The coordination compounds were known since 18th century but no satisfactory theory was available to explain the observed properties of these compounds. Alfred Werner in 1893 put forward his concept of auxillary (secondary) valency for advancing a correct explanation for the characteristics of the coordination compounds. The fundamental postulates of Werner's theory which was actually a result of dream and made Werner a nobel prize winner in chemistry may be summarised as below.
1. Metals possess two types of valencies, namely primary (principal) or ionizable valency and secondary (auxillary) or nonionizable valency.
Primary valencies are those which a metal normally exhibit in the formation of its simple salts.
Thus in the formaton of \(\mathrm{PtCl}_4, \mathrm{CuSO}_4\) and AgCl the primary valencies of \(\mathrm{Pt}, \mathrm{Cu}\) and Ag are 4, 2 and 1 respectively. Primary valencies are satisfied by negative ions.
Secondary valencies are those which a metal cation exercises towards a neutral molecule or negative group in the formation of its complex ions. Thus secondary valencies may be satisfied by negative ions, neutral molecules having lone electron pair (e.g. \(\mathrm{H}_2 \mathrm{O}, \mathrm{NH}_3,\) etc.) or even sometimes by some positive groups. In every case, the coordination number of the metal must be fulfilled.
In modern terminology the primary valency corresponds to oxidation number and the secondary to coordination number. Primary valencies are shown by dotted lines while secondary by thick lines.
2. Every metal has a fixed number of secondary valencies, e.g. cobalt \({ }^{3+}\) and platinum \({ }^{4+}\) were recognized to have six secondary valencies and copper\({ }^{2+}\) four. The total number of secondary valencies required by a metal is more commonly known as coordination number (C.N.).
3. The secondary valencies are always directed towards fixed positions in space about the central metal ion. Thus the number and arrangement of ligands2 in space determines the sterochemistry of a complex. Thus in case of six secondary valencies the arrangement of secondary valencies was as directed to the apices of a regular octahedral while in case of four secondary valencies arrangement might be either in a planar or a tetrahedral manner and thus this postulate predicted the possibilities of a variety of type of isomerism in coordination compounds (see later). Remember that primary valencies are non-directional.
To distinguish between the two types of valencies, Werner introduced the square brackes [ ] to enclose atoms making up the coordination complex and which are therefore not ionized. The portion enclosed in the bracket is known as coordination sphere and the portion present outside the bracket as ionising sphere.
The important aspect of structure of five different complexes of \(\mathrm{PtCl}_4\) with ammonia prepared by Werner are given in Table.
Coordination Compounds of PtCl4 with NH3
| Complex |
Modern formula |
No. of Cl- ions precipitated |
Total number of ions |
| \(PtCl_4.6NH_3\) |
\([Pt (NH_3)_6]Cl_4\) |
4 |
5 |
| \(PtCl_4.5NH_3\) |
\([Pt(NH_3)_5 Cl]Cl_3\) |
3 |
4 |
| \(PtCl_4 .4NH_3\) |
\([Pt(NH_3)_4 Cl_2] * Cl_2\) |
2 |
3 |
| \(PtCl_4.3NH_3\) |
\([Pt(NH_3)_3 Cl_3]Cl\) |
1 |
2 |
| \(PtCl_4.2NH_3\) |
\([Pt(NH_3)_2 Cl_4]\) |
0 |
0 (non-electrolyte) |
In all these compounds, platinum exhibits a primary valency (oxidation number) of four and secondary valency (coordination number) of six.
