Post on 01-Mar-2021
Class- XII
Chemistry
Chapter-9
Coordination Compounds
Coordination compounds
contain a central atom (or cation) which is coordinated to a suitable number of anions
or neutral molecules and usually retain their identity in solution as well as in solid
state. These may be a positively charged, negatively charged or a neutral species,
[Co (NH3)6]3+, [NiCl4]2-, [Ni (CO)4] etc.
Werner’ Theory of Coordination Compounds:
Werner in 1898, propounded his theory of coordination compounds. The main
postulates are:
1. In coordination compounds metals show two types of linkages (valences)-primary
and secondary.
2. The primary valences are normally ionisable and are satisfied by negative ions.
3. The secondary valences are non-ionisable. These are satisfied by neutral molecules
or negative ions. The secondary valence is equal to the coordination number and is
fixed for a metal.
4. The ions/groups bound by the secondary linkages to the metal have characteristic
spatial arrangements corresponding to different coordination numbers.
In modern formulations, such spatial arrangements are called coordination polyhedra.
The species within the square bracket are coordination entities or complexes and the
ions outside the square bracket are called counter ions.
5. Octahedral, tetrahedral and square planar geometrical shapes are more common in
coordination compounds of transition metals. Thus, [Co (NH3)6] 3+ [CoCl (NH3)5] 2+ and
[CoCl2(NH3)4] + are octahedral entities, while [Ni (CO)4] and [PtCl4] 2– are tetrahedral
and square planar, respectively.
Difference between a double salt and a complex
Double salts dissociate into simple ions completely when dissolved in water. such as
carnallite, KCl.MgCl2.6H2O, Mohr’s salt,
FeSO4. (NH4)2SO4.6H2O, potash alum, KAl (SO4)2.12H2O, etc.
Complex ions do not dissociate into simple ions completely when dissolved in water
such as [Fe (CN)6] 4– of K4Fe (CN)6, do not dissociate into Fe2+ and CN– ions.
Some Important Terms Pertaining to Coordination Compounds:
(a) Coordination entity:
A coordination entity constitutes a central metal atom or ion bonded to a fixed
number of ions or molecules. For example, [CoCl3(NH3)3] is a coordination entity
in which the cobalt ion is surrounded by three ammonia molecules and three
chloride ions.
(b) Central atom/ion:
In a coordination entity, the atom/ion to which a fixed number of ions/groups
are bound in a definite geometrical arrangement around it, is called the central
atom or ion. For example, the central atom/ion in the coordination entities:
[NiCl2(H2O)4], [CoCl (NH3)5]2+ and [Fe (CN)6]3– are Ni2+, Co3+ and Fe3+, respectively.
These central atoms/ions are also referred to as Lewis acids.
(c) Ligands
The ions or molecules bound to the central atom/ion in the coordination entity
are called ligands. These may be simple ions such as Cl
H2O or NH3, larger molecules such as H
macromolecules, such as proteins.
Types of ligands
Unidentate ligand:
When a ligand is bound to a metal ion through a
NH3, the ligand is said to be unidentate
Bidentate ligand:
When a ligand can bind through two donor atoms as in H
diamine) or C2O42– (oxalate), the ligand is said to be
Polydentate:
When several donor atoms are present in a single ligand as in N(CH
ligand is said to be polydentate. Ethylenediaminetetraacetate ion (EDTA
important hexadentate ligand. It can bind through two nitrogen and four oxygen
atoms to a central metal ion.
The ions or molecules bound to the central atom/ion in the coordination entity
These may be simple ions such as Cl–, small molecules such as
, larger molecules such as H2NCH2CH2NH2 or N(CH
macromolecules, such as proteins.
When a ligand is bound to a metal ion through a single donor atom, as with Cl
unidentate.
When a ligand can bind through two donor atoms as in H2NCH2CH2NH
(oxalate), the ligand is said to be didentate.
When several donor atoms are present in a single ligand as in N(CH
ligand is said to be polydentate. Ethylenediaminetetraacetate ion (EDTA
important hexadentate ligand. It can bind through two nitrogen and four oxygen
The ions or molecules bound to the central atom/ion in the coordination entity
small molecules such as
or N(CH2CH2NH2)3 or even
single donor atom, as with Cl–, H2O or
NH2 (ethane-1,2-
When several donor atoms are present in a single ligand as in N(CH2CH2NH2)3, the
ligand is said to be polydentate. Ethylenediaminetetraacetate ion (EDTA4–) is an
important hexadentate ligand. It can bind through two nitrogen and four oxygen
Ambidentate ligand:
Ligand which can ligate through two different atoms is called ambidentate ligand.
