Werner put forward a set of ideas which are known as Werner’s coordination theory, to explain the nature of bonding in complexes. His theory has been a guiding principle in Inorganic Chemistry and in the concept of valence.
1. Metals exhibit two types of valence.
Primary or ionizable valence is satisfied by negative ions and corresponds to oxidation state of the metal. The secondary or non-ionizable valence, which is satisfied by negative, positive or neutral groups, is equal to the coordination number of metal ion.
Every metal tends to satisfy both its primary and secondary valence.
2. The secondary valence is directed toward fixed positions in space i.e. this has spatial arrangement corresponding to different coordination number.