The bonding between the metal and the ligands and also the properties of complexes are explained by some theories.
EARLY STRUCTURAL THEORIES
Blomstrand-Jorgensen's Chain Theory
This theory is of only historical importance. Blomstrand along with his student Jorgensen, developed the chain theory in order to explain the existence of metal complexes. At that time, there was a belief that elements had only one kind of valence. Hence, considering CoCl3.6NH3, they proposed that there could be only three bonds to cobalt(III) in its complexes. As a consequence, a chain structure (a) was suggested to account for the additional six ammonia molecules in CoCl3.6NH3.
Since the three chlorides are separated by some distance from cobalt, they are assumed to be less tightly bound to cobalt. Hence all the three chlorides are readily precipitated as AgCl on the addition of AgNO3 solution. This is in agreement with the experimental fact. On the basis theory, CoCl3.5NH3 is represented as (b).
In this structure, the chloride which is directly attached to cobalt is very tightly bound to it. Hence only 2/3 of the chloride content of the complex should get precipitated as AgCl on treatment with AgNO3 solution. This is also in agreement with experimental facts. Structure (c) for CoCl3.4NH3 is also in accord with experiments which show that two of the chlorides are more tightly held than the third one. Blomstrand and Jorgensen did not succeed in preparing the next member of this series, namely, CoCl3.3NH3.
However, they were able to prepare the analogous iridium complex, IrCl3.3NH3 (d).
The chloride that is not directly linked to iridium should less tighlty bound to the central metal according to this theory. This means that 1/3 of its chloride content should be precipitated as AgCl on the addition of AgNO3 solution. But a solution of this compound did not give any precipitate of AgCl. Thus, this chain theory was proved to be incorrect in this case and also in several other metal complexes.