Examples of such ligands are the NO
through nitrogen or through oxygen to a
can coordinate through the sulphur or nitrogen atom.
(d) Chelate:
When a di- or polydentate ligand uses its two or more donor atoms to bind a single
metal ion, it is said to be a chelate ligand. The number of such
the denticity of the ligand. Such complexes, called chelate complexes tend to be more
stable than similar complexes containing unidentate ligands
(e) Coordination number
Ligand which can ligate through two different atoms is called ambidentate ligand.
Examples of such ligands are the NO2– and SCN– ions. NO2– ion can coordinate either
through nitrogen or through oxygen to a central metal atom/ion. Similarly, SCN
can coordinate through the sulphur or nitrogen atom.
or polydentate ligand uses its two or more donor atoms to bind a single
metal ion, it is said to be a chelate ligand. The number of such ligating groups is called
the denticity of the ligand. Such complexes, called chelate complexes tend to be more
stable than similar complexes containing unidentate ligands
Ligand which can ligate through two different atoms is called ambidentate ligand.
ion can coordinate either
central metal atom/ion. Similarly, SCN– ion
or polydentate ligand uses its two or more donor atoms to bind a single
ligating groups is called
the denticity of the ligand. Such complexes, called chelate complexes tend to be more
The coordination number (CN) of a metal ion in a complex can be defined as the
number of ligand donor atoms to which the metal is directly bonded. For example, in
the complex ions, [PtCl6] 2– and [Ni (NH
6 and 4 respectively.
(f) Coordination sphere
The central atom/ion and the ligands attached to it are enclosed in square bracket and
is collectively termed as the coordination sphere. The ionisable groups are written
outside the bracket and are called counter io
K4[Fe(CN)6], the coordination sphere is [Fe(CN)
(g) Coordination polyhedron
The spatial arrangement of the ligand atoms which are directly attached to the central
atom/ion defines a coordination polyhedron about the central atom. The most
common coordination polyhedra are octahedral, square planar and tetrahedral. For
example,
[Co (NH3)6]3+ is octahedral, [Ni (
(h) Oxidation number of central
The oxidation number of the central atom in a complex is defined as the charge it
would carry if all the ligands are removed along with
with the central atom. The oxidation number is represented by a Roman numeral in
parenthesis following the name of the coordination entity. For example, oxidation
number of copper in [Cu (CN)
(i) Homoleptic and heteroleptic complexes:
in which a metal is bound to only one kind of donor groups, e.g.,
[Co (NH3)6] 3+, are known as homoleptic. Complexes in which a metal is bound to more
than one kind of donor groups, e.g.,
[Co (NH3)4Cl2] +, are known as heteroleptic.
Naming of Mononuclear Coordination Compounds
The following rules are used when naming coordination compounds:
The coordination number (CN) of a metal ion in a complex can be defined as the
number of ligand donor atoms to which the metal is directly bonded. For example, in
and [Ni (NH3)4] 2+ the coordination number of Pt and Ni are
The central atom/ion and the ligands attached to it are enclosed in square bracket and
is collectively termed as the coordination sphere. The ionisable groups are written
outside the bracket and are called counter ions. For example, in the complex
], the coordination sphere is [Fe(CN)6]4– and the counter ion is K
Coordination polyhedron
The spatial arrangement of the ligand atoms which are directly attached to the central
atom/ion defines a coordination polyhedron about the central atom. The most
common coordination polyhedra are octahedral, square planar and tetrahedral. For
Ni (CO)4] is tetrahedral and [PtCl4]2– is square planar
Oxidation number of central atoms
The oxidation number of the central atom in a complex is defined as the charge it
would carry if all the ligands are removed along with the electron pairs that are shared
with the central atom. The oxidation number is represented by a Roman numeral in
parenthesis following the name of the coordination entity. For example, oxidation
number of copper in [Cu (CN)4]3– is +1 and it is written as Cu(I).
