Non-valence Compounds and Homoionic Bonds

The network equations (14a) and (14b) can only be used if the graph of the bond network is bipartite, that is, if every bond has a cation at one end and an anion at the other. In inorganic compounds, and particularly in organic compounds, this condition is not always satisfied. Although this restricts the application of the bond valence theory, the core-and-valence-shell picture of the atom is still valid, as is the description of the chemical bonds this picture gives. 

There are a variety of ways in which a non-bipartite bond graph can be converted to a bipartite graph, although some information is usually lost along the way. 

The two cations, or two anions, that form the bond can be considered as a single pseudo-atom. This makes the bond graph bipartite. This solution works well for cations like Hg2 which are traditionally described in this way. In the mercurous cation, the Hg-Hg bond is formed by an electron pair and has a valence of 1.0 vu. If the Hg2 cation is in an asymmetric environment, the valence of the external bonds formed by the individual Hg atoms may not be the same, in which case the two mercury atoms may contribute different numbers of electrons to the Hg-Hg bond. Although this violates the equal contribution mle (3), the valence of the bond is correctly given by the average of the contributions of the two mercury atoms. Hg-Hg bonds are known in a number of mercury complexes, and not all of these are electron pair bonds, but as expected, the length of the bond is found to correlate with its valence (the number of electron pairs that form it) in the same way as any other bond [26]. 

An alternative approach to the mercurous cation is to treat the bonding electron pair as a pseudo-anion, Inserting such an anion into the bond again makes the graph of the bond bipartite and does not require that the two Hg-E bonds have the same valence. Since it is not possible to locate the pseudo-anion, the individual valences of the Hg-E bonds cannot be found from their bond lengths. 

With the N-O bonds having a valence of 1.7 vu, the residual valence oti each of the oxygen atoms is reduced from 0.5 vu in the free nitrite ion to 0.3 vu and the residual valence of the nitrite group as a whole is then 

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