CO-ORDINATION COMPOUNDS
Double salt and Complex salt
A salt that keeps its identity only in solid state is called a double salt. In solution they dissociate into
component ions.
E.g.: Mohr’s salt [FeSO4.(NH4)2SO4.6H2O],
Carnalite [KCl.MgCl2.6H2O],
Potash alum [K2SO4.Al2(SO4)3.24H2O].
The salt that keeps its identity both in solid and solution states is called a complex salt.
e.g. K4[Fe(CN)6] , [Cu(NH3)4]SO4, K2[PtCl4], [Ni(CO)4] etc.
Some important terms related to coordination compounds
(i) Coordination entity: A complex compound that constitutes a central metal (atom or ion) linked
with a fixed number of ions or molecules.
For example, [Ni(CO)4], [COCl3(NH3)3], [PtCl2(NH3)2], [Fe(CN)6]4- etc.
(ii) Central atom/ion: The atom or ion to which a fixed number of ions/groups are bound in a
certain geometrical arrangement around it. Since it accepts a lone pair of electrons for the formation
of coordinate bond, it is also referred to as Lewis acids.
For example, Fe3+ and Ni2+ are the central ions in the coordination compounds [Fe(CN)6]3- and
[NiCl2(H2O)4] respectively.
(iii) Ligands: The atoms, ions or molecules which donate a pair of electrons to the metal atom to
form a coordinate bond, are called ligands.
For example,NH3, H2O, Cl-, CN-, Br-, C2O42-, etc.
Depending on the number of donor atoms, a ligand can be of following types:
(a) Unidentate or Monodentale ligand: It contains only one donor atom.
Eg :H2O and NH3 in which O and N are the donor atoms
(b) Didentate or Bidentate ligand: When a ligand has two donor atoms,
Eg: ethane-1,2-diamine (H2NCH2CH2NH2), notated as ‘en’ in which the two nitrogen atoms of the
amino group act as donor atoms and oxalate ion (C2O42–) has two donor atoms.
(c) Polydentate or Multidentate ligand: When several donor atoms are present in a single ligand,
for example EDTA4- (Ethylenediaminetetraacetate), is an important hexadentate ligand which can
bind through two nitrogen and four oxygen donor atoms to a central metal ion.
1
,Ligands are also classified as:
a) Ambidentate ligands: They are unidentate ligands which contain more than one donor atoms.
They can coordinate through two different atoms.
Eg: NO2--, SCN--etc.
NO2-- ion can coordinate either through nitrogen or through oxygen atom to the central metal
atom/ion. If the donor atom is N, it is written as NO2-- and is called nitrito (N) and if it is O, it is
written as ONO-- and is called nitrito(O).
Similarly, SCN-- ion can coordinate either through sulphur atom (←SCN – thiocyanato) or through
nitrogen atom (←NCS – isothiocyanato).
b) Chelating Ligands: Di- or polydentate ligands can bind to the central atom through two or more
donor atoms and form ring complexes. Such complexes are called chelates and such types of ligands
are said to be chelating ligands. Complexes containing chelating ligands are more stable than those
containing unidentate ligands.
For e.g. the complex [Co(en)3]3+ is a chelate and ethane-1,2-diamine (en) is a chelating ligand.
(iv). Denticity: The number of donor atoms of a particular ligand that are directly bonded to the
central atom is called denticity. For unidentate ligands, the denticity is 1, for didentate ligands it is 2
and so on.
(v) Coordination number (C.N.): The co-ordination number (C.N) of a metal ion in a complex can
be defined as the total number of ligand donor atoms (unidentate ligands) to which the metal is
directly bonded.
2
, Eg: In the complex ions, [Ag(NH3)2]2+, [Zn(CN)4]2- and [Ni(NH3)6]2+ the coordination number of
Ag, Zn and Ni are 2, 4 and 6 respectively.
When the bonded ligands are didentate the coordination number is double the number of ligands
because the number of bonds linked to the central metal becomes double.
