question_answer 11)

 Explain the bonding in coordination compounds in terms of Werner’s postulates.

Answer:

1. Werner?s theory of co-ordination compounds. The main points of postulates to explain the bonding are: (1) In co-ordination compounds, the metals posses two types of valencies. (i) Primary valencies are those which a metal posses in the formation of its simple salts and it corresponds to its oxidationstate. The primary valency of metal ion is always satisfied by a negative ion. For example, the primary valencies of Ag, Cu, Co and Pt in the formation of their simple salts AgCl, CuSO4, CoCl3 and PtCl4 are 1, 2, 3 and 4 respectively. The primary valencies are indicated by dotted lines, since they are non-directional in nature. Primary valencies are ionisable.  (ii) Secondary valencies are those which a metal posses during the formation of its complex ion. Secondary valencies are directional in nature and are non-ionisable. For example, in complex compounds : [Ag(NH3)2Cl], [Co (NH3)6] Cl3 and [Pt(NH3)] Cl4 the secondary valencies of Ag+, Co3+ and Pt4+ are 2, 6 and 6 respectively. The secondary valency corresponds to the coordination number of the metal ion or atom. These are indicated by thick lines. On the basis of the Werner theory, the complex CoCl3.6NH3 of [Co(NH3)6] Cl3 i.e. hexaamminceobalt(III) chloride can be represented as; where three dotted lines indicate the three primary valency (or oxidation state) of cobalt; while six thick lines indicate its secondary valencies (or co-ordination number). 2. The ligands which satisfy secondary valuency occupy the fixed position in space. Thus the geometry of the complex is determined by the number and arrangement of the ligands around the central metal atom (or ion).