Werner in 1893 put forward a theory, known as Werner's co-ordination theory. Some important postulates of this theory are as below:

(1) Each metal ion possesses two types of valency

(i) Primary or principle or ionisable valency

(ii) Secondary or subsidary or non-ionisable valency.

(2) Primary valencies are satisfied by anions only. The number of primary valencies depend upon the oxidation state of central metal. These are represented by dotted lines in between the central metal atom and anion.

(3) The secondary valencies are satisfied by the ions or the neutral electron pair donor molecules. These are represented by solid lines. Secondary valency is also known as co-ordination number.

(4) The ions attached by secondary valencies do not ionise when the complex is dissolved in a solvent.

What is the co-ordination number of Co in [Co(H₂O)₆]Cl₃ ?

6

Co-ordination number of Co in [Co(H₂O)₆]Cl₃ is 6 as six neutral water molecules act as ligands.