Certain elements exist in more than one oxidation state and thus show variable valency. For example, transition metals show wide range of valence states, e.g., Fe2+, Fe3+; Cr2+, Cr3+ Co2+, CO3+, (more common), Co4+ and Co5+ (less common) etc. Similarly, normal metals, e.g., Pb2+ and Pb4+ ; Sn2+ and Sn4+ also show variable valencies. Certain non-metals are also found to show more than one valence states e.g., P3+ and P5+.
Variable valency may be due to different reasons and would be discussed over here accordingly.
In transition elements (elements in which d orbitals are in the process of completion) the variable valency is due to the partially filled ‘ d’ orbitals, state of hybridization and type of reactants (usually called ligands). Such elements can involve different number of electrons in compound formation and would show variable valencies. The stability of the particular valence state would also depend upon the nature of the reacting species and the number of d electrons present.
Iron (Fe) shows normally Fe2+ and Fe3+ valence states. Atomic number of Fe is 26 and its electronic configuration would be: 1s2 2s2 2p6 3s2 3p6 3d6 4s2.
Loss of two electrons from 4s orbitals would not leave behind an inert gas configuration. Therefore, more chances of electrons to be pulled out exist. Since `cl’ orbitals would be more stable when half-filled (with d5 configuration), so loss of one electron from 3d orbital along with two electrons from 4s orbitals would give a more stable state of the ion. Therefore, iron in Fe3+ state would be more stable. Let us show the valence shell having 3d orbitals and arrange the electrons in accordance with valence bond theory (See Chapter 4). In order to get next inert gas configuration more electrons would be required which are supplied by reacting species (Lewis bases or ligands) and the ions are thus stabilized. Mostly water will be acting as Lewis base and due to this reason most of the transition metal salts exist in stable state as hydrated species.
Elements of lanthanides and actinides show variable valencies due to the involvement of ‘f’ orbitals.
Some of the `1,’ block elements show variable valency due to the involvement of an ‘inert pair’ of electrons. The ‘inert pair’ of electrons are present in the ‘s’ orbitals and do not take part in chemical reactions under the prevailing conditions. Under such conditions only `p’ orbitals would take part in bond formation. However, if the ‘inert pair’ of electrons present in `s’ orbitals is actuated to take part in bond formation, an increase in valence state by two units will be observed. It is due to the `inert pair’ of electrons that the difference in valence states of such elements is always by two units.
Although the inert pair effect is quite marked for bivalent tin compounds but tetravalent tin is more stable of the two. Hence, bivalent tin is readily converted to tetravalent state, and thus the former ion is a reducing agent.
Sn(II) — 2e- Sn(IV)
The `13′ block elements which have vacant ‘cl’ orbitals available in their electronic configuration also show variable valency by utilizing these orbitals in addition to the corresponding ‘s’ and `ps’ orbitals. Thus the involvement of vacant ‘cr orbitals would be responsible for the variation in valence state of such elements. Let us consider the first two members of Vth group namely, nitrogen and phosphorus. There is no chance for the presence and involvement of `cr ‘orbitals in nitrogen as indicated by its electronic configuration (1s2 2s2 2p3) i.e., no ‘ar orbital is available in the 2nd shell.