Valence bond theory multiple bonding in polyatomic molecules

3 Valence bond theory multiple bonding in polyatomic molecules 

In the previous section, we emphasized that hybridization of some or all of the valence atomic orbitals of the central atom in an XY species provided a scheme for describing the X—Y f7-bonding. In, for example, the formation of sp, sp and sp d hybrid orbitals, some p ox d atomic orbitals remain unhybridized and, if appropriate, may participate in the formation of TT-bonds. In this section we use the examples of C2H4, HCN and BF3 to illustrate how multiple bonds in polyatomic molecules are treated within VB theory. Before considering the bonding in any molecule, the ground state electronic configurations of the atoms involved should be noted. 

Worked example 5.2 Valence bond treatment of the bonding in [NO3] 

There is an additional electron from the negative charge giving a total of 24 valence electrons. 

Both N and O obey the octet rule and so the most important resonance forms are expected to be  

Ethene, C2H4, is a planar molecule (Fig. 5.8a) with C—C—H and H—C—H bond angles of 121.3° and 117.4° respectively. Thus, each C centre is approximately trigonal planar and the cr-bonding framework within C2H4 can be described in terms of an sp hybridization scheme (Fig. 5.8b). The three 

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LCAO-MO theory is specially appropriate for describing polyatomic molecules in which multiple bonds occur. Strict application of the theory often demands very complex calculations in which all the valence orbitals in a field formed by the nuclei and the rest of the electrons should be considered. However there are some simplifications that make its application easier. 

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