Electron delocalization metal reduction

There is an extensive chemistry of the nickel(I) ion generated by pulse radiolysis which is beyond the scope of this review. Complexes with saturated amines such as 1,2-diaminoethane have been studied by this method and by the y radiolysis of aqueous glasses, but the species formed have no more than a transient existence. The imine ligands phen and bpy offer a more attractive environment for nickel(I) by allowing electron delocalization over the ligand n system (178,179). A number of complexes of these ligands have been reported in y-radiolysis studies. The EPR spectra indicate that reduction is primarily metal centered with a significant orbital contribution. Electrochemical reduction of [NiH(bpy)3]2+ in anhydrous acetonitrile results in [Ni (bpy)3] +, which can be detected by EPR methods. The reduction potential is reported to be —1.55 V but the complex is thermodynamically unstable with... 

After reduction with dithionite, this dendrimer was cast into a film the electrical properties of which were isotropic. (This means that on the molecular and macroscopic levels there is a three-dimensional electron delocalization.) The conductivity was humidity dependent (water may take part in long-distance electron transfer). At 95% humidity, a mixed-valence film (0.55 electron per diimide) showed conduction at room temperature around 11 fl 1-cm 1 (Miller Mann 1996). As shown later, partially reduced mixed-valence materials are required for organic metals of high electrical conductivity. 

The cyclic peralkylsilane oligomers, (R2Si) with n = 4-6, manifested especially strong electron delocalization.5 These rings are structurally analogous to those of the cycloalkanes, since the silicon atoms form four sigma bonds. However, the electronic properties of the cyclosilanes more nearly resemble those of aromatic hydrocarbons such as benzene. One example of such behavior is their reduction to anion radicals. Aromatic hydrocarbons such as naphthalene can be reduced, electrolytically or with alkali metals, to deeply colored anion radicals in which an unpaired electron occupies the lowest unoccupied molecular orbital (LUMO) of the hydrocarbon (equation (2)). 

The results for Cr34 and the 3d5 cations Fe3+ and Mn2+ show that it is possible to derive values of the Racah B parameter for transition metal compounds from absorption bands in their crystal field spectra, enabling comparisons to be made with field-free ion values. In all cases, there is a decrease of the Racah B parameter for the bonded cations relative to the gaseous ions, which is indicative of diminished repulsion between 3d electrons in chemical compounds of the transition metals. This reduction is attributable to electron delocalization or covalent bonding in the compounds. Such decreases of Racah B parameters are expressed as the nephelauxetic Greek , cloud expanding) ratio, p, given by... 

Polynuclear metal complexes are more suited for water oxidation catalyst because of their nature to act as multielectron transfer reagents in addition to the fact that charge delocalization can lead to stabilization of the catalyst rather than decomposition during the process. The trinuclear ruthenium complexes Ru-red and Ru-brown, [(NH3)sRu-0-Ru(NH3)4-0-Ru(NH3)j] - (Ru "-Ru" -Ru" ) and [(NH3)sRu-0-Ru(NH3)4-0-Ru(NH3)5] + (Ru -Ru" -Ru ), respectively, have been shown to be efficient water oxidation catalysts for oxygen evolution with high turnover numbers When Ru-red was dissolved in an acidic aqueous solution, it underwent one-electron oxidation with the formation of Ru-brown. When Ru-brown was dissolved in a basic solution, the complex underwent reduction to produce Ru-red. The one-electron oxidation and reduction of the Ru-red and Ru-brown has already been well established (Eq. 11) 6 65-6 )... 

The mechanism of dissolving metal reductions depends on the nature of the solvent and the nature of the substrate. The proposed mechanism for the reduction of dialkylacetylenes by sodium in HMPA in the presence of a proton donor is illustrated in equation (18). The addition of an electron to the triple bond of (45) is proposed to produce the rran -sodiovinyl radical (46), or the corresponding radical anion (47), which undergoes protonation by the added alcohol to produce the radical (48). Further reduction of (48) by sodium produces the rrans-sodiovinyl compound (49), which on protonation produces the trans-a -kene (50). In the absence of a proton donor, the reduction of (45) with sodium in HMPA results in the formation of a mixture of cis- and trans-2- and 3-hexenes. Control studies showed that the isomerization products 2- and 3-hexene are not formed by rearrangement of the cis- or frans-3-hexenes. It was concluded that the starting alkyne (45) acts as a reversible proton donor reacting with an intermediate anion or radical anion to produce the delocalized anion (51) which is then protonated to produce the al-lene (52). Reduction of the allene (52), or further rearrangement to the alkyne (53) followed by reduction, then leads to the formation of the mixture of the cis- and trans-2- and 3-hexenes (equation 19). ... 

Acenaphtylene (82 ) dianion is obtained by alkali metal reduction (e.g. lithium or sodium) in an ether solvent. Early 3H NMR studies were reported by Lawler and Ristagno 10, n). This dianion was subject to detailed studies which concentrated on its mode of electron delocalization and the ion-pairing equilibrium. The mode of electron delocalization84b) is mainly deduced from H and 13C NMR chemical shifts in tetrahydrofuran (THF) (Fig. 8) while the ion-solvation equilibrium was deduced from 1H, 13C and 7Li shifts in THF-dg, 2-MeTHF, Et20 and THF-HMPA-d8 83a). 

The higher homologs of acenaphthylene dianion 82 are the aceanthrylene dianion 33 and acephenanthrylene dianion 34 122,123). The convenient synthesis of the the hydrocarbon enabled a detailed investigation of their metal reduction and the exploration of their patterns of delocalization. Despite their being (4n + 2)ji-electron systems, these anions are not diatropic as one may expect from counting their jc-electrons. 

Isolated carbon-carbon double bonds are not normally reduced by dissolving metal reducing agents. Reduction is possible when the double bond is conjugated, because the intermediate anion can be stabilized by electron delocalization. The best reagent is a solution of an alkali metal in liquid ammonia, with or without addition of an alcohol - the so-called Birch reduction conditions. Under these conditions conjugated alkenes, a,p-unsaturated ketones and even aromatic rings can be reduced to dihydro derivatives. 

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