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Complex salt; main postulates of Werner's theory

Short answer

A complex salt contains a complex ion that keeps its identity in solution. Werner’s theory: metals have primary (ionisable, oxidation state) and secondary (coordination number) valencies; secondary valencies are directed in space, giving definite geometry; ligands inside the coordination sphere satisfy secondary valency and do not ionise.

What is complex salt? Give the main postulates of Werner's coordination theory.

Complex salt

A complex salt contains a complex ion, formed by a central metal ion bound to ligands by coordinate bonds. The complex ion keeps its identity in solution and does not give the usual tests of its metal and ligands. Example: K₄[Fe(CN)₆], which gives K⁺ and [Fe(CN)₆]⁴⁻ (not Fe²⁺ or CN⁻) in water.

Main postulates of Werner's theory

  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.

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.

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