Werner’s postulates explain the bonding in coordination compounds as follows:
(i) A metal exhibits two types of valencies ‘ namely, primary and secondary valencies. Primary valencies are satisfied by negative ions while secondary valences are satisfied by both negative and neutral ions. (In modem terminology the primary valency corresponds to the oxidation number of the metal ion, whereas the secondary valency refers to the coordination number of the metal ion)
(ii) A metal ion has a definite number of secondary valences around the central atom. Also, these valences project in a specific direction in the space assigned to the definite geometry of the coordination compound.
(iii) Primary valences are usually ionisable, while secondary valences are non ionisable.
