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Karnataka Board PUCPUC Science 2nd PUC Class 12

Werner’s Theory of Coordination Compounds

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Estimated time: 5 minutes
CBSE: Class 12

Theory: Werner’s Theory of Coordination Compounds

Alfred Werner (1893) proposed the first systematic theory to explain the structure and bonding in coordination compounds. His key postulates are:

Main Postulates:

  1. In coordination compounds, metals show two types of valencies — Primary (ionisable) and Secondary (non-ionisable).
  2. Primary valences are normally ionisable. They are satisfied by negative ions (counter ions/anions). They correspond to the metal's oxidation state.
  3. Secondary valences are non-ionisable. They are satisfied by neutral molecules or negative ions (ligands). The secondary valency is equal to the coordination number and is constant for a metal.
  4. Ion groups bound by secondary valencies to the metal have a characteristic spatial arrangement (geometry). This geometry is decided by the secondary valences, not the primary valences.

Werner's Formula Examples:

  • CrCl₃·6H₂O: In [Cr(H₂O)₆]Cl₃, all three Cl⁻ are outside the coordination sphere and hence ionisable.
  • CrCl₃·5H₂O: [Cr(H₂O)₅Cl]Cl₂ — two Cl⁻ are ionisable, one is inside the coordination sphere.
  • CrCl₃·4H₂O: [Cr(H₂O)₄Cl₂]Cl — one Cl⁻ ionisable.
  • CrCl₃·3H₂O: [Cr(H₂O)₃Cl₃] — no ionisable Cl⁻, no precipitate with AgNO₃.

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Coordination Compounds part 2 (Werner's Experiment) [00:12:48]
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