Bond properties three-center

Boranes are typical species with electron-deficient bonds, where a chemical bond has more centers than electrons. The smallest molecule showing this property is diborane. Each of the two B-H-B bonds (shown in Figure 2-60a) contains only two electrons, while the molecular orbital extends over three atoms. A correct representation has to represent the delocalization of the two electrons over three atom centers as shown in Figure 2-60b. Figure 2-60c shows another type of electron-deficient bond. In boron cage compounds, boron-boron bonds share their electron pair with the unoccupied atom orbital of a third boron atom [86]. These types of bonds cannot be accommodated in a single VB model of two-electron/ two-centered bonds. 

Hydrogen bonds may be considered special types of 3c/4e interactions, closely related to other forms of hypervalency in main-group (Section 3.5) and d-block (Section 4.6) compounds. However, the fundamental nB— oah interaction of B - HA hydrogen bonding displays unusual characteristics compared with other three-center MO phenomena, due mainly to the unique properties of the H atom, whose valence shell contains only the isotropic Is orbital for construction of ctah and ctah NBOs. 

A wide range of thermochemical properties can be measured, including not only proton affinity or gas-phase basicity, but also electron affinity, ionization energy, gas-phase acidity and cation affinity Entropy changes upon attachment of an ion to a molecule are also accessible and provide information on both the nature of the bonding and fragmentation mechanisms in cluster ions, especially in biological compounds. Thermochemical determinations by the kinetic method also provide very useful structural information e.g., two-electron three-center bond has been observed in the gas phase by means of the kinetic method. " In the last years, the kinetic method has been also applied to characterize chiral ions in the gas phase. 

These three-center bonds are invariably unsymmetrical with one strong bond (H 0, 1.9 to 2.4 A N-ft A, 145-170°) and one weak bond (H 0, 2.4 to 2.8 A N-ft A, 105-135 °). These unsymmetrical chelated structures are more a property of the molecular configuration and crystal packing than of the hydrogen bonds. In the absence of crystal packing forces, i.e., in solution, we doubt whether such chelation would be energetically favorable. 

Three-center (bifurcated) hydrogen bonding between adjacent base pairs explains unusual properties of poly(dA) poly(dT). The stability of the B-form of this duplex (and of its ribo analog) formed by two homopolymers might be a consequence of the extreme propeller twist (Box 20.1) reported for the central portions (the A-tracts) of the double helical dodecamer d(CGCAAAAAAGCG) [702] and d(CGCAAATTTGCG) [703]. In both crystal structures the DNA molecules show the same unusual three-center bonds linking adjacent base pairs. 

The VSEPR model works at its best in rationalizing ground state stereochemistry but does not attempt to indicate a more precise electron distribution. The molecular orbital theory based on 3s and 3p orbitals only is also compatible with a relative weakening of the axial bonds. Use of a simple Hiickel MO model, which considers only CT orbitals in the valence shell and totally neglects explicit electron repulsions can be invoked to interpret the same experimental results. It was demonstrated that the electron-rich three-center bonding model could explain the trends observed in five-coordinate speciesVarious MO models of electronic structure have been proposed to predict the shapes and other properties of non-transition element... 

The Ugand-to-metal bonding represented by 34, of course, differs from that represented by 30. In 34, the cyclobutadiene ligand is shared by two metal centers, thus reflecting the bonding properties of a three-centered bond. There will, therefore, be an extra set of orbitals corresponding to those in Fig. 7, which are nonbonding in character. 

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