Werner suggested the theory of coordination compound to explain structures and formation of compounds.
Werner was first inorganic chemist to be awarded with Nobel prize in chemistry.
Important postulate of Werner’s theory of coordination compound are-
1. Central metal atom exhibit two types of valencies
a) primary valency / ionizable valency.
b) secondary valency / non ionizable valency.
In modern terminology primary valency corresponds to the oxidation number and secondary valency to coordination number of Central metal atom.
Primary valency- primary valencies of Central metal ions are satisfied by negative ions.
e.g. In simple salt such as CoCl3 , CuSO4 and AgCl, the primary valencies of Co, Cu and Ag are 3, 2, 1 respectively.
In complex compounds its attachment to metal ion is shown by dotted lines.
Secondary valency- secondary valencies are satisfied either by negative or neutral molecules or both. Its attachment to metal is shown by thick lines.
2. Every metal ion has a tendency to satisfy both of its primary and secondary valency.
3. Every metal ion has a fixed number of secondary valencies.
4. Secondary valencies (ligands satisfying secondary valencies) are always directed towards fixed position in space around Central metal ion and geometry of complex is determined by number and arrangement of such ligands in space.
e.g. If a metal ion has secondary valency of 4, the geometry of complex can be tetrahedral or square planar and if secondary valency is 6, complex is expected to be octahedral. The primary valency is non directional and play no role in determining geometry of a complex. Thus, according to Werner’s theory the complex CoCl₃.6NH₃ may be represented as
Limitations of Werner’s theory of coordination compound.
It does not explain-
1. Why only certain metal atom form coordination compound.
2. Why coordination sphere has a definite geometry.
3. Why these compounds possess definite magnetic and optical properties.