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Chapter-09: Coordination Compounds
When solution, of two or more simple, stable compounds in molecular proportions are allowed
to evaporate crystals of new compounds may be formed. These compounds are known as
molecular or addition compounds. These are of two types-
1. Double salt or lattice compounds- The addition compounds which are stable in solid
state but are broken into individual constituents when dissolved in water. Example –
Mohr salt [FeSO4.(NH4) 2SO4.6H2O] and potash alum [K2SO4.Al2(SO4)3.24H2O].
2. Complex compounds- The addition compounds which retain their identity in solid state
as well as in solution are termed as coordination or complex compounds. In these
compounds at least one complex ion is present.
Note: Complex compounds are mainly formed by transition metal but to small extent by other metals such
as Magnesium in chlorophyll, Transition metals form complexes due to small size, high nuclear charge,
number of vacant orbitals of equivalent energy where the electron donate by ligands are accommodated.
Some Basic Definitions:
Coordination Chemistry: The branch of chemistry dealing with the study of coordination
compounds is known as coordination chemistry.
Coordination compounds: The compounds in which the central metal atom is linked to a number
of ions or neutral molecules by coordinate bonds i.e. by donation of lone pairs of electrons by
these ions or neutral molecules to the central metal atom.
Ligands: The neutral molecules anions or cations which are directly linked with central metal
atom are called ligands. Ligands are classified on the basis of donor sites. They are termed as
monodentate or unidentate, bidentate, Tridentate etc. The ligands having two or more donor
sites are called polydentate.
Polydentate ligand is called chelating ligand if on coordination it results in form of closed or
cyclic ring structure. The complexes thus formed are called chelates. Word ligand is derived
from Latin word ‘ligare’ known as ‘to-bind’.
Coordination Polyhedun: Spatial arrangement of the ligand atoms which are directly attached o
the central atom/ion is called coordination polyhedron around the central atom/ion.
Coordination number: The number of atoms of the ligands that are directly bonded to central
metal atom or ion by coordinate bond is known as coordination number of metal atom/ion or
ligancy.
Coordination sphere or Coordination entity: The central atom and the ligands which are directly
attached to it are enclosed in square brackets are collectively termed as coordination sphere.
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Classification of ligands based on charge:
1. Neutral ligand. Example- H2O, NH3 etc.
2. Anionic ligand (have –ve charge). Example- F-, Cl-, NO3- etc.
3. Ambidentate ligand: Unidentate ligands containing more than one coordinating atoms
are called Ambidentate ligand. Example-
4. Cationic ligands: They are having positive charge. Example-
Classification of ligands based on denticity: The number of coordinating or ligating groups
present in ligand is called the denticity of that ligand.
a) Mono/Unidentate (uni means one and dent means tooth ): Denticity is one i.e. with one
donor site. Example- NH3, OH- etc.
b) Bidentate or Didentate: With one donor sites. Example- C2O42-, SO42- etc.
c) Tridentate: With three donor sites. Example- Diethylene triamine
d) Tetradentate: With four donor sites. Example- Triethylene tetra amine
e) Pentadentate: With five donor sites. Example- ethylene diamine tri acetate
f) Hexadentate: With six donor sites. Example- EDTA (ethylene diamine tetra acetate)
Note:
1. Ligands with more than one donor sites are collectively called multidentate ligand or
polydentate ligand.
2. Chelates: The bidentate or polydentate ligand when attached with metal form a ring like
structure called chelates. Such ligands are called chelating ligands.
Note:
a) Greater the denticity of the ligand more stable is the complex (chelate) form.
b) Ligands with large groups form unstable rings than the ligands with smaller groups due to
steric hinderance.
3. Chelating effect: Chelating ligands increase the stability of coordination compounds.
Chapter-09: Coordination Compounds
When solution, of two or more simple, stable compounds in molecular proportions are allowed
to evaporate crystals of new compounds may be formed. These compounds are known as
molecular or addition compounds. These are of two types-
1. Double salt or lattice compounds- The addition compounds which are stable in solid
state but are broken into individual constituents when dissolved in water. Example –
Mohr salt [FeSO4.(NH4) 2SO4.6H2O] and potash alum [K2SO4.Al2(SO4)3.24H2O].
2. Complex compounds- The addition compounds which retain their identity in solid state
as well as in solution are termed as coordination or complex compounds. In these
compounds at least one complex ion is present.
Note: Complex compounds are mainly formed by transition metal but to small extent by other metals such
as Magnesium in chlorophyll, Transition metals form complexes due to small size, high nuclear charge,
number of vacant orbitals of equivalent energy where the electron donate by ligands are accommodated.
Some Basic Definitions:
Coordination Chemistry: The branch of chemistry dealing with the study of coordination
compounds is known as coordination chemistry.
Coordination compounds: The compounds in which the central metal atom is linked to a number
of ions or neutral molecules by coordinate bonds i.e. by donation of lone pairs of electrons by
these ions or neutral molecules to the central metal atom.
Ligands: The neutral molecules anions or cations which are directly linked with central metal
atom are called ligands. Ligands are classified on the basis of donor sites. They are termed as
monodentate or unidentate, bidentate, Tridentate etc. The ligands having two or more donor
sites are called polydentate.
Polydentate ligand is called chelating ligand if on coordination it results in form of closed or
cyclic ring structure. The complexes thus formed are called chelates. Word ligand is derived
from Latin word ‘ligare’ known as ‘to-bind’.
Coordination Polyhedun: Spatial arrangement of the ligand atoms which are directly attached o
the central atom/ion is called coordination polyhedron around the central atom/ion.
Coordination number: The number of atoms of the ligands that are directly bonded to central
metal atom or ion by coordinate bond is known as coordination number of metal atom/ion or
ligancy.
Coordination sphere or Coordination entity: The central atom and the ligands which are directly
attached to it are enclosed in square brackets are collectively termed as coordination sphere.
, 2
Classification of ligands based on charge:
1. Neutral ligand. Example- H2O, NH3 etc.
2. Anionic ligand (have –ve charge). Example- F-, Cl-, NO3- etc.
3. Ambidentate ligand: Unidentate ligands containing more than one coordinating atoms
are called Ambidentate ligand. Example-
4. Cationic ligands: They are having positive charge. Example-
Classification of ligands based on denticity: The number of coordinating or ligating groups
present in ligand is called the denticity of that ligand.
a) Mono/Unidentate (uni means one and dent means tooth ): Denticity is one i.e. with one
donor site. Example- NH3, OH- etc.
b) Bidentate or Didentate: With one donor sites. Example- C2O42-, SO42- etc.
c) Tridentate: With three donor sites. Example- Diethylene triamine
d) Tetradentate: With four donor sites. Example- Triethylene tetra amine
e) Pentadentate: With five donor sites. Example- ethylene diamine tri acetate
f) Hexadentate: With six donor sites. Example- EDTA (ethylene diamine tetra acetate)
Note:
1. Ligands with more than one donor sites are collectively called multidentate ligand or
polydentate ligand.
2. Chelates: The bidentate or polydentate ligand when attached with metal form a ring like
structure called chelates. Such ligands are called chelating ligands.
Note:
a) Greater the denticity of the ligand more stable is the complex (chelate) form.
b) Ligands with large groups form unstable rings than the ligands with smaller groups due to
steric hinderance.
3. Chelating effect: Chelating ligands increase the stability of coordination compounds.
