Electron organometallic anions

Scheme 1 Electron-donor molecules that have been utilized as components of cation radical salts with organometallic anions...

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... 

The net charge on the complex is also an important factor, and organometallic anions are among the most effective electron donors currently available (10). Conversely, the positively charged organometallic cations are useful electron acceptors in measure with the magnitudes of their electron affinities (11). 

The establishment of electron-transfer equilibria is allowed by the unique stabilizations inherent in the organometallic anion TpMo(CO)3 [where Tp = hydridotris(3,5-dimethylpyrazolyl)borate] (85), in which the 17-electron radical (m20 is persistent (56). Since other members of the group VIB metals, namely, TpW(CO)3 and TpCr(CO)3-, are also available (87), this triad of anions enables the structural effects to be systematically examined,... 

Electron-transfer equilibria of the organometallic anions TpM(CO)3- can be examined by coupling them with a graded series of triarylpyrylium cations (TaP+) for which 1-electron reduction potentials are known to be strongly dependent on the substituents (88). The evaluation of the constant K for the electron-transfer equilibrium in Eq. (39) requires the quantitative analysis of the anionic organometallic redox couple in Eq. (38), as well as that of triarylpyrylium cation. 

Direct evidence for the involvement of the organometallic anions TpM(CO)3 in contact ion pairs obtains from the electronic transitions associated from proximate cation-anion pairs and delineated as charge-transfer excitations by Mulliken (7). This consideration is brought out by the dramatic colors obtained on the exposure of the TpM(CO)3- anions to selected pyridinium cations with reduction potentials °d that are sufficiently negative to obviate spontaneous electron transfer. Thus, the mixing of a dilute, pale yellow solution of Bu4N+ TpMo(CO)3 in dichloromethane with a colorless solution of A-methyl-4-phenylpyridinium (MPP+) triflate with an value of —1.3 V versus SCE immediately produces a deep purple coloration. The quantitative effect of this color change is... 

Many other species are stabilized in 18-electron organometallic complexes car-benes and carbynes, enyls and polyenyls (XL ligands), o-xylylene (o-quinodime-thane), trimethylenemethane, benzyne, norbornadiene-7-one, cyclohexyne, 1,2-di-hydropyridines (intermediates in biological processes), thermodynamically unfavorable organic tautomers such as vinyl alcohols [less stable by 14 kcafrmol (58.5 kJ mol ) than their aldehyde tautomers], aromatic anions resulting from deprotonation in juxta-cyclic position such as tautomers of phenolates and benzylic carbanions. All these species have a specific reactivity that can lead to synthetic applications in the same way as cyclobutadiene above. 

As a result of the systematic application of coordination-chemistry principles, dozens of previously unsuspected stnicture types have been synthesized in which polyhedral boranes or their anions can be considered to act as ligands which donate electron density to metal centres, thereby forming novel metallaboranc elusters, ". Some 40 metals have been found to act as acceptors in this way (see also p. 178). The ideas have been particularly helpful m emphasizing the close interconnection between several previously separated branches of chemistry, notably boron hydride clu.ster chemistry, metallaboranc and metallacarbaborane chemistry (pp. 189-95). organometallic chemistry and metal-metal cluster chemistry. All are now seen to be parts of a coherent whole. 

Three possible mechanisms may be envisioned for this reaction. The first two i.e. 1) Michael addition of R M to the acetylenic sulfone followed by a-elimination of LiOjSPh to yield a vinyl carbene which undergoes a 1,2 aryl shift and 2) carbometallation of the acetylenic sulfone by R M followed by a straightforward -elimination, where discarded by the authors. The third mechanism in which the organometallic reagent acts as an electron donor and the central intermediates is the radical anion ... 

Many other TT-organometallic compounds have been prepared. In the most stable of these, the total number of electrons contributed by the ligands (e.g., four for allyl anions and six for cyclopentadiene anion) plus the valence electrons on the metal atom or ion is usually 18, to satisfy the effective atomic number rule.31 ... 

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