Homoleptic and heteroleptic complexes:
in which a metal is bound to only one kind of donor groups, e.g.,
are known as homoleptic. Complexes in which a metal is bound to more
than one kind of donor groups, e.g.,
, are known as heteroleptic.
Naming of Mononuclear Coordination Compounds
The following rules are used when naming coordination compounds:
The coordination number (CN) of a metal ion in a complex can be defined as the
number of ligand donor atoms to which the metal is directly bonded. For example, in
the coordination number of Pt and Ni are
The central atom/ion and the ligands attached to it are enclosed in square bracket and
is collectively termed as the coordination sphere. The ionisable groups are written
ns. For example, in the complex
and the counter ion is K+.
The spatial arrangement of the ligand atoms which are directly attached to the central
atom/ion defines a coordination polyhedron about the central atom. The most
common coordination polyhedra are octahedral, square planar and tetrahedral. For
is square planar.
The oxidation number of the central atom in a complex is defined as the charge it
the electron pairs that are shared
with the central atom. The oxidation number is represented by a Roman numeral in
parenthesis following the name of the coordination entity. For example, oxidation
are known as homoleptic. Complexes in which a metal is bound to more
The following rules are used when naming coordination compounds:
(i) The cation is named first in both positively and negatively charged coordination
entities.
(ii) The ligands are named in an alphabetical order before the name of the central
atom/ion.
(iii) Names of the anionic ligands end in –o, those of neutral and cationic ligands are
the same except aqua for H2O, ammine for NH3, carbonyl for CO and nitrosyl for NO.
These are placed within enclosing marks ( ).
(iv) Prefixes mono, di, tri, etc., are used to indicate the number of the individual ligands
in the coordination entity. When the names of the ligands include a numerical prefix,
then the terms, bis, tris, tetrakis are used, the ligand to which they refer being placed
in parentheses. For example, [NiCl2(PPh3)2] is named as
dichlorobis(triphenylphosphine)nickel(II).
(v) Oxidation state of the metal in cation, anion or neutral coordination entity is
indicated by Roman numeral in parenthesis.
(vi) If the complex ion is a cation, the metal is named same as the element. For
example, Co in a complex cation is called cobalt and Pt is called platinum. If the
complex ion is an anion, the name of the metal ends with the suffix – ate. For example,
Co in a complex anion, [Co (SCN) 4]-2 is called cobaltate. For some metals, the Latin
names are used in the complex anions, e.g., ferrate for Fe.
(vii) The neutral complex molecule is named similar to that of the complex cation.
Bonding in Coordination Compounds
Valence bond theory:
It was given by Pauling in 1931
According to this theory, the metal atom or ion under the influence of ligands
can use its (n-1) d, ns, np or ns, np, nd orbitals for hybridisation to yield a set of
equivalent orbitals of definite geometry such as octahedral, tetrahedral, and
square planar.
These hybridised orbitals are allowed to overlap with ligand orbitals that can
donate electron pairs for bonding.
Number of Orbitals and Types of Hybridisations
ordination Number Type of hybridisation Shape of hybrid
4 sp3 Tetrahedral
4
5
6
6
Magnetic properties of coordination compounds:
A coordination compound is paramagnetic in nature if it has unpaired electrons and
diamagnetic if all the electrons in the coordination compound are paired.
Magnetic moment
For example:
For any coordination compound: To find the shape using valence bond theory
following steps to be followed
Remove the electrons from the metal and form it the ion
Rearrange metal electrons if necessary
Hybridization
Overlapping of hybrid orbitals of metal with ligand
Example: [Co (NH3)6]3+
In this central metal atom Co atomic no. is 27.
The electronic configuration of Co = (Ar)
Co3+=(Ar)183d6
Example [Fe (Co)5]: (inner orbital complex and diamagnetic)
dsp2 Square planar
sp3d Trigonal bipyramidal
sp3d2 Octahedral
d2sp3 Octahedral
Magnetic properties of coordination compounds:
A coordination compound is paramagnetic in nature if it has unpaired electrons and
diamagnetic if all the electrons in the coordination compound are paired.
where n is number of unpaired electrons
For any coordination compound: To find the shape using valence bond theory
following steps to be followed
Remove the electrons from the metal and form it the ion
electrons if necessary
Overlapping of hybrid orbitals of metal with ligand
In this central metal atom Co atomic no. is 27.
