation of a radical anion by a redox process in solution produces an ion pair and that any hopping of the electrons will thus be bound to the migration of the cation, which then becomes rate-limiting (Gerson et al., 1972, 1974, 1990). The, ion-pair structure of the radicals is mainly affected by the size of the counterions and the ion-solvating capability of the solvent (Hogen-Esch, 1977 Szwarc, 1968). 

Most of the salts based on decamethylmetallocenium radical cations and on planar metal bisdichalcogenate radical anions reported so far present crystal structures with mixed linear chain basic motives. The only known exception is [Fe(Cp )2][Ni (mnt)2], which exhibits another type of crystal structure based on a D+ [A2]2 D+ repeat unit [28]. In the case of this compound the magnetic behavior is dominated by the intradimer antiferromagnetic interactions. 

As most of the work with this type of salts was essentially motivated by the results obtained with the salts based on decamethylmetalocenium cations and polynitrile planar radical anions, the use of different metallocenium derivatives was limited to a small number of compounds. Among these only [Fe (C5Me4SCMe3)2][M(mnt)2], M = Ni and Pt, present crystal structures based on mixed linear chain motives. 

The structures of the radical anion of dibenzo[a,c]naphthacene (135) and the corresponding radical cation were investigated by ESR and proton-ENDOR spectroscopies. ... 

Almost all reactions of alkylidenecycloproparenes lead to opening of the cyclopropane ring. A notable exception to this is the reversible electrochemical reduction of237 and 240 which leads to the stablfe radical anions 396 and 397, with half-wave potentials of -2.32 and -1.93 V, respectively, and their oxidation to the quasi-stable radical cations 398 and 399 ( i/2(ox) = +0.68 and +0.81). The cations may be further oxidized to the corresponding very short-lived dications. In contrast, the photoelectron spectra of 237 and 240 reveal practically identical first-oxidation potentials of both compounds, which indicates that the difference in half-wave potentials for oxidation (in condensed phase) of 237 and 240 does not exist in the gas phase. This has been attributed to structure-specific solvation energies in the radical cations 398 and 399. °... 

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