The Formation of Complexes in Solution
It is implied above that all the constituent species from M and L to MLn may exist together in solution.
The solution composition will depend on the nature of M and L and the amounts present together with
the relative values of Kf. As a general rule Kf values show a steady decrease as shown in the Cd2+ + CN–
system (Table 3.7). This steady decrease is brought about by three major factors: statistical; coulombic;
steric. Once the first ligand group is attached the next stage will decrease in probability because there
are less sites available. Furthermore, the positive charge characteristically present on the metal atom
will be reduced, and with it the coulombic attraction for the ligand. This may even be converted to a
repulsion as subsequent ligand groups are attached. The third factor concerns the bulky nature of many
ligands which will place a steric restriction on the reaction. This restriction reaches its ultimate in the
EDTA type of ligand (p. 205); which forms a cage around the acceptor atom and prevents the
attachment of any further ligands, irrespective of the nature of the acceptor ion (Figure 5.4).
Table 3.7
MLn Kf/mol–^1 dm^3Cd(CN)+ 5.0 × 105
Cd(CN) 2 1.3 × 1054.3 ×^1043.5 ×^103The effect of this variation in values of Kf can be seen by returning to the Cd2+ + CN– system.
Commonly a ligand is added to a solution of the acceptor ion, first forming ML then ML 2 , ML 3 and
finally ML 4 , each becoming the predominant species in turn until ML 4 is formed to the exclusion of all
others when an excess of the ligand has been added. Figure 3.2 illustrates this pattern graphically.
The Chelate Effect
Many ligand molecules contain more than one donating centre and are called multidentate ligands. If
two or more dative links are formed with the same acceptor ion, a ring compound is produced.
Compounds of this type