Homoionic bond

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. 

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... 

In most of these cases the homoionic bond can be assigned a valence, but this does not always correlate with bond length as the examples of the trifluoroacetate ion and S-bonded dmso show. However, a correlation is expected for Cu(N02)g and has been found for Hg-Hg bonds. In addition to the well-known mercurous ion (Hg ), cations such as ngj" ", Hg4+, (Hg +) (infinite chains), and (Hg ) (infinite sheets) are also known. The Hg-Hg bonds in these cations show a considerable variation in length which correlates well with the bond valence, as shown in Fig. 3.8 (Brown et al. 1984). 

A bond network with this property is said to have a bipartite graph, and a corollary to this definition is that the bond network of a valence compound crmtains only even-membered rings since an odd-membered ring necessarily contains a homoionic bond. 

Heteroionic bond A bond formed between a cation and an anion Homoionic bond A bond formed between two cations or between two anions Ideal bond valences Bond valences predicted for a valence compound using the network equations... 

An alternative, widely used, approach to charge assisted networks is based on the exploitation of direct acid-base reactions. There are, broadly speaking, essentially two different means to obtain charge-assisted interactions, which depend on whether the network is constructed of ions of the same charge (homoionic hydrogen bonded networks) or of ions of opposite charge (hetero-ionic networks). These two limiting situations are shown in Fig. lc,d. The utilization of... 

It should be noted, however, that the distinction between the two strategies is only instrumental to the discussion, since intermediate situations whereby homoionic and heteroionic hydrogen-bonding interactions may co-exist, for example, and —0 can be obtained by means of a large acid base ratio. 

Although the itMiic model is often thought to apply orfly to compounds composed of ionic bonds. Sect 5 of [1] shows that its application is almost universal only homoionic and delocalized bonds are excluded. The ionic model describes covalent and ionic bonds without distinction because the electrostatic flux depends only on the number of valence electrons and not on whether the bonding electrons lie closer to the anion or cation, or somewhere in between. It has been customary to label strong bmids as covalent, but attempts to define a covalent bond quickly run into problems. 

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