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Co-ordination Compounds

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Co-Ordination Compounds PDF Notes, Important Questions and Formulas

Coordination Compound

Molecular/Addition Compound:
Molecular/Addition compounds are formed when stoichiometric amounts of two or more simple compounds join together. Molecular/Addition compounds are of two types.

Double salts: Those which don't retain their identity in solutions are called double salts.
For example.
KCl + MgCl2 + 6H2O → KCl.MgCl2. 6H2O
K2SO4 + Al2 (SO4)3 + 24 H2O → K2SO4.Al2 (SO4)3.24H2
                                                          Opotash alum

Complex compounds: Those which retain their identity in solution (complexes).

For example.

CuSO4 + 4 NH→ CuSO4.4NH3 or [Cu (NH3)4] SO4 
                           tetrammine copper (II) sulphate

Fe (CN) 2 + 4 KCN → Fe(CN)2. 4KCN or K4[Fe (CN)6]

Potassium ferrocyanide

When crystals of carnallite are dissolved in water, the solution shows properties of K+, Mg2+ and Cl ions. In a similar way, a solution of potash alum shows the properties of K+, Al3+ and SO42– ions. These are both examples of double salts which exist only in the crystalline state. When the other two examples of coordination compounds are dissolved they do not form simple ions, Cu2+/Fe2+ and CN, but instead their complex ions are formed.

Representation of complex Ion:

begin mathsize 12px style left square bracket ML subscript straight x right square bracket to the power of straight n to the power of plus-or-minus end exponent end style


M=Central Metal atom/ion (usually of d-block)


X=No.of ligands

n±=charge on coordination

Outside region apart from coordination sphere is called ionisation sphere.

  1. Central metal atom/ion: Central ion acts as an acceptor (Lewis acid) and has to accommodate electron pairs donated by the donor atom of the ligand, it must have empty orbitals. This explains why the transition metals having empty d-orbitals form coordination compounds readily. Thus, in complexes
    [Ni (NH3)6]+2 and [Fe (CN) 6]2+, Ni2+ and Fe3+
    Respectively are the central metal ions.

  2. Ligands: Species which are directly linked with the central metal atom/ ion in a complex ion are called ligands. The ligands are attached to the central metal atom/ion through co-ordinate or dative bond free ligands have at least one lone pair.

    The ligands are thus Lewis bases and the central metal ions/atoms are Lewis acids.

Ligands can be of following types depending on the number of donor atoms present in them.

  1. Mono/Unidentate Ligands: They have one donor atom, i.e. they can donate only one electron pair to the central metal atom /ion eg. F-, Cl-, Br-, H2O, NH3, CN-, NO2-, OH-, CO etc.
  2. Bidentate Ligands : Ligands which have two donor atoms and have the ability to link with the central metal atom/ion at two position are called bidentate ligands e.g.


  3. Tridentate Ligands: Ligands having three donor atoms are called tridentate ligands. Examples are

  4. Tetradentate Ligands: These ligands possess four donor atoms. Example are

                              Ethylendiamine triacetate ion
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