The electronic configuration of Co = (Ar)183d74s2
orbital complex and diamagnetic)
Square planar
Trigonal bipyramidal
Octahedral
Octahedral
A coordination compound is paramagnetic in nature if it has unpaired electrons and
diamagnetic if all the electrons in the coordination compound are paired.
where n is number of unpaired electrons.
For any coordination compound: To find the shape using valence bond theory
EXAMPLE: in [CoF6]3-…… (outer orbital complex and paramagnetic)
Drawbacks of valence bond theory:
This theory couldn’t have valid reason behind that why some complexes of
metal oxidation state is inner orbital while in some other complexes the same
metal atom ion in same state form outer orbital complex.
The magnetic behaviour explained wasn’t satisfactory
This theory couldn’t give the information about colour of compounds
This theory failed to distinguish between strong and weak ligand.
Crystal field splitting theory
It was given by Hans Bethe Ans John van vleck
Postulates
It assumes the central metal atom and ligands as point charges
When a complex is formed: central metal atom positive charge
Ligands –have negative charge
This theory considers the interaction between central metal atom and ligand is
purely electrostatic
When a complex is formed the central metalatom is surrounded by oppositely
charged ligands
No hybridization takes place
To form a bond the ligand molecule must approach towards central metal atom
In absence of external magnetic field, the d orbital of central metal atom is
degenerate but this degeneracy breaks when ligand approaches.
The d orbital splits into two sets:
Axial set non-axial set
dxy,dyz, dzx dx2-y2,dz2
This is crystal field splitting
Repulsive forces occur between electrons of metal and with lone pair ligands due to
which energy of electron fluctuate or changes.
For octahedral complexes
To form octahedral complex the ligands, have to approach central metal atom along
the coordination axis. During the approach the d orbitals whose lobes lie along the axis
will experience more repulsion due to this their energy will increase and the other
non-axial set will suffer less repulsion. as a result, the non-axial will have less energy as
compare to axial set (eg greater than t2g)
Tetrahedral complex:
The ligands have to approach central metal atom in between the coordinationaxis.
during the approach the d orbital’s whose lobes lie along the axis will experience less
repulsion due to this their energy will increase and the other non-axial set will suffer
more repulsion. as a result, the non-axial will have more energy as compared to axial
set (t2g greater than eg)
Square planar complex:
In the different order is seen i.e.
dx2-y2,dxy,dz2,dyz,dzx
Please note for all the complexes:
for strong ligands: the CFSE is more therefore pairing will occur
for weak ligands: the CFSE Is less
Spectrochemical series:
In general, ligands can be arranged in a series in the order of increasing field strength
as given below:
I –< Br–< SCN–< Cl–< S2–< F–< OH–< C2O42–< H2O < NCS–< edta4–< NH3< en < CN–< CO
Such a series is termed as spectrochemical series.
Please note for all the complexes:
for strong ligands: the CFSE(Δo) is more therefore pairing will occur
for weak ligands: the CFSE(Δo) Is less
(j) If Δo< P, the fourth electron enters one of the eg orbitals giving the configuration
t2g3eg
1. Ligands for which Δo< P are known as weak field ligands and form high spin
complexes.
(ii) If Δo> P, it becomes more energetically favourable for the fourth electron to occupy
a t2g orbital with configuration t2g4 eg
0. Ligands which produce this effect are known as
strong field ligands and form low spin complexes.
The limitations of crystal field theory:
The assumption that the interaction between metal-ligand is purely electrostatic
cannot be said to be very realistic.
This theory takes only d-orbitals of a central atom into account. The s and p
orbits are not considered for the study.
The theory fails to explain the behaviour of certain metals which cause large
splitting while others show small splitting. For example, the theory has no
explanation as to why H2O is a stronger ligand as compared to OH–.
The theory rules out the possibility of having p bonding. This is a serious
drawback because is found in many complexes.
The theory gives no significance to the orbits of the ligands. Therefore, it cannot
explain any properties related to ligand orbitals and their interaction with metal
orbitals.
Colour in Coordination Compounds
The crystal field theory attributes the colour of the coordination compounds to d-d
transition of the electron, i.e., the transition of electron from t2g level to the higher
eg level which accompanies the absorption of light in visible spectrum.
