Hypovalent three-center bonds

To this point, we have considered only default features of NBO analysis, those performed on every input species without keywords or other user intervention. In this section, we encounter the first of many keyword options that allow further exploration of specialized molecular species or properties. 

In this section, we wish to explore the bonding challenges presented by hypo valent ( electron deficient ) species such as diborane (B2H6) and other boron 

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. 

Such novel 3c/2e (three-center/two-electron) connectors allow two of the H s to form Tbhb bridges between boron atoms, while the remaining four H atoms are linked by ordinary 2c/2e ctbh bonds as depicted below  

Introduction of the 3c/2e r-bond as a novel structural element of Lewis structure diagrams allowed Lipscomb and others to successfully rationalize the bonding and geometry of many hypovalent species. 

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The NBO donor-acceptor theory of hypovalency three-center bridge bonds... 

Note that despite the superficial resemblance to the H-bridging geometry of diborane (B2H6, Section 4.4), the optimal NBO description of AI2CI6 reflects a quite different bonding pattern, with no appreciable three-center hypovalent bond character in the latter case. Thus, AI2CI6 provides a fairly direct intramolecular comparison between two-center cr bonds of distinct covalent versus coordinate character, with the latter bonds responsible for the unusual four-membered ring motif. 

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