Coordination Compounds

Werner's Theory of Coordination Compounds

• Coordination Compounds − Complex compounds in which transition metal atoms are bound to a number of anions or neutral molecules

Postulates of Werner’s Theory of Coordination Compounds

• In coordination compounds, there are two types of linkages (valences) − primary and secondary.

• The primary valences are ionisable, and are satisfied by negative ions.

• The secondary valences are non-ionisable, and are satisfied by negative ions or neutral molecules. The secondary valence is equal to the coordination number of a metal, and remains fixed for a metal.

• Different coordination numbers have characteristic spatial arrangement of ions or groups bound by the secondary linkages.

Such spatial arrangements are called coordination polyhedra. The species within the square brackets are called coordination entities or complexes, and the ions outside the square brackets are called counter ions.

Difference between a Double Salt and a Complex

• In water, a double salt dissociates completely to give simpler ions. Examples of double salt: carnallite (KCl.MgCl_2.6H₂O), Mohr’s salt [FeSO_4.(NH₄)₂SO₄.6H₂O]

• Complex ions do not dissociate further to give simpler ions; for example, [Fe(CN)₆]⁴⁻, [Fe(C₂O₄)₃]³⁻.

Definition of Important Terms in Coordination Compounds

• Coordination Entity

• Constitutes a central metal atom or ion bonded to a fixed number of ions or molecules

• Example: [CoCl₃(NH₃)₃] is a coordination entity

• Central Atom or Ion

  • The atom or ion to which a fixed number of ions/groups are bound in a definite geometrical arrangement around it in a coordination entity
  • Example: Ni²⁺ in [NiCl₂(H₂O)₄] and Fe³⁺ in [Fe(CN)₆]³⁻

• Ligands

  • Ions or molecules bound to the central metal atom or ion in a coordination entity
  • Unidentate- A ligand bound to the central metal atom through one donor atom only. Example: ${\mathrm{Cl}}^{-}$, NH₃.

  • Didentate- A ligand bound to central metal atom/ion through two donor atoms. Example: ethylene diamine, oxalate ion etc.

• Polydentate- A ligand bound to central metal atom/ion through multiple donor …