Three center-four electron bond

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. 

Answer. As a crude approximation, let the Xe atom in XeF2 use one rip AO to form one three-center-four-electron bond with one p orbital from each of the F atoms. The other nine ns and np AOs contain 18 of the 22 valence electrons as lone pairs. Figure 1.14 with the remaining four electrons occupying the bonding and non-bonding three-center orbitals applies. Thus, at the Xe center, we have three... 

In the vast majority of cases in which six coordination is observed, the bonding can be viewed as arising from the interaction of all three cr -orbitals with a halide anion, i.e., all three in S. Because the three orbitals are all trans to the primary E-X bonds, such a situation leads naturally to octahedral coordination. Moreover, in cases in which the primary and secondary bonds are the same length, i.e., where A = 0 and a three-center, four-electron bonding model is appropriate, a regular octahedron is the result. Such a structure is clearly at odds with simple VSEPR theory, which is predicated on the lone pair(s) occupying specific stereochemical sites, but stereochemical inactivity of the lone pair tends to be the rule rather than the exception in six-coordinate, seven-electron pair systems Ng and Zuckerman (102) have reviewed this topic for p-block compounds in general. 

Hypervalent bonds in compounds of hexacoordinated silicon are formed in a similar fashion. Thus, in the anions of the SiF and SiFg type there are two three-center four-electron bonds ... 

Figure 5 Molecular orbital description of a three-center, four-electron bonding arrangement...

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