Valence-Bond Structures and Quinquevalent Nitrogen Atoms

For a number of electron-excess molecules that involve atoms of first-row elements (in particular, nitrogen atoms), an older type of increased-valence structure is sometimes used to represent their electronic structures. Since the 1860s and until the introduction of the Lewis-Langmuir octet theory, nitrogen atoms were often represented with valencies of 3 or 5 in valence-bond structures. For example, valence-bond structures for N2O, MCjNO, and N2O4 were written as structures (l)-(3). 

Using structure (1) for N2O, with an N-N triple bond and an N-0 double bond, we would be let to predict that the bond-lengths are similar to the 1.10 A and 1.21 A for N2 and CH3N=0. The experimental lengths of 1.13 A and 1.19 A confirm this expectation (Section 2-3 (b)), and on this basis structure (1) is a suitable valence-bond structure for N2O. But the Lewis theory, with electron-pair bonds, does not permit a valence of five for first-row atoms, provided that only the 2s and three 2p orbitals of these atoms are valence orbitals for bonding. Therefore, in the Lewis theoiy, the quinquevalent structures are replaced by octet stractures such as structures (4)-(6) for N2O, and (7) and (8) (together with equivalent resonance forms) for MesNO and N2O4. 

Chapter 16 Classical Valence-Bond Structures and Quinquevalent Nitrogen Atoms 

In these latter structures, the valencies of N , N, and hT are 2, 3 and 4, respectively. We may note that each of the structures (1), (2) and (3) seems to have one more bond than have the corresponding Lewis structures, and therefore we might also designate structures (1), (2) and (3) as increased-valence structures. Alternatively, we may say that the quinquevalent nitrogen atom has increased its valence relative to the maximum of four which is allowed in the Lewis theory. Sometimes, the valence-bond structures such as (1), (2) and (3) are designated as classical valence structures , and we shall refer to them as such here. 

Although the use of octet structures such as (5)-(8) is extremely widespread, it is by no means universal Sometimes, the classical valence structures are used to account for certain empirical information, and the quantum mechanical basis for them is not discussed, i.e. it is not suggested how the nitrogen atom forms five covalent bonds. However, there have been three major attempts to explain how a nitrogen atom (or other first-row atoms - in particular, a carbon atom) may acquire an apparent valence of five, and we shall describe them briefly here. 

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If in structure (9), y and b are the carbon 2p atomic orbitals, and a is the nitrogen 2p atomic orbital, then from them we may construct the two-centre bond-orbitals ( )l =y+ a and ( )r =b-i- a with A > 0. We may locate the four N-C-N -electrons of structure (10) into these two orbitals, to afford the bond-orbital configuration (y -i-A a) (b-i-A a). It is satisfactory to do this, provided it is recognised that the two bond-orbitals are not orthogonal, and therefore, when they are both donbly occnpied, they cannot represent two separate, independent C-N %-bonds. Therefore for this theory, the nitrogen atom does not have a true valence of five - it is only apparently quinquevalent... 

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