Coordination Compounds — Chemistry STD 12 Science — Question

1. Metals possess two types of valencies called​​​​​​​

1. Primary valency which are ionisable;
2. secondary valency which are non- ionisable

2. Primary valency is satisfied by the negative ions and it is that which a metal exhibits in the formation of its simple salts.
3. Secondary valencies are satisfied by neutral ligand or negative ligand and are those which metal exercises in the formation of its complex ions. Every cation has a fixed number of secondary valencies which are directed in space about central metal ion in certain fixed directions, e.g,, In CoCl₃-6NH₃, valencies between Co and Cl are primary valencies and valencies between Co and NH₃ are secondary. In COCl₃-6NH₃, six ammonia molecules linked to Co by secondary valencies are directed to six corners of a regular octahedron and thus account for structure of COCl₃-6NH₃ as follows:

In modern theory, it is now referred as coordination number of central metal atom or ion.