1.    Every element exhibits two types of valencies :
(a)    primary valency ; and
(b)    secondary valency.

(a)    Primary valency corresponds to the oxidation state of the metal atom or ion. The primary valency of the central metal ion must be satisfied by a negative ion. We are already familiar with this valency of the metal ion. For example, in compounds CoCI₃, CuSO₄ and AgNO₃ the primary valencies of Co, Cu and Ag are 3, 2 and 1, respectively. Werner showed the attachment of negative ions to metal atom by dotted lines.

(b)    Secondary valency corresponds to the coordination number of the metal atom or ion. The secondary valencies may be satisfied by either negative ions or neutral molecules and are called ligands.

These ligands which satisfy the coordination number are directly attached to the metal atom or ion and were represented in the diagram as thick lines. While writing down the formulae, ligands satisfying the secondary valences are always shown in square brackets i.e., they form the coordination sphere of the metal atom. The ions which are outside the coordination sphere are ionisable.

2.    Every metal atom has a fixed number of secondary valencies, i.e., its coordination number.

3.    Every element wants to satisfy both its primary and secondary valencies. In order to meet this requirement a negative ion may often have a dual behavior, i.e., it mat satisfy both the primary and secondary valency.

4.    Those ligands which satisfy the secondary valency, i.e., coordination number of metal atom are always directed towards fixed positions in space. The stereochemistry of a complex is determined by the number and arrangement of such ligands in space. For a metal ion with a secondary valency equal to six the geometry of the complex ion was assumed by Werner to be octahedral. If the secondary valency is four, then two geometries are possible around the central metal ion, i.e., either tetrahedron or square planar.

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