Class 12 Chemistry coordination compounds

COORDINATION COMPOUNDS

The compounds in which the central atom is linked to a number of ions or neutral
molecules by coordinate bonds are called coordination compounds. Coordination
compounds must contain a complex ion and hence also known as complexes. The branch
of in-organic chemistry dealing with study of coordination compounds is known as
coordination chemistry.

The complexes can be classified into three types like
i) Cationic complex: The complexes in which the complex ion carries a net positive
charge is called cationic complex.

[Co(NH3 )6 ]3+ , [Ni(NH3 )6 ]2+

ii) Anionic complex: The complexes in which the complex ion carries a net negative
charge is called anionic complex.

[Fe(CN)6 ]4- , [Ag(CN)2 ]- etc

iii) Neutral complex: The complex which carries no charge is called a neutral
complex. Ex. [Ni(CO)4 ], [Co(NH3 )3 Cl3 ] etc

Difference between double salt and coordination compounds:
Double salt Coordination compounds

1) Double salts contain two simple salts in 1) In complex compounds the simple salts
equimolar proportions. may or may not be in equimolar proportions.
2) In aqueous solution they completely 2) The complex ion doesn’t dissociate into
dissociate into ions. ions in aqueous solution.
3) Double salts don’t contain any 3) The complex ion always contains
coordinate bond. coordinate bonds.
4) The properties of double salts are same 4) The properties of coordinate compounds
with the constituent compounds. are different from its constituent ions.
e.g. e.g.
Carnalitie – KCl.MgCl2.6H2O K4[Fe(CN)6]
Potash alum- K2SO4 .(Al2SO4)3 .24H2O [Co(NH3)6]Cl3
Mohr’s Salt–FeSO4(NH4)2SO4.6H2O [Ni(CO)4] etc
IMPORTANT TERMS OF COORDINATION COMPOUNDS:

1. Ligands: The ions or molecules which can donate a pair of electrons to the metal
atom or ion and form a coordinate bond with it are called Ligands.



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, The metal atom or ion to which the Ligands are attached is called central metal
atom or ion. Thus ligands are Lewis bases and central metal atom or ion is Lewis
acid.

Types of Ligands:

i). Monodentate Ligands: If a ligand contains one donor atom then it is called
monodentate ligand.

e.g. OH–, CN–, NO2–, H2O, NH3, CO, NO etc.

ii). Bidentate Ligands: Ligands which have two donor atoms are called as
bidentate ligands.

CH2 - NH2
e.g. Ethylenediamine (en) – |
CH2 - NH2


COO-
Oxalate(ox) ion – | (C2O24- )
COO-

CH2 - NH2
Glycinate(gly) ion – |
COO-

Carbonate ion CO32-

DmgH CH3 –C=N-O-

|

CH3-C=N-OH

iii). Tridentate ligands: Ligands having three donor atoms are called tridentate
ligands

.e.g. diethylinetriamine(dien)

iv). Tetradentate ligands: Ligands having four donor atoms are called tetradentate
ligands.

e.g. Triethylenetetramine (trien)
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, v) Pentadentate ligand: Ligands having five donor atoms are called pentadentate
ligands.

e.g. ethylene diamine triacetate ion.

vi). Hexadentate ligands:

Ligands having six donor atoms are called hexadentate ligands.

e.g. ethylenediaminetetraacetate(edta) ion




Chelating Ligands: When a bidentate or polydentake ligand is attached by two or more
donor atoms to the same central metal atom or ion forming a ring structure then the ligand
is called chelating ligand and the complex is called a chelate compound. The process is
called chelation.

e.g. Ethylenediamine (en)

[Cu(en)2]2+

Ambidentate ligands:

Monodentate ligands containing more than one co-ordinating atoms are called
ambidentate ligands.

e.g. NO-2 , CN–, SCN– etc.

Bridging ligands: The ligands which can simultaneously bond itself to more than
one metal ion are called bridging ligands.

e.g. OH–, Cl–, NH– etc.

2. Co-ordination number:

The number of co-ordinate bonds formed by the ligands with the central metal atom
or ion is known as co-ordination number.

e.g. i) ln[Cu(NH4)4]SO4, the coordination no of Cu is 4.


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