Radical three-electron

Stabilizing resonances also occur in other systems. Some well-known ones are the allyl radical and square cyclobutadiene. It has been shown that in these cases, the ground-state wave function is constructed from the out-of-phase combination of the two components [24,30]. In Section HI, it is shown that this is also a necessary result of Pauli s principle and the permutational symmetry of the polyelectronic wave function When the number of electron pairs exchanged in a two-state system is even, the ground state is the out-of-phase combination [28]. Three electrons may be considered as two electron pairs, one of which is half-populated. When both electron pahs are fully populated, an antiaromatic system arises ("Section HI). 

Allyl radical is a conjugated system in which three electrons are delocalized over three carbons The resonance structures indicate that the unpaired electron has an equal probability of being found at C 1 or C 3 C 2 shares none of the unpaired electron... 

Both CSs and CSs were also successfully generated by the fragmentation of ionized 4,5-dioxo-2-thioxo-l,3-dithione (65) and 2-thioxo-l,3-dithiole (66) (90JA3750). Tire three sulfur atoms in the anion and cation radicals were chemically equivalent, suggesting that they take the D h (or C2u) form (67 or 68). On the other hand, under similar conditions, 3-thioxo-1,2-dithiole (69) yielded two isomeric cation radicals the (or 2 ) form and the carbon disulfide 5-sulfide form (70). Ab initio calculations on three electronic states of CS3 at the 6-31G -l-ZPVE level indicated that the C21, form (68) was more stable than the carbon disulfide 5-sulfide form (70) in the neutral (both singlet and triplet states) and the anion radical states, but 68 was less stable than 70 in the radical cation state. 

Addition of small amounts of (CH3)2S (5 x 10 5 — 5 x 10"4m) to deoxygenated solutions of 3m HC104 and 0.5m DMSO leads to replacement of the 285 nm absorption by 465 nm absorption which is known to belong to the complexed three-electron-bonded radical cation of the disulfide47. 

Dihydro-l,3-diborolenes are accessible as pentaorgano derivatives " and serve as ideal precursors of the diborolyl radieal. The formation of the radical and its interaction with a metal center is formally described as a metal oxidation. The resulting 2,3-dihydro-1,3-diborolyI ligand is either a three-electron donor or an anionic four-electron donor. l,3,4,5-Tetraethyl-2-methyl-2,3-dihydro-l,3-diborole reacts with [T -CpNi(CO)2] or (T7 -Cp>2Ni in hot mesitylene to yield a sandwich complex and a triple-decked complex ... 

The S-S bond dissociation energies of H2S2, H2S3 and H2S4 have been studied by Steudel et al. at the GCSD(T)//6-311 G(2df,p) level [42]. The calculated enthalpies AH° for the dissociation at the central bonds at 298 K are 247, 201 and 159 kJ mol respectively. The lower stability of the tri- and tetrasulfanes towards homolytic S-S cleavage is attributed to the stability of the generated HSS radical which is stabilized by the formation of a three-electron n bond. 

Fig. 4 Structures of the three-electron hemibonded radical cations [R2S.. SR2] - The S-S bond lengths (HF/6-31G, with the MP2/6-31G values in parenthesis) are given in pm...

In sharp contrast to the stable [H2S. .SH2] radical cation, the isoelectron-ic neutral radicals [H2S.. SH] and [H2S. .C1] are very weakly-bound van der Waals complexes [125]. Furthermore, the unsymmetrical [H2S.. C1H] radical cation is less strongly bound than the symmetrical [H2S.. SH2] ion. The strength of these three-electron bonds was explained in terms of the overlap between the donor HOMO and radical SOMO. In a systematic study of a series of three-electron bonded radical cations [126], Clark has shown that the three-electron bond energy of [X.. Y] decreases exponentially with AIP, the difference between the ionisation potentials (IP) of X and Y. As a consequence, many of the known three-electron bonds are homonuclear, or at least involve two atoms of similar IP. 

Asmus et al. unambiguously identified a variety of [R2S.. SR2] radical cations in solution and measured their optical absorption spectra using pulse radiolysis techniques [133]. They proposed that the spectrum of [H2S. .SH2] arises from the transition in the three-electron S.. S... 

Matsika S, Yarkony DR (2003) Conical intersections of three electronic states affect the ground state of radical species with little Or no symmetry pyrazolyl. J Am Chem Soc 125 12428... 

The picket fence Ni11 complex [Ni(TpivPP)] ((70) TpivPP meso-a-a-a-a-lclrakis-(o-pivalamidophenyl)porphyrin) is reversibly oxidized by three electrons in two steps and generates as final product [Ni(Tipw)]3+ which was characterized as a Nilv cation radical in solution by both UV-visible and EPR spectroscopy.289... 

The activation energy of radical abstraction is influenced by the so-called triplet repulsion in the transition state. This influence is manifested by the fact that the stronger the X—R bond towards which the hydrogen atom moves in the thermally neutral reaction X + RH, the higher the activation energy of this reaction. The triplet repulsion is due to the fact that three electrons cannot be accommodated in the bonding orbital of X—C therefore, one electron... 

Nitric oxide formation from hydroxyurea requires a three-electron oxidation (Scheme 7.15) [114]. Treatment of hydroxyurea with a variety of chemical oxidants produces NO or NO-related species , including nitroxyl (HNO), and these reactions have recently been extensively reviewed [114]. Many of these reactions proceed either through the nitroxide radical (25) or a C-nitroso intermediate (26, Scheme 7.15) [114]. The remainder of the hydroxyurea molecule may decompose into formamide or carbon dioxide and ammonia, depending on the conditions and type of oxidant (one-electron vs. two electron) employed. 

There are two other mechanistic possibilities, halogen atom abstraction (HAA) and halonium ion abstraction (EL), represented in Schemes 4.4 and 4.5, respectively, so as to display the stereochemistry of the reaction. Both reactions are expected to be faster than outer-sphere electron transfer, owing to stabilizing interactions in the transition state. They are also anticipated to both exhibit antiperiplanar preference, owing to partial delocalization over the C—C—Br framework of the unpaired electron in the HAA case or the electron pair in the EL case. Both mechanisms are compatible with the fact that the activation entropies are about the same as with outer-sphere electron donors (here, aromatic anion radicals). The bromine atom indeed bears three electron pairs located in two orthogonal 4p orbitals, perpendicular to the C—Br bond and in one s orbital. Bonded interactions in the transition... 

Fig. 39). The ESR data for dimethylselenide dimer cation radical 107c are consistent with two-center three-electron bonding, in which two electrons are in a cr bond between the two chalcogens, while the third electron is in a o orbital. 

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