Figure 3.2
The relation between solution composition and the
amount of cyanide added in the Cd2+ + CN– system.are known as chelates and the ligands are chelating agents. A typical example is ethylenediamine which
reacts with many metal ions to form stable 5-membered ring compounds,
Other important examples are 'oxine' and EDTA. Almost invariably, the formation of a ring system
leads to a large increase in stability relative to the comparable non-ring compound. Thus Ni(II)
ethylenediamine [Nien]2+ may be compared with [Ni(NH 3 ) 2 ]2+. For each, the number of ligand-acceptor
bonds is the same, and furthermore, both involve nitrogen donor groups. It is tempting to compare the
stability quantitatively by the stability constants β which are 107.51 and 105.00 respectively. However,
such a comparison is not valid because of the different dimensions of the two constants and such
stability comparisons can only be made in a loose sort of way. To rationalize the experimentally
observed stability it is necessary to consider the formation reaction in a slightly more detailed way.
Stage 1 of this reaction involves the breaking of Ni—H 2 O bonds in the [Ni(H 2 O) 6 ]2+ species and stage 2
the formation of Ni—N bonds. Both stages are similar in chelate and non-chelate systems so it is
unlikely that or will be significantly different. Comparisons of between the two
formation/reactions, however, show differences, as equations (3.32) and (3.33) illustrate.
In the first equation the disorder remains the same, whilst in the second case it increases, and is
accompanied by an entropy increase, i.e. ∆S becomes