Werner's theory and structure of complexes

Werner's theory (1893) explains the structure of complexes through these postulates:

1. A metal in a complex shows two kinds of valency: primary (ionisable) valency, which is equal to the oxidation state and is satisfied by negative ions; and secondary (non-ionisable) valency, which is equal to the coordination number and is satisfied by negative ions or neutral molecules (ligands).
2. Every metal tries to satisfy both its primary and secondary valencies.
3. The secondary valencies are directed in space, so each complex has a definite geometry: coordination number 6 gives an octahedral complex, and 4 gives a tetrahedral or square planar complex.
4. Ligands that satisfy the secondary valency are attached directly to the metal inside the coordination sphere (written in square brackets) and do not ionise; the primary valency is satisfied by ions outside the sphere that ionise in water.

Structures predicted

 Octahedral (CN 6)      Square planar (CN 4)
        L                  L         L
        |                    \     /
    L --M-- L                  M
       /|\                   /     \
      L | L                 L         L
        L

Example: CoCl₃·6NH₃, formula [Co(NH₃)₆]Cl₃. Primary valency 3 is satisfied by 3 Cl⁻ (outside), secondary valency 6 by 6 NH₃ (inside, octahedral). All 3 Cl⁻ precipitate with AgNO₃. Werner supported this by conductivity and AgNO₃ precipitation experiments, and by isolating the predicted cis/trans isomers of [Co(NH₃)₄Cl₂]⁺.