If absorption occurs in the visible region of the spectrum, then the transmitted light
bears a colour complementary to the colour of the light absorbed. The relationship
between the absorbed and transmitted wavelengths can be readily understood from
the following,
In the absence of ligands, crystal field splitting does not occur and
is colourless.
[Ti(H2O)6] 3+, which is violet in colour.
Anhydrous CuSO4 is white, but CuSO
[Ni(H2O)6] 2+(aq)+ en (aq) = [Ni(H
green pale blue
[Ni(H2O)4 (en)]2+(aq) + en (aq) = [Ni(H
blue/purple
[Ni(H2O)2(en)2] 2+(aq) + en (aq) = [Ni(en)
violet
Metal carbonyl
Metal carbonyls are homoleptic complexes in which carbon monoxide (CO) acts as the
ligand. Metal carbonyl, any coordination or complex compound consisting of a
heavy metal such as nickel, cobalt, or iron surrounded by
common metal carbonyls include:
Fe(CO)5, and octa carbonyl dicobalt Co
Considering bonding in metal carbonyls:
These compounds possess both s and p characters. The M
donation of lone pair of electrons
metal. The M-C pi bond is formed by the donation of pair of electrons from the filled d
orbital of metal into vacant antibonding pi orbital of carbon monoxide. The metal to
ligand bonding creates a synergic effect which strengthens the bond between CO and
metal as shown below: -
In the absence of ligands, crystal field splitting does not occur and hence the substance
which is violet in colour.
is white, but CuSO4.5H2O is blue in colour.
= [Ni(H2O)4(en)]2+(aq) + 2H2O
+ en (aq) = [Ni(H2O)2(en)2] 2+(aq) + 2H2O
+ en (aq) = [Ni(en)3] 2+(aq) + 2H2O
Metal carbonyls are homoleptic complexes in which carbon monoxide (CO) acts as the
Metal carbonyl, any coordination or complex compound consisting of a
such as nickel, cobalt, or iron surrounded by carbonyl (CO) groups. Some
include: tetracarbonyl nickel Ni(CO)4, pentacarbonyl iron
dicobalt Co2(CO)8.
Considering bonding in metal carbonyls:
hese compounds possess both s and p characters. The M-C sigma bond is formed by
f electrons from the carbonyl carbon into a vacant orbital of
pi bond is formed by the donation of pair of electrons from the filled d
orbital of metal into vacant antibonding pi orbital of carbon monoxide. The metal to
ligand bonding creates a synergic effect which strengthens the bond between CO and
hence the substance
Metal carbonyls are homoleptic complexes in which carbon monoxide (CO) acts as the
Metal carbonyl, any coordination or complex compound consisting of a
(CO) groups. Some
pentacarbonyl iron
C sigma bond is formed by
the carbonyl carbon into a vacant orbital of
pi bond is formed by the donation of pair of electrons from the filled d
orbital of metal into vacant antibonding pi orbital of carbon monoxide. The metal to
ligand bonding creates a synergic effect which strengthens the bond between CO and
Stability of Coordination Compounds
The stability of complex in solution refers to the degree of association between the
two species involved in the state of equilibrium. It is expressed as stability constant (K).
The factors on which stability of the complex depends:
(i) Charge on the central metal atom as the magnitude of charge on metal atom
increases, stability of the complex increases.
(ii) Nature of metal ion the stability order is 3d < 4d < 5d series.
(iii) Basic nature of ligands Strong field ligands form stable complex.
The instability constant or the dissociation constant of compounds is defined as the
reciprocal of the formation or stability Constant.
NCERT INTEXT QUESTIONS
Page No. 244
Answer.1
(i) [CO(NH3)4(H2O)2]Cl3.
(ii)K2[Ni(CN)4]
(iii)[Cr(en)3]Cl3
(iv)[Pt (NH3) Br Cl (N02)]–
(v)[PtCl2(en)2](N03)2
(vi)Fe4[Fe(CN)6]3
Answer.2
(a) hexaamminecobalt (III) chloride
(b) pentaamminechloridocobalt (III) chloride
(c) potassium hexacyanoferrate (III)
(d) potassium trioxalatoferrate (III)
(e) potassium tetrachloridoplatinum (II)
(f) diamminechlorido (methylamine) platinum(II) chloride.
