Coordination Compounds: Werner՚s Coordination Theory and Some Important Terms (For CBSE, ICSE, IAS, NET, NRA 2022)

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Coordination Compounds

Coordination compounds are compound in which a central metal ion is attached to a group of surrounding ligands by coordinate covalent bond.

Werner՚s Coordination Theory

A stable salt like reacts with varying number of stable molecules or compounds such as ammonia to give several new compounds.

Postulates of Werner՚s theory explains these compound are as follows:

Metals exhibit two types of valence:

  • Primary Valence (ionizable)
  • Secondary Valence (non-ionizable)

Primary or ionizable valence is satisfied by negative ions and corresponds to oxidation state of metals.

  • The secondary or non-ionizable valence is satisfied by negative, positive or neutral groups, is equal to the coordination number of metal ion.
  • Every metal tends to satisfy both its primary and secondary valence.
  • The secondary valence is directed toward fixed positions in space i.e.. this has spatial arrangement corresponding to different coordination number.
  • For example, on the basis of Werner՚s theory the structure of is
Werner՚s Coordination Theory
  • Primary Valence (ionizable)
  • Secondary Valence (non-ionizable)

One of the three chloride ions satisfy both primary and secondary valence

  • He also postulated that octahedral, tetrahedral and square planar shapes are more
  • common for coordination compounds of transition elements.
  • For example, six coordinated complexes such as and are octahedral and four coordinated such as and are tetrahedral and square planar respectively.

Some Important Terms


  • The molecules or ions that are attached to the metal in a complex ion are called ligands.
  • The interaction between a metal atom and the ligands can be thought of as Lewis acid-base reaction.
  • A Lewis base is a substance capable of donating one or more electron pairs, every ligand has at least one unshared pair of valence electron. For example
Some Important Terms: Ligands
  • Nitrogen is the donor atom and is the acceptor atom in the complex ion.
  • Ligands can be defined as monodentate, bidentate or polydentate on the basis of number of donor atom present.
  • and are monodentate ligands with only one donor atom in each.
  • Ethylenediamine is a bidentate ligand.
Some Important Terms: Ligands
  • The two nitrogen atoms can coordinate with a metal atom.
  • Bidentate and polydentate ligands are also called chelating agents because of their ability to hold the metal atom like a claw.
  • Example of polydentate (hexadentate) ligand is ethylenediaminetetraacetate ion (EDTA) .
Some Important Terms: Ligands

Coordination Number

  • The coordination number in coordination compounds is defined as the number of ligand (donor) atoms/ions surrounding the central metal atom in the complex.
  • For example, the coordination number of cobalt in is six, coordination number of in is 2, in is 4 and in is 6.

Coordination Sphere

  • The central metal atom and the ligands which are directly attached to it are enclosed in a square bracket and are collectively termed as coordination sphere.
  • The ligands and the metal atom inside the square brackets behave as a single constituent units.
Coordination Sphere

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