Postulates
1. In co-ordination compounds, central metal atoms exhibit primary valency and secondary valency.
The primary valency is ionizable. Secondary valency is not ionizable.
The primary valency corresponds to oxidation state.
The secondary valency corresponds to coordination number. (the central metal ion and ligands are not ionizable)
2. Every metal atom has a fixed number of secondary valencies (coordination number(s)).
3. The metal atom tends to satisfy boths its primary valency as well as its secondary valency. Primary valency is satisfied by negative ions (metal ion has a positive charge) whereas secondary valency (coordination number) is satisfied either by negative ions or by neutral molecules. (In certain case a negative ion may satisfy both types of valencies).
4. The coordination number or secondary valencies are always directed towards the fixed positions in space and this leads to definite geometry of the coordination compound.
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