Page No. 254
Answer.5
Outer electronic configuration of nickel (Z = 28) in ground state is 3d84s2. Nickel in this
complex is in + 2 oxidation state. It achieves + 2 oxidation state by the loss of the two
4s-electrons. The resulting Ni2+ ion has outer electronic configuration of 3d8. Since CN–
ion is a strong field, under its attacking influence, two unpaired electrons in the 3d
orbitals pair up.
Outer electronic configuration of nickel (Z = 28) in ground state is 3d84s2 Nickel in this
complex is in + 2 oxidation state. Nickel achieves + 2 oxidation state by the loss of two
4s-electrons. The resulting Ni2+ ion has outer electronic configuration of 3d8. Since CP
ion is a weak field ligand, it is not in a position to cause electron pairing.
Answer.6
ion is a weak field ligand, it is not in a position to cause electron pairing.
ion is a weak field ligand, it is not in a position to cause electron pairing.
Answer.7
Answer.8
Answer.9
Answer.10
Mn(II) ion has 3d5 configuration. In the presence of H
ligands, the distribution of these five electrons is
unpaired to form a high spin complex. However, in the presence of CN
strong field ligands, the distribution of these electrons is
contain paired electrons while the third t
complex formed is a low spin complex.
Page No.256
Answer.11
Overall stability constant (β4) = 2.1 x 1013.
Thus, the overall dissociation constant is
NCERT EXERCISES
Page No.258
Answer.1
The main postulates of Werner’s theory of coordination compounds are as follows:
(a)Metals possess two types of valences called
(i) primary valency which are ionisable; (ii) secondary valency which are non
(b)Primary valency is satisfied by the negative ions and it is that which a metal exhibits
in the formation of its simple salts.
(c)Secondary valences are satisfied by neutral ligand or negative ligand and are those
which metal exercises in the formation of its complex ions. Ev
number of secondary valences which are directed in space about central metal ion in
configuration. In the presence of H2O molecules acting as weak field
distribution of these five electrons is t32ge
2g i. e., all the electrons remain
unpaired to form a high spin complex. However, in the presence of CN
strong field ligands, the distribution of these electrons is t52ge
0g i.e., two t
ain paired electrons while the third t2g orbital contains one unpaired electron. The
complex formed is a low spin complex.
Overall stability constant (β4) = 2.1 x 1013.
Thus, the overall dissociation constant is
The main postulates of Werner’s theory of coordination compounds are as follows:
(a)Metals possess two types of valences called
(i) primary valency which are ionisable; (ii) secondary valency which are non
satisfied by the negative ions and it is that which a metal exhibits
in the formation of its simple salts.
(c)Secondary valences are satisfied by neutral ligand or negative ligand and are those
which metal exercises in the formation of its complex ions. Every cation has a fixed
number of secondary valences which are directed in space about central metal ion in
O molecules acting as weak field
i. e., all the electrons remain
unpaired to form a high spin complex. However, in the presence of CN– acting as
i.e., two t2g orbitals
orbital contains one unpaired electron. The
The main postulates of Werner’s theory of coordination compounds are as follows:
(i) primary valency which are ionisable; (ii) secondary valency which are non- ionisable
satisfied by the negative ions and it is that which a metal exhibits
(c)Secondary valences are satisfied by neutral ligand or negative ligand and are those
ery cation has a fixed
number of secondary valences which are directed in space about central metal ion in
certain fixed directions, e.g. In CoCl
valences and valences between Co and NH3
ammonia molecules linked to Co by secondary valences are directed to six corners of a
regular octahedron and thus account for structure of COCl
In modern theory, it is now referred as coordination number of central metal atom
ion.
Answer.2
When FeSO4 and (NH4)2SO4 solutions are mixed in 1: 1 molar ratio, a double salt known
as Mohr’s salt is formed. It has the formula FeSO
solution, the salt dissociates as:
The solution gives the tests for all
when CuSO4 and NH3 are mixed in the molar ratio of 1: 4 in solution, a complex
[Cu(NH3)4]SO4 is formed. Since the Cu
in square bracket), it will not give th
Answer.3
Coordination entity: It constitutes of a central atom/ion bonded to fixed number of
ions or molecules by coordinate bonds
e.g. [COCl3(NH3)3], [Ni (CO)4] etc.
