Werner's co-ordination theory

Werner's Co-ordination Theory : The postulates of this theory are -

• Each metal in co-ordination compound possesses two type of valencies :

i)Principal valency or Primary valency or ionisable valency.

ii)Secondary valency or non-ionisable valency.

• Primary valencies are satisfied by anion only. The number of primary valencies depends upon the oxidation state of the central metal.These are represented by dotted lines between central metal atom and anion.
• Secondary valencies are satisfied only by electron pair donor, the ions or the neutral species. These are represented by thick lines.
• Every central ions tends to satisfy its primary as well as secondary valencies.
• The secondary valencies are directional and are directed in space about the central metal ions. The primary valencies are non-directional.
• The presence of secondary valencies gives rise to stereoisomerism in complexes.
• The ions attached to the primary valency possesses ionizing nature whereas, the ions attached to secondary valencies do not ionizes when the complex is dissolved in a solvent.
•  Initially, Werner had pointed out co-ordination number of a metal to be four or six.

Note: Now it has been proposed that coordination number of a metal may be any whole number between 2 and 9.

Sidgwick theory and EAN rule : 

Sidgwick suggested that the metal ion will continue accepting electron pair till the total number of electron in the metal ion and those donated by ligands is equal to that to the next noble gas. The total number of electron is called EAN of the metal.

                                                                                 https://youtu.be/O3D2belxKBs