Oxygen three-electron bonds

Group VI No room temperature example has been reported for oxygen-oxygen three-electron bonds. They are apparently extremely unstable. In case of the hypothetical (H2OOH2)" ", as opposed to the relatively stable (H2S. . SH2)" ", theory has provided a special clue in that the thermodynamically favored species is not the three-electron bonded species but a (H30 - 0H) ion-... 

The revision leads to a difference of 0.06 A. between the interatomic distance in the normal oxygen molecule and the sum of the double-bond radii. This may be attributed to the presence of an unusual structure, consisting of a single bond plus two three-electron bonds. We assign this structure both to the normal 2 state, with ro = 1.204 A., and to the excited 2 state, with ro = 1.223 A., the two differing in the relative spin orientations of the odd electrons in the two three-electron bonds. We expect for the double-bonded state the separation n 1.14 A. 

The onium form of the ethyl acetate cation-radical is more stable by 50 kJ moH than the corresponding carbonyl form (Rhodes 1988). The CHj fragment is stabilized by the three-electron bonding with the neighboring oxygen in the following manner -O.. CH2. Oxidation of the carbonyl... 

A few molecules in addition to those discussed in the preceding sections can be assigned structures involving one or two three-electron bonds. The normal states of the molecules SO, St, Se2, and Tea are 82 states, like that of the normal oxygen molecule, and it is probable that the electronic structures with a single bond plus two three-electron bonds are satisfactory for these molecules. The- observed values of interatomic distances, 1.493, 1.888, 2.152, and 2.82 A, respectively, are about those expected. 

Let us now consider the formation of three-electron bonds between different atoms. Stabilization of an oxidized sulfur atom can, in principle, be achieved in cases of its interaction with other heteroatoms if they provide free (preferably p-) electron pairs. Nitrogen, oxygen, and halogens (except fluorine) can be mentioned as such heteroatoms (Anklam et al. 1988 Carmichael 1997). The stability of these bonds is generally not as high as that of a symmetric S.. S system. An important reference for the enhanced stability of symmetrical three-electron bonds is Clark s (1988) calculations. 

Bond energies for commonly occurring diatomic molecules range from 135 kcal per mole for HF to 36 kcal per mole for I2. Similar measurements on No and O2 set the energy of the NssN triple bond at 170 kcal per mole and the energy of the system of one single bond plus two three-electron bonds in oxygen (Chap, 4) as 158 kcal per mole. 

One formulation of an A type layer of the high Tc superconductor YBa2Cu307 involves a. ..(CuO)(CuO)+(CuO)(CuO)+... arrangements of copper and oxygen ions [30], Each (CuO) component involves a three-electron bond, which arises from the overlap of the singly-occupied 3dx2.y2 AO of Cu2+ with a... 

As a striking exanqrle, photo-initiated chlorination (Scheme 21) of 3 mM (19) with 1.5 equiv. PhICb led to the 9-chloro derivative (20) in >98% yield with Ag this was converted to the A -alkene. Again the template-directed reaction overcomes the normal reactivity of the substrate, but at 21 mM (19) undirected reactions start to compete and some 6-chloro steroid is also formed. A pyridine A(-oxide template, that can use three-electron bonding to complex a chlorine to the oxygen atom, seems to be almost as effective. Furthermore, an imidazole template in compound (21) directs chlorination at C-9 with similar efficiency to the templates previously examined, and (21) is particularly easily prepared using caibonyldiimidazole. 

The structure with the three electron bond between nitrogen and oxygen is the best description of the bonding situation in nitric oxide. 

The effect of N-acetyl substitution in methionine on the nature of transients formed after one-electron oxidation was studied as a function of pH and NAM concentration. The observed absorption bands with X = 290 nm, 360 nm, and 490 nm were respectively assigned to a-(alkylthio)alkyl, hydroxysulfuranyl and dimeric radical cations with intermolecular three-electron bond between sulfur atoms. N-acetylmethionine amide (NAMA) (Chart 7) represents a simple chemical model for the methionine residue incorporated in a peptide. Pulse radiolysis studies coupled to time-resolved UV-Vis spectroscopy and conductivity detection of N-acetyl methionine amide delivered the first experimental evidence that a sulfur radical cation can associate with the oxygen of an amide function vide infra). ... 

The force of attraction between the oxygen atoms is much greater than that expected for a single covalent bond. This shows that the unpaired electrons arc really in volved in the formation of bonds of a special sort. The oxygen molecule may be said to ntain a single covalent bond plus two three-electron bonds, and its structure may be written as 0—rrr-O ... 

The odd-electron molecule is very reaetive. When warmed it explodes. CIO2 has a bond angle of 118° and a very short bond length of 0.149 nm with six electrons on each of the oxygen atoms and seven on the chlorine. This means that there are two electrons shared between one Cl-0 bond and three electrons between the other Cl-O bond. The three electron bond resonates between these two bonds. This extra electron is really a free radical that probably accounts for the instability of chlorine dioxide and its reactivity as an oxidizing agent. 

The stability constants for the 2o/10 species seem to be mainly controlled by the activation energies and, in turn, rate constants for the dissociation of the three-electron bond (back reaction of eq. 40/40a). The respective values, for the two systems with all-methyl and all-i-propyl substitution are 57 and 17 kJ mol and 1.5x10 and 5.6x10 s . 6 xhe latter are certainly in line with the S S bond energies but, again, a direct correlation is not justified because in aqueous solution the reaction of interest is not simply the dissociation of the 3-e-bond but, in fact, a displacement process which involves also a water molecule. (See section on sulfur-oxygen interactions ). 

The >S 0(0)C- species touches on another aspect of general interest, namely, the establishment of a three-electron bond between two elements of very different electronegativity. Any sulfur-oxygen interaction, in principle, constitutes an extreme situation with respect to an asymmetry in the MO energy diagram. One of the most relevant examples is probably the association of a water molecule to an oxidized sulfur radical cation, as formulated in eq. 58.56... 

One triumph of molecular orbital theory is its prediction of two unpaired electrons for O2. Oxygen had long been known to be paramagnetic, but early explanations for this phenomenon were unsatisfactory. For example, a special three-electron bond was proposed. The molecular orbital description directly explains why two unpaired electrons are required. In other cases, experimental observations (paramagnetic B2, diamagnetic C2) require a shift of orbital energies, raising o-g above 7t , but they do not require major modifications of the model. 

Make a calculation, similar to that of the preceding Exercise, of the resonance energy of the three-electron bond and that of interchange of the two kinds of bonds to the two oxygen atoms in NO2. This resonance energy is expected to be about twice that for the three-electron bond in NO. 

Apart from one a bond and one a lone pair on each oxygen atom, the dioxygen molecule has six n electrons to be distributed in the two n planes, say Kx and Ky. The question is What is the most favorable mode of distribution Is it 25 in which three electrons are placed in each n plane, or perhaps is it 26 where two electrons are allocated to one plane and four to the other (Scheme 10) Obviously, 25 is a diradical structure displaying one three-electron bond... 

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