Three center four electron bond model

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. 

Of course, the octet is usually not actually violated. Multicenter bonding models require some MOs that are essentially nonbonding and concentrated only on the substituents, and thus, the number of electrons in the valence shell of the central atom rarely exceeds the octet. However, here we should distinguish, between what Musher [61] more than 40 years ago termed hypervalent compounds of first and second kind, respectively. In the first class, the central atom is not in its maximum oxidation state, and thus, the central-atom s-character concentrates in a Ip. Then, as we have discussed in detail above, the bonds are made mainly from np-orbitals of the central atom, and thus, the assumptions of the usual three-center-four-electron bonding models are nicely fulfilled. In contrast, hypervalent compounds of the second kind exhibit the maximum oxidation state and, thus, necessarily involve the ns-orbitals fuUy in bonding. One thus sees (i) extensive hybridization defects... 

Let us compare the MO descriptions of six and four sep R4E4 tetrahedral clusters. In Figure 2.5 the cluster MO energies for E = C and E = Ga are shown alongside the localized descriptions discussed above. The number of filled and unfilled MOs is equal for E = C a consequence of the two-center-two-electron bond model. Conversely, the number of filled MOs is less than the number of empty MOs for E = Ga a consequence of the three-center-two-electron bond model. Note also that an e symmetry pair of orbitals lies in between lower-energy filled orbitals and... 

Finally, as a further contribution to understanding of bonding in hypervalent molecules. Sun has offered an alternative model of bonding in hexacoordinated molecules, eg SFg and PFej", which does not involve d-orbital participation but employs the concept of the three center, four electron bond. The model was supported by the use of a partial charge analysis using Allen s electronegativity approach. ... 

Models to describe them extend from a simple 4p 4d 5s hybridization scheme, with the inert pair shielding the bonding electrons, up to a molecular orbital-based model of three orthogonal three-center-four-electron bonds. 

Ponec, R., Yuzhakov, G., and Cooper, D.L. (2004) Multicenter bonding and the structure of electron rich molecules. Model of the three-center four-electron bonding reconsidered. Theor. Chem. 

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,... 

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,... 

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. 

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