Electron donor properties of the

The electron-donor property of the enol form is indicated by the magnitude of its ionization potential (IP in the gas phase) or its oxidation potential ( °x in solution).4 8 Conversely, the relatively electron-poor keto form is a viable acceptor in measure with the magnitude of its electron affinity (EA in the gas phase) or its reduction potential in solution). 

The passage from a stepwise (the phenylthiolate ion as a nucleophile) to a concerted mechanism (the nitropropanate ion as a nucleophile) is a consequence of the low driving force offered by the poor electron-donor properties of the nucleophile. 

Studies of medium effects on hexacyanoferrate(II) reductions have included those of dioxygen,iodate, peroxodisulfate, - [Co(NH3)5(DMSO)] +, and [Co(en)2Br2]+. Rate constants for reaction with dioxygen depended strongly on the electron-donor properties of the organic cosolvent. Rate constants for reduction of peroxodisulfate in several binary aqueous media were analyzed into their ion association and subsequent electron transfer components. Rate constants for reduction of [Co(en)2Br2] in methanol water and dioxan water mixtures were analyzed by a variety of correlatory equations (dielectric constant Grunwald-Winstein Swain Kamlet-Taft). 

Very interesting oxidation patterns are obtained for decanuclear compounds (Scheme 1 and Table 1). For example, in compound lOC, which contains one Os " and nine Ru " ions, the Os " ion is expected to be oxidized at less positive potentials than the nine Ru ions. Furthermore, because of the different electron donor properties of the ligands, the six peripheral Ru ions are expected to be oxidized at less positive potentials than the three intermediate Ru " ions. In agreement with these expectations, the differential pulse voltammogram of IOC (Figure 12) shows... 

Several additional studies have confirmed interannular electronic interactions in ferrocene. Early investigations by Nesmeyanov and coworkers and by others demonstrated the electron-donor properties of the ferrocenyl group (71). Aminoferrocene, for example, is approximately 20 times as strong a base as aniline, while benzoic acid is several times more acidic than ferrocenecarboxylic acid. Moreover, acidities and basicities of this sort are markedly affected by substituents on the opposite cyclopentadienyl ring. 

Increasing the electron-donor properties of the R, substituent in the case of dihydro derivatives of 1,2,4-triazolo[l, 5-a]pyrimidines [X, Z are N, Y is C(R)], tetrazolo[l, 5-a]pyrimidines (X, Y, Z are N), and pyrimidofl, 2-a]benzimidazoles (X, Y are o-C() I4, Z is N) tends to stabilize the B form. This can be explained by the conjugation of Rj with the electron-withdrawing azomethine group and the azole moiety. Reduction of the electron-accepting properties of the azole fragment (imidazopyrimidines and pyrazolopyrimidines) decreases the influence of the Ri substituent on tautomeric composition. 

No detailed evaluation of the different factors contributing to this binding has been made yet. One quantitative relationship relevant to the binding has nevertheless been obtained. Thus we have been able to show that the solvent power of a series of purines toward, say, a given hydrocarbon runs parallel to the electron donor properties of the purines, as measured by the value of the energy coefficient of their highest filled molecular orbital. These values are reproduced in Table I for the essential biochemical purines and... 

CIDEP studies indicated that photo-ejection reactions probably occurred from triplet anions33. Oxidation potentials (—0.29 to —0.90 V, versus SCE)113 confirmed the electron-donor properties of the anions. 

Competitive reactions and essentially competitive hydrogenations were often used to discuss the extent of the electronic transfers induced by poison adsorption. For instance, two model molecules with different electronic densities are chosen, e.g., benzene and toluene. In this case the electronic donor properties of the methyl group increase the electronic density on the insaturated bonds. During competitive hydrogenation of benzene and toluene, sulfur adsorption poisons the two reactions but is less toxic for toluene than for benzene hydrogenation (94-96). Sulfur, by its adsorption as an electron acceptor, is able to decrease the electronic density of the unpoisoned metallic surface area and could favor the adsorption of the reactant with the highest donor properties enhancing the hydro-... 

AHa, will be solvent-dependent unless IA//3I = IA Hi + AH2, where A Hi and A H2 are the heats of solvation of the electron donor and the hydrogen compound, respectively, and AH3 is the heat of solvation of the complex. The enthalpy of the formation of the adduct in the gas phase, which would give the actual strength of the hydrogen bond, will be different from that in solution unless the solvation energies cancel. In many cases, this is questionable because of the electron donor properties of the solvent. There is no doubt that many of the hydrogen bond enthalpies reported in the literature may be in error because of this. Moreover, it has been shown that the extent of self-association of... 

Dipole moments of the chalcones containing heterocyclic radicals confirm the electron-donor properties of the five-membered heterocycles in their ground states. The moments in these compounds are dominated by the carbonyl polarization which is stronger the more electron-donating is the radical bonded to the carbonyl. Thus, all the physical data on the chalcone analogs suggest that the electron-donating effect of the heteroaromatic radicals decreases in the order a-selenienyl > a-thienyl > a-furyl.100... 

As shown in Table 20, the overall order of stability is as follows dpype < dppf < (-l-)-diop < dppb < dppe < dppp < dppv < dpmcb < dope < dmpe < dcpe. As the Tohnan electronic scale predicts, the relative stability of the resulting complexes is strongly influenced by the electronic donor properties of the bidentate phosphine ligand if no larger steric factors are present. For example, better a donors (see a-Donor), such as, dmpe, depe, and dcpe, give more thermodynamically stable complexes while the isosteric (see Isosteric Groups) ligand dpype, which also... 

These ions have received considerable attention and have been extensively reviewed 13). The chemistry of such ions is thus outside the scope of this review additional information may be found in the latest papers of Nugent and Richards (55). However, the more quantitative aspects of the electron donor properties of the ferrocenyl and related groups, such as those provided by determination of their Hammett [Pg.108]

Ferrocene forms charge-transfer complexes with a number of acceptor molecules (4, 16, 17 21 j 22, 46, 65), of which that with tetracyanoethylene isolated as a green crystalline substance 4.6, 65) is perhaps the best characterized. In view of the electron-donor properties of the metal atom, it has been suggested that its structure might best be represented by XIX, rather than by the more classical formulation XVIII (4 ). 

It is generally agreed that the extracted uranium species is U02(N03)2(TBP)2 in which, in contrast to the species extracted into ether, the extractant molecules are directly coordinated to the metal ion. The extraction of uranium(VI) by neutral organophosphorus compounds increases with the number of C—P bonds (as opposed to C—0—P bonds) and with the increased branching of the substituent alkyl groups, as would be expected from the effect of these changes on the electron-donor properties of the phosphoryl group. The extracted thorium(IV) complex has been formulated as Th(N03)4-2S and as Th(N03)4-3S,2 2 and its composition may depend to some extent on the conditions of extraction. The extraction of thorium (like that of... 

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