Werner's Coordination Theory
Several theories were proposed to explain the observed properties of Co(III) ammines and of other similar compounds like Pt(IV) ammines which had been prepared by then. It was only in 1893, that Werner presented a theory known as Werner's coordination theory which could explain all the observed properties of complex compounds. Important postulates of this theory are:
Most elements exhibit two types of valencies:
(a) Primary valency
This corresponds to the oxidation state of the metal ion. This is also called principal, ionisable or ionic valency. It is satisfied by negativeions and its attachment with the central metal ion is shown by dotetd lines.
(b) Secondary valency
It is also termed as coordination number of the central metal ion. It is non-ionic or non-ionisable. this is satisfied by either negative ions or neutral molecules. TThe ligands, which satisfy the coordination nuber are directly to the metal atom or ion and are shown by thick lines.
Every element tends to satisfy both its primary and secondary valencies. In order to meet this requirement a negative ion may often show a dual behaviour.