Three-center bonds concept

THE THREE-CENTER BOND CONCEPT TYPES OF THREE-CENTER BONDS... 

In order to explain the hydrogen bridges in diborane and their apparent electron deficiencies, the concept of three-center bonding, in which one pair of electrons links three atoms, was introduced by Hugh C. Longuet-Higgins and Ronald P. Bell. Lipscomb and co-workers extended the concept to multicenter bonds in general. 

The generalized concept of carbocations and electrophilic reactions indicates that the initial interactions of electrophiles with 7r-donor systems (olefins, acetylenes, aromatics) involves three-centered bond carbonium ion formation. The 7r-bond provides the bonding electron pair which interacts with the empty orbital of the electrophile. Thus in principle there is no difference between the electrophilic reactivity of... 

The concept of bridged ions provides an alternative explanation of stereospecificity. The equilibrating pair of carbocations, (2) (2), is replaced by a o-delocalized ( nonclassical ) intermediate (10). A two-electron, three-center bond is characteristic of (10). Other compounds have structures which indicate delocalized o bonds. Well known examples are diborane and the dimer of trimethylaluminum. Because of... 

This concept of Kekule structures can be extended to the 3D deltahedral borane anionsB H " (6Kekule structures make use of three-center B-B-B bonds instead of the carbon-carbon double bonds in benzenoid Kekule structures. Lipscomb s semitopological method"" " for studying the electron and orbital balance in boron networks containing mixtures of B-B two-center and B-B-B three-center bonds is essential for extending the concept of Kekule structures from 2D benzenoid hydrocarbons to 3D deltahedral boranes. 

Finally, the three-center bond MO theory together with the differential orbital utilization concept prove valuable in rationalizing acceptor behavior of organotellurium trihalides (39). Direct extension of the theory to sulfur and selenium analogs is possible (though space limitations does not permit this here) and provides structural predictions for possible adducts and intermediates. 

HF and HSO3F (or other superacid) solutions (see subsequent discussion). Higher alkanes and isoalkanes gave yields of 5-10% and adamantane was nitrated in 30% yield. Data indicate that nitratioa (nitrolysis) of alkanes with nitronium salts proceeds in accordance with the generalized concept of electrophilic reactions of single bonds [77] involving two-electron, three-center bond carbocationic intermediates (transition states) as illustrated with case of the nitration of methane. 

Many people make the mistake of equating the study of three-center bonds to the study of the boron hydrides. Certainly, the Colonel was very successful in applying 3-center bond concepts to boron hydrides. However, we should see the larger applicability of the general concept to noble gas compounds, localized orbitals for species such as COa, metals, metal- cluster compounds, etc. Someplace between the localized 2e-2c bonds beginning students (and some elderly chemists) love and cherish, and the fully-delocalized molecular orbitals we so often find useful, lies a localized multicenter bond. This approach to localized bonds looks like it will be very useful to chemists when it is more fiilly developed. They have some resemblance to the intuitive "best" valance structure selected from resonance hybrids. One should be able to do chemistry with these orbitals. Fractional bonds may be uncomfortable at first, but note that the octet rule applies to electrons, not orbitals. One of the most inqjortant contributions... 

In both compounds each carbon atom appears to be bonded to five instead of the customary four atoms. To circumvent this anomaly the concept of carbon participating in delocalized two- and three-center bonds is invoked (77, 316). From the H NMR a shielding constant of about 1.1 has been determined for the l,6-dicarbaclovohexaboranyl(6) group bonded at a skeletal carbon atom 315). 

The resolution of this bonding paradox was achieved with the three-center bond (r-bond) concept, symbolically represented by a Y-bond connector between TABC bonded atoms A, B, C, namely. 

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-... 

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