Three-center, four-electron

It is difficult to give a localized orbital description of the bonding in a period 3 hypervalent molecule that is based only on the central atom 3s and 3p orbitals and the ligand orbitals, that is, a description that is consistent with the octet rule. One attempt to do this postulated a new type of bond called a three-center, four-electron (3c,4e) bond. We discuss this type of bond in Box 9.2, where we show that it is not a particularly useful concept. Pauling introduced another way to describe the bonding in these molecules, namely, in terms of resonance structures such as 3 and 4 in which there are only four covalent bonds. The implication of this description is that since there are only four cova-... 

The Three-Center, Four-Electron (3c, 4e) Bond Model... 

The difference in the axial and equatorial bond lengths has been discussed in terms of the three-center, four-electron model (Box 9.2), but this model was postulated on the basis of the known difference in axial and equatorial bond lengths and so does not provide an explanation of this difference in bond lengths,... 

However, a different view of the bonding in XeF2 is provided by a molecular orbital approach in which a p orbital on Xe combines with a p orbital from each F to form a three-center, four-electron linear bond. Actually, the three atomic orbitals form three molecular orbitals, but only the bonding and nonbonding orbitals are populated. This population of the orbitals places nonbonding electron density on the F atoms to give polar Xe-F bonds. 

Pimentel employed this three-center, four-electron (3c/4e) MO model to discuss the bonding in triiodide (I3-), bifluoride (FHF-), and other prototypical hypervalent species. In triiodide and other trihalides, for example, the relevant AOs are the (pa, Pb, Pc) orbitals along the bonding axis,... 

Figure 17. Alternative valence bond pictures of the catalyst-enamide adduct Only one of the four three center-four electron resonance structures is shown (far right). Electron withdrawing substituents prefer the basal position of the square pyramidal metallocyclopropane structure, while electron donating substituents prefer the axial position.

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). ... 

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... 

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. 

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. 

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