For example, the coordination number of Fe in [Fe(C2O4)3]3- is 6, because C2O42- is a didentate
ligand.
(vi) Coordination polyhedron: It describes the spatial arrangement of the ligand atoms which are
directly attached to the central atom/ion. The most common co-ordination polyhedra are octahedral,
square planar and tetrahedral.
Eg: The coordination polyhedra of [Ni(CO)4], [Co(NH3)6]3+ and [PtCl4]2- are tetrahedral, octahedral
and square planar respectively.
(vii) Coordination sphere: The coordination complex which constitutes the central atom/ion and the
ligands, are represented in a square bracket, collectively termed as coordination sphere. The ionisable
groups are written outside the bracket, called counter ions.
Eg: In the complex K4[Fe(CN)6], the coordination sphere is [Fe(CN)6]4- and the counter ion is K+.
(viii). Oxidation number of central atom: The oxidation number of the central atom in a complex
is defined as the residual charge on it, if all the ligands are removed along with their electron pairs
that are shared with the central atom. The oxidation number is represented by a Roman numeral in
simple brackets.
Eg: Oxidation number of copper in [Cu(CN)4]3– is +1
(ix) Homoleptic and Heteroleptic complexes: The complex in which metal ion is bound to only
one kind of ligands is called the homoleptic complex and when more than one kind of ligands are
bound to the metal, it is called heteroleptic complex.
Eg: Homoleptic complex : [Co(NH3)6]3+, [Fe(CN)6]4-, etc
Heteroleptic complex : [CoCl2(NH3)4]+, [PtCl(NH3)5]3+, etc
(x) Charge on a complex ion: The charge carried by a complex ion is the algebraic sum of charges
carried by the central metal ion and the coordinated groups or ions.
For example,
[Fe(CN)6]3-Fe3+ = +3 charge; 6CN- = 6 × (-1) = -6 charge
Thus charge on Fe(CN)6 = +3 - 6 = -3.
xi) Types of complex ion:
Cationic complex : [Cr(H2O)4Cl2]+
Anionic complex : [Fe(CN)6]3-
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Double salt and Complex salt
A salt that keeps its identity only in solid state is called a double salt. In solution they dissociate into
component ions.
E.g.: Mohr’s salt [FeSO4.(NH4)2SO4.6H2O],
Carnalite [KCl.MgCl2.6H2O],
Potash alum [K2SO4.Al2(SO4)3.24H2O].
The salt that keeps its identity both in solid and solution states is called a complex salt.
e.g. K4[Fe(CN)6] , [Cu(NH3)4]SO4, K2[PtCl4], [Ni(CO)4] etc.
Some important terms related to coordination compounds
(i) Coordination entity: A complex compound that constitutes a central metal (atom or ion) linked
with a fixed number of ions or molecules.
For example, [Ni(CO)4], [COCl3(NH3)3], [PtCl2(NH3)2], [Fe(CN)6]4- etc.
(ii) Central atom/ion: The atom or ion to which a fixed number of ions/groups are bound in a
certain geometrical arrangement around it. Since it accepts a lone pair of electrons for the formation
of coordinate bond, it is also referred to as Lewis acids.
For example, Fe3+ and Ni2+ are the central ions in the coordination compounds [Fe(CN)6]3- and
[NiCl2(H2O)4] respectively.
(iii) Ligands: The atoms, ions or molecules which donate a pair of electrons to the metal atom to
form a coordinate bond, are called ligands.
For example,NH3, H2O, Cl-, CN-, Br-, C2O42-, etc.
Depending on the number of donor atoms, a ligand can be of following types:
(a) Unidentate or Monodentale ligand: It contains only one donor atom.
Eg :H2O and NH3 in which O and N are the donor atoms
(b) Didentate or Bidentate ligand: When a ligand has two donor atoms,
Eg: ethane-1,2-diamine (H2NCH2CH2NH2), notated as ‘en’ in which the two nitrogen atoms of the
amino group act as donor atoms and oxalate ion (C2O42–) has two donor atoms.
(c) Polydentate or Multidentate ligand: When several donor atoms are present in a single ligand,
for example EDTA4- (Ethylenediaminetetraacetate), is an important hexadentate ligand which can
bind through two nitrogen and four oxygen donor atoms to a central metal ion.
1
,Ligands are also classified as:
a) Ambidentate ligands: They are unidentate ligands which contain more than one donor atoms.
They can coordinate through two different atoms.
Eg: NO2--, SCN--etc.
NO2-- ion can coordinate either through nitrogen or through oxygen atom to the central metal
atom/ion. If the donor atom is N, it is written as NO2-- and is called nitrito (N) and if it is O, it is
written as ONO-- and is called nitrito(O).
Similarly, SCN-- ion can coordinate either through sulphur atom (←SCN – thiocyanato) or through
nitrogen atom (←NCS – isothiocyanato).
b) Chelating Ligands: Di- or polydentate ligands can bind to the central atom through two or more
donor atoms and form ring complexes. Such complexes are called chelates and such types of ligands
are said to be chelating ligands. Complexes containing chelating ligands are more stable than those
containing unidentate ligands.
For e.g. the complex [Co(en)3]3+ is a chelate and ethane-1,2-diamine (en) is a chelating ligand.
(iv). Denticity: The number of donor atoms of a particular ligand that are directly bonded to the
central atom is called denticity. For unidentate ligands, the denticity is 1, for didentate ligands it is 2
and so on.
(v) Coordination number (C.N.): The co-ordination number (C.N) of a metal ion in a complex can
be defined as the total number of ligand donor atoms (unidentate ligands) to which the metal is
directly bonded.
2
, Eg: In the complex ions, [Ag(NH3)2]2+, [Zn(CN)4]2- and [Ni(NH3)6]2+ the coordination number of
Ag, Zn and Ni are 2, 4 and 6 respectively.
When the bonded ligands are didentate the coordination number is double the number of ligands
because the number of bonds linked to the central metal becomes double.
For example, the coordination number of Fe in [Fe(C2O4)3]3- is 6, because C2O42- is a didentate
ligand.
(vi) Coordination polyhedron: It describes the spatial arrangement of the ligand atoms which are
directly attached to the central atom/ion. The most common co-ordination polyhedra are octahedral,
square planar and tetrahedral.
Eg: The coordination polyhedra of [Ni(CO)4], [Co(NH3)6]3+ and [PtCl4]2- are tetrahedral, octahedral
and square planar respectively.
(vii) Coordination sphere: The coordination complex which constitutes the central atom/ion and the
ligands, are represented in a square bracket, collectively termed as coordination sphere. The ionisable
groups are written outside the bracket, called counter ions.
Eg: In the complex K4[Fe(CN)6], the coordination sphere is [Fe(CN)6]4- and the counter ion is K+.
(viii). Oxidation number of central atom: The oxidation number of the central atom in a complex
is defined as the residual charge on it, if all the ligands are removed along with their electron pairs
that are shared with the central atom. The oxidation number is represented by a Roman numeral in
simple brackets.
Eg: Oxidation number of copper in [Cu(CN)4]3– is +1
(ix) Homoleptic and Heteroleptic complexes: The complex in which metal ion is bound to only
one kind of ligands is called the homoleptic complex and when more than one kind of ligands are
bound to the metal, it is called heteroleptic complex.
Eg: Homoleptic complex : [Co(NH3)6]3+, [Fe(CN)6]4-, etc
Heteroleptic complex : [CoCl2(NH3)4]+, [PtCl(NH3)5]3+, etc
(x) Charge on a complex ion: The charge carried by a complex ion is the algebraic sum of charges
carried by the central metal ion and the coordinated groups or ions.
For example,
[Fe(CN)6]3-Fe3+ = +3 charge; 6CN- = 6 × (-1) = -6 charge
Thus charge on Fe(CN)6 = +3 - 6 = -3.
xi) Types of complex ion:
Cationic complex : [Cr(H2O)4Cl2]+
Anionic complex : [Fe(CN)6]3-
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