Bond networks with non-bipartite graphs

Hitherto it has been assumed that the bond graph is bipartite, i.e. bonds only occur between a cation and an anion with no cation-cation or anion-anion bonds present. While the majority of inorganic compounds have bipartite bond graphs, there are a few, such as mercurous and peroxy compounds, that contain homoionic bonds. It is easy to see that there can be no electric flux linking two cations or two anions, so the ionic model predicts that no bond will exist between them. 

However, we do not need to abandon the bond valence model for those few inorganic compounds which contain homoionic bonds since there are a number of ways of adapting the model depending on the nature of the structure. If the two cations or two anions that form the bond are equivalent by symmetry, as the two Hg cations are, for example, in the tetragonal crystals of Hg2Cl2 (65441, Fig. 3.4), the normal rules still apply. In this compound the two Hg  

However, if the atoms are not related by symmetry, the normal rules break down. The homoionic N-N bond in the hydrazinium ion is an electron pair bond, but one in which N1 contributes 1.25 and N2 0.75 electrons. How can we apply the bond valence model in such cases where no solution to the network equations is possible One approach is to isolate the non-bipartite portion of the graph into a complex pseudo-atom. Thus in the hydrazinium ion the homoionic bond and its two terminating N atoms are treated as a single pseudo-anion which forms six bonds with a valence sum equal to the formal charge of —4. 

The trifluoroacetate ion CF3CO2 (Fig. 3.5(a)) is similar (Brown 1980 ). F and are both anions so the two C atoms are both formally cations, each with a valence of +4. As before, we treat the C-C unit as a single pseudocation, reserving one electron pair for the C-C bond. It is instructive to compare this with the closely related acetate ion, [H C — whose 

A homoionic bond that needs a different treatment is the cation ation bond formed by a cation with a stereoactive lone electron pair, a situation modelled in more detail in Section 8.2. An example of this kind of bond is the Cu N bond 

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