Electroneutrality principle

    Pauling’s electroneutrality principle is an approximate method of estimating the charge distribution in molecules and complex ions. It states that the distribution of charge in a molecule or ion is such that the charge on any single atom is within the range ‏1 to ^_1 (ideally close to zero).

    Let us consider the complex ion [Co(NH₃(₆]³⁺‏. Figure 1.1a gives a representation of the complex which indicates that the coordinate bonds are formed by lone pair donation from the ligands to the Co(III) centre. It implies transfer of charge from ligand to metal, and Figure 1.1b shows the resulting charge distribution. This is clearly unrealistic, since the cobalt(III) centre becomes more negatively charged than would be favourable given its electropositive nature.

Fig. 1.1 The complex cation [Co(NH₃)₆]^3‏+: (a) a conventional diagram showing the donation of lone pairs of electrons from ligands to metal ion; (b) the charge distribution that results from a 100% covalent model of the bonding; (c) the charge distribution that results from a 100% ionic model of the bonding; and (d) the approximate charge distribution that results from applying the electroneutrality principle.

   At the other extreme, we could consider the bonding in terms of a wholly ionic model (Figure 1.1c): the 3‏+ charge remains localized on the cobalt ion and the six NH₃ ligands remain neutral. However, this model is also flawed; experimental evidence shows that the [Co(NH₃)₆]^3+‏ complex ion remains as an entity in aqueous solution, and the electrostatic interactions implied by the ionic model are unlikely to be strong enough to allow this to happen. Thus, neither of the extreme bonding models is appropriate. If we now apply the electroneutrality principle to [Co(NH₃)₆]³⁺‏, then, ideally, the net charge on the metal centre should be zero. That is, the Co³⁺‏ ion may accept a total of only three electrons from the six ligands, thus giving the charge distribution shown in Figure 1.1d.

Table 1.1 Oxidation states of the d-block metals; the most stable states are marked in blue. Tabulation of zero oxidation states refers to their appearance in compounds of the metal. In organometallic compounds, oxidation states of less than zero are encountered. An oxidation state enclosed in [ ] is rare.

The electroneutrality principle results in a bonding description for the [Co(NH₃)₆]³⁺‏ ion which is 50% ionic (or 50% covalent).