Coordination compounds of COCl3 with NH3
Dotted line represents the primary valency. Normal line represents the secondary valency.
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 auxiliary (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 noble prize winner in chemistry may be summarized as below.
1. Metals possess two types of valencies, namely primary (principal) or ionizable valency and secondary (auxiliary) or non–ionizable valency.
Primary valencies are those which a metal normally exhibit in the formation of its simple salts. Thus, in the formation of PtCl4, CuSO4 and AgCl the primary valencies of Pt, 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., H2O, NH3, etc.) or even sometimes by some positive groups. In every case, the coordination number of the metal must be fulfilled.
| Complex | Modern formula | No. of Cl− ions precipitated | Total number of ions |
|---|---|---|---|
| PtCl4.6NH3 | [Pt(NH3)6]Cl4 | 4 | 5 |
| PtCl4.5NH3 | [Pt(NH3)5Cl]Cl3 | 3 | 4 |
| PtCl4.4NH3 | [Pt(NH3)4Cl2]Cl2 | 2 | 3 |
| PtCl4.3NH3 | [Pt(NH3)3Cl3]Cl | 1 | 2 |
| PtCl4.2NH3 | [Pt(NH3)2Cl4] | 0 | 0 |
| (non electrolyte) |
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., cobalt3+ and platinum4+ were recognized to have six secondary valencies and copper2+ 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 ions. Thus, the number and arrangement of ligands in space determines the stereochemistry of a complex. Thus, in case of six secondary valencies the arrangement of secondary valencies may be as directed to the apices of a regular octahedral while in case of four secondary valencies arrangement might be either a planar or a tetrahedral manner and thus, this postulate predicted the possibilities of a variety of type of isomerism in coordination compounds. Remember that primary valencies are non–directional.
To distinguish between the two types of valencies, Werner introduced the square brackets [ ] 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 ionizing sphere.