Four-electron bond

A common interpretation of the interaction of chalcogens with nucleophiles considers donation of electron density from a lone pair on the donor atom into the o- (E-X) orbital (Figure 15.1). As the degree of covalency increases, a hypervalent three-centre four-electron bond is formed. Real systems fall somewhere between secondary interactions and hypervalent (three centre - four electron) bonds. The two extremes can be distinguished by the correlation of X-E and E D distances.In the hypervalent case both bond distances decrease simultaneously, whereas in the secondary bond the distances are anticorrelated. This concept has been applied in a study of selenoquinones 15.17 (R = Ph, Me) with short Se 0 contacts,for... 

Thus there are five bonding electrons giving a bond order of 2.5, consistent with the bond length of 115 pm, versus 121 pm for the four-electron bond in O2 and 110 pm for the six-electron bond in N2. For these and other related molecules, the double-quartet model is a convenient and useful alternative to the conventional molecular orbital model. Moreover, it shows that for a singly bonded terminal atom such as F or Cl there is a ring of six nonbonding electrons rather than three separate lone pairs. As we will see in Chapters 7 and 8, this conclusion is confirmed by the analysis of electron density distributions. 

In the tellurium(IV) oxidation state, the 5p orbital of the tellurium(fV) atom is involved in the three-center, four-electron bonding to the halides and cannot interact with the carbon rr-framework. Long-wavelength absorption maxima for 6-11 are found between 515 and 565 nm in dichlromethane. Reductive elimination generates the tellurium(II) oxidation state in compounds 12 and 13. The tellurium(II) 5p orbital can now overlap with the carbon 7r-framework and long-wavelength absorption maxima for 12 and 13 are observed between 765 and 830 nm in dichloromethane. The rate of loss of the 515-565 nm band and the rate of appearance of the 765-830 nm band are identical, which is consistent with reductive elimination as the rate-determining step of this process. 

Organotellurium(II) compounds can also contain a three-center, four-electron bond as shown for 39 to 42 in Fig. 19. Typically, these molecules contain an odd number ligands around the central atom and an electronegative atom helps to stabilize a tellurenyl halide or selenenyl halide bond through chelation to form a four, five, or six membered ring. " Such molecules are described as lO-Te-3 and lO-Se-3... 

The corresponding selenium(IV) derivatives are less stabilized by the three-center, four-electron bonds in the chalcogen(rV) complex than the tellurium(lV) derivatives. As a consequence, the selenium(lV) derivative is more easily reduced than the tellurium(lV) derivative. Diphenylselenium(lV) dibromide (1) undergoes a two-electron reduction with Ep at - - 0.40 V (vs. SCE) while diphenyltellurium(lV) dibromide (2) undergoes a two-electron reduction with at-1-0.05 V (vs. SCE). ... 

FIGURE 7. Molecular orbitals for a three-centre, four electron bond (a) antibonding (n) non-bonding (b) bonding... 

Recent detailed discussions229,283,284,286,289,290 of the bonding in phosphoranes has usually taken for granted the three-centre, four-electron bond model described above and have concentrated on other related issues of importance in these systems, viz. apicophilicity, pseudorotation and the oxaphosphetanes involved in the Wittig reaction. 

Two points of view are applicable to these species, as they also are to the isoelectronic noble gas fluorides (1) a valence bond approach with promotion of electrons to d orbitals and (2) three-center, four-electron bonds. The same arguments, pro and con. apply as given previously, so they will not be repeated here. Independent of die alternative approaches via VB or MO theory, all are agreed that Madelung energy ( ionic character ) is very important in stabilizing both the polyhalide tons and the polyhalogens.27... 

Show how UieFHF ion (Chapter 8) can be treated as a ihree-center. four-electron bond. 

The homologous series known as alkenes is also called unsaturated hydrocarbons or olefins. The general formula for the alkene family is CnH2 . The distinguishing feature of the alkene structure is the carbon-carbon double bond which, as we have discussed previously, is a four-electron bond formed by the sharing of two electrons from each of two carbon atoms. 

It should be noted that the q2-C=N, 2-C—N (four electron) bonding mode has been proposed for Fe(CO)3[Me2CN(Me)C(Me)C(Me)N(Me)] in which the N atoms are coupled together via a CMe2 bridge.122... 

Five-Center Four-Electron Bonding Structures. The potential for the existence of 5-center 4-electron (5c-Ae) bonding structures 558 have recently been surveyed by Tantillo and Hoffmann980 [calculations at the B3LYP/6-31G(d) level], A cation with three anthracenyl units joined around the C -H-C -H--C core with two approximately trigonal pyramidal carbon atoms and one five-coordinate trigonal bipyramidal carbon was found to have 5c-Ae bonding. The anticipated existence of... 

The bonding in I5+ (and in Br5+) can be described in terms of valence bond structures 325a and 325b showing a formal bond order of 1 for the terminal I—I bonds.788 The bond order of the central bonds is 0.5, and these bonds may be considered as three-center four-electron bonds. 

In translating this MO description into VB language, we speak of a three-centre, four-electron bond, often abbreviated as (3c,4e) three orbitals rather than four are used to hold the three atoms together. An equivalent description in VB language involves resonance structures ... 

The presence of a hypervalent Te—O bond that is part of the three center-four electron bond (3c-4e) in the linear Cl—Te—O fragment does not markedly affect the reactivity of the 1,2-oxatellurolyl-l-ium chlorides 21, 25, which behave as expected of true tellurenyi halides. Thus, on treatment of 21 with LiBr, Nal, and CF3COOAg (83JA875) substitution of chloride by bromide, iodide, or trifluoroacetate is readily achieved. When AgBF4 is used 1,2-oxatellurolyl-l-ium fluorides are obtained. 

XeF4 is square planar as expected for a molecule with four electron bonding pairs and two unshared electron pairs on Xe. Crystals of XeF4 are monoclinic (P2i/n) with two molecules per cells (a = 5.05, b = 5.92, c = 5.77 A, and (3 = 99.6°). It is a body-centered cell and the notation is 3 2PTOT(t). 

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