Ligand: The ions/molecules bound to central atom/ion in coordination entity are called
ligands. Ligands in above examples are C
Coordination number: This is the number of bonds formed by central atom/ion with
ligands.
Coordination polyhedron: Spatial a
complex. E.g. Co and Ni polyhedron are octahedral and tetrahedral in [CoCl3
and [Ni(CO)4] respectively.
Homoleptic: Metal is bound to only one kind of ligands e
Heteroleptic: Metal is bound to more than one kind of ligand
In CoCl3-6NH3, valences between Co and Cl are primary
valences and valences between Co and NH3 are secondary. In COCl
ammonia molecules linked to Co by secondary valences are directed to six corners of a
regular octahedron and thus account for structure of COCl3-6NH3 as follows:
In modern theory, it is now referred as coordination number of central metal atom
solutions are mixed in 1: 1 molar ratio, a double salt known
as Mohr’s salt is formed. It has the formula FeSO4. (NH4)2SO4.6H2O. In aqueous
solution, the salt dissociates as:
The solution gives the tests for all the ions including Fe2+ ions. On the other hand,
are mixed in the molar ratio of 1: 4 in solution, a complex
is formed. Since the Cu2+ ions are a part of the complex entity (enclosed
in square bracket), it will not give their characteristic tests as are given by Fe
: It constitutes of a central atom/ion bonded to fixed number of
ions or molecules by coordinate bonds
] etc.
The ions/molecules bound to central atom/ion in coordination entity are called
ligands. Ligands in above examples are Cl, NH3, CO
: This is the number of bonds formed by central atom/ion with
Spatial arrangement of ligands defining the shape of
Co and Ni polyhedron are octahedral and tetrahedral in [CoCl3
Metal is bound to only one kind of ligands e.g. Ni in[Ni(CO)
und to more than one kind of ligands e.g. Coin [CoCl
, valences between Co and Cl are primary
are secondary. In COCl3-6NH3, six
ammonia molecules linked to Co by secondary valences are directed to six corners of a
as follows:
In modern theory, it is now referred as coordination number of central metal atom or
solutions are mixed in 1: 1 molar ratio, a double salt known
O. In aqueous
the ions including Fe2+ ions. On the other hand,
are mixed in the molar ratio of 1: 4 in solution, a complex
ions are a part of the complex entity (enclosed
eir characteristic tests as are given by Fe2+ ions.
: It constitutes of a central atom/ion bonded to fixed number of
The ions/molecules bound to central atom/ion in coordination entity are called
: This is the number of bonds formed by central atom/ion with
rrangement of ligands defining the shape of
Co and Ni polyhedron are octahedral and tetrahedral in [CoCl3 (NH3)3]
Ni in[Ni(CO)4]
Coin [CoCl3(NH3)3]
Answer.4
Unidentate ligand: A molecule or an ion which has only one donor atom to form one
coordinate bond with the central metal atom is called unidentate ligand, e.g. Cl- and
NH3.
Bidentate ligand: A molecule or ion which contains two donor atoms and hence forms
two coordinate bonds with the central metal atom is called a Bidentate ligand.
Ambidentate ligand: A molecule or an ion which contains two donor atoms but only
one of them forms a coordinate bond at a time with the central metal atom is called
Answer.5
Answer.6
(a) [Zn(OH)4]2-
(b) [Pt(NH3)6]4+
(c) K2[PdCl4]
(d) [Cu(Br)4]2-
(e) [CO(NH3)6](SO4)3
(f) K2[Ni(CN)4]
(g) K3[Cr(OX)3]
(h) [CO(NH3)5ONO]2+
(i) [Pt(NH3)2Cl2]
(j) [CO(NH3)5NO2]2+
Answer.7
(i) Hexamminecobalt (III) chloride.
(ii) Diamminechlorido (methylamine) platinum (II) chloride.
(iii) Hexaaquatitanium (III) ion.
(iv) Tetraamminechloridonitrito
(v)Hexaaquamanganese (II) ion.
(vi)Tetrachloridonickelate (II) ion.
(vii)Hexaamminenickel (II) chloride.
(viii)Tris (ethane -1,2-diamine) cobalt (III) ion.
(ix) Tetra carbonyl nickel (0).
Page No.259
Answer.13
Aqueous CuS04 solution exists as [Cu(H
[Cu(H20)4]2+ ions.
