Three-center, four-electron bonding interactions

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. 

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. 

The transannular interaction in germatranes is explained by the model of hypervalency that assumes the formation of a three-center four-electron N Ge—X hypervalent bond . This model successfully explains the fraw -inhuence in such fragments, i.e. the inverse interdependence and the constancy of the sum of Ge—X and Ge—N bond orders. The calculated energies of the transannular bonds in some germatranes are higher than those in the corresponding silatranes . The three-center N Ge—X bond has a predominantly a-nature with a minor contribution from jt-interaction of the 4d-orbitals of the Ge atom. 

Each Be atom is connected to four H atoms, and each H atom is connected to two Be atoms. Since Be has two valence electrons, and H only one, obviously there are insufficient electrons to fulfill the traditional organic two-center, two-electron bonding description. Alternatively, three-center interactions persist, whereby banana-shaped molecular orbitals (a three-center, two-electron bond, see Three-center Bond) connect the Be-H-Be atoms, each containing two electrons (see Figure 2 for the molecular orbital scheme see Molecular Orbitals) for this type of Delocalized... 

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