Bond properties hypervalent

Three basis sets (minimal s-p, extended s-p and minimal s-p with d functions on the second row atoms) are used to calculate geometries and binding energies of 24 molecules containing second row atoms, d functions are found to be essential in the description of both properties for hypervalent molecules and to be important in the calculations of two-heavy-atom bond lengths even for molecules of normal valence. 

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. 

The exceptions to the octet rule described in the previous section, the xenon compounds and the tri-iodide ion, are dealt with by the VSEPR and valence bond theories by assuming that the lowest energy available d orbitals participate in the bonding. This occurs for all main group compounds in which the central atom forms more than four formal covalent bonds, and is collectively known as hypervalence, resulting from the expansion of the valence shell This is referred to in later sections of the book, and the molecular orbital approach is compared with the valence bond theory to show that d orbital participation is unnecessary in some cases. It is essential to note that d orbital participation in bonding of the central atom is dependent upon the symmetry properties of individual compounds and the d orbitals. 

In applying an electron-pair bond model for the hypervalent molecules studied, the (apical, equatorial) and (equatorial, equatorial) isomers, namely 23-eq and 23b, respectively, are both local minima. This means that the tetracoordinated species are less strained than the pentacoordinated ones, due to less antibonding properties of the electron delocalization between the geminal ring bonds on the hypervalent sulfur atom. Yet, the more strained (eq, eq) isomers are, surprisingly, more stable than the (ap, eq) isomers in most of the hypervalent three-membered rings studied <2001PCA10711>. 

The properties of the formal double bonds in the hypervalent compounds H3ASO and... 

The year has seen a considerable extension of the principles of hypervalent phosphorus chemistry into the fields of other elements notably Si, Sn, Ge and Sb and in this context the well-known Martin ligand has proved to be especially valuable. Particularly interesting contributions have been provided by Ju et al on the relative rates of reaction of pentoses and hexoses with pentacoordinate phosphorus, by Bentrude et on the mechanism of the reaction of hy-dridophosphoranes with dimethyl disulfide and by Kawashima et alP on the isolation of two carbaphosphatranes containing covalent P-C bonds. Holmes et al report on the fluxional properties of propeller shaped phosphoranes and Buono et have also demonstrated the utility of hydridophosphoranes in some highly diastereoselective reactions with isocyanates. In summary, although the number of reports was small, the year was noteworthy for some very sophisticated contributions to the field. 

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