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Coordination Compounds

Werner's Theory

• 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.MgCl2.6H2O), Mohr’s salt [FeSO4.(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 atoms. Example: ethylenediamnietetraacetate ion.

• Ambidentate- A ligand which can bind through two different atoms. For example thiocyanate ion.

• Coordination Number

• Number of ligand-donor atoms bonded directly to the metal

• Example: The coordination number of Pt and Ni in [PtCl6]2− and [Ni(NH3)4]2+ are 6 and 4 respectively.

• Coordination Sphere

• The central atom or ion and the ligands attached to it are enclosed in square brackets, which are collectively known as the coordination sphere.

• Example: In the complex K4[Fe(CN)6], the coordination sphere is [Fe(CN)6]4−.

• Coordination Polyhedron

• The spatial arrangement of the ligand atoms which are directly attached to the central atom or ion

• Example: Octahedral, square planar, tetrahedral

• Oxidation Number of Central Atom

• The charge central metal atom would carry if all the ligands are removed along with the electron pairs that are shared with the central atom

• Example: Oxidation number of copper in [Cu(CN)4]3− is 1.

• Homoleptic and Heteroleptic Complexes

• Homoleptic − Complexes in which the metal is bound to only one kind of donor group. Example: [Co(NH3)6]3+

• Heteroleptic − Complexes in which the metal is bound to more than one kind of donor groups. For example: [Co(NH3)4Cl2]+

• 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…

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