(i) When KF is added, the weak H
ions which is a green precipitate.
(ii)When KCl is added, Cl– ligands replace the weak H
which has bright green colour.
Answer.14
First cupric cyanide is formed which decomposes to give cuprous cyanide and
cyanogen gas. Cuprous cyanide dissolves in excess of potassium cyanide to form the
complex, K3[Cu(CN)4],
Thus, coordination entity formed in the above reaction is [Cu(CN)4]3
strong ligand, the complex ion is highly stable and does not dissociate/ionize to give
Cu2+ ions. Hence, no precipitate,with H
(iii) Hexaaquatitanium (III) ion.
(iv) Tetraamminechloridonitrito-N-cobalt (IV) chloride.
(v)Hexaaquamanganese (II) ion.
(II) ion.
(vii)Hexaamminenickel (II) chloride.
diamine) cobalt (III) ion.
solution exists as [Cu(H20)4]S04 which has blue colour due to
KF is added, the weak H20 ligands are replaced by F– ligands forming [C
ions which is a green precipitate.
ligands replace the weak H20 ligands forming [CuCl
which has bright green colour.
c cyanide is formed which decomposes to give cuprous cyanide and
cyanogen gas. Cuprous cyanide dissolves in excess of potassium cyanide to form the
Thus, coordination entity formed in the above reaction is [Cu(CN)4]3
trong ligand, the complex ion is highly stable and does not dissociate/ionize to give
ions. Hence, no precipitate,with H2S is formed.
which has blue colour due to
ligands forming [CuF4]2-
0 ligands forming [CuCl4]2- ion
c cyanide is formed which decomposes to give cuprous cyanide and
cyanogen gas. Cuprous cyanide dissolves in excess of potassium cyanide to form the
Thus, coordination entity formed in the above reaction is [Cu(CN)4]3-. As CN– is a
trong ligand, the complex ion is highly stable and does not dissociate/ionize to give
Answer.15
Answer.16
Answer.17
The crystal field splitting, ∆0, depends upon the field produced by the ligand and
charge on the metal ion. Some ligands are able to produce strong fields in which, the
splitting will be large whereas others produce weak fields and consequently result in
small splitting of d-orbitals. In general, ligands can be arranged in a series in the order
of increasing field strength as given below :
Answer.18
When the ligands approach a transition metal ion, the d
one with lower energy and the other with higher energy. The difference of energy
between the two sets of orbitals is called crystal field splitting energy (Δ0
octahedral field). If Δ0 < P (pairing energy), the fourth electron enters
one of the e°g, orbitals giving the
complexes. Such ligands for which Δ0
fourth electron pairs up in one of the t
t42ge1g thereby forming low spin complexes. Su
strong field ligands.
∆0, depends upon the field produced by the ligand and
charge on the metal ion. Some ligands are able to produce strong fields in which, the
splitting will be large whereas others produce weak fields and consequently result in
orbitals. In general, ligands can be arranged in a series in the order
of increasing field strength as given below :
When the ligands approach a transition metal ion, the d-orbitals split into two sets,
and the other with higher energy. The difference of energy
between the two sets of orbitals is called crystal field splitting energy (Δ0
< P (pairing energy), the fourth electron enters
, orbitals giving the configuration t32ge
1g, thus forming high spin
which Δ0 < P are called weak field ligands. If Δ0
fourth electron pairs up in one of the t2g orbitals giving the configuration
thereby forming low spin complexes. Such ligands for which Δ0> P are called
∆0, depends upon the field produced by the ligand and
charge on the metal ion. Some ligands are able to produce strong fields in which, the
splitting will be large whereas others produce weak fields and consequently result in
orbitals. In general, ligands can be arranged in a series in the order
orbitals split into two sets,
and the other with higher energy. The difference of energy
between the two sets of orbitals is called crystal field splitting energy (Δ0 for
< P (pairing energy), the fourth electron enters
, thus forming high spin
< P are called weak field ligands. If Δ0 > P, the
orbitals giving the configuration
ch ligands for which Δ0> P are called
Answer.19
Answer.20
In [Ni(H20)6]2+, Ni is in + 2 oxidation state and having 3d
which there are two unpaired electrons which do not pair in the presence of the wea
H20 ligand. Hence, it is coloured. The d
complementary light emitted is green.
In [Ni(CN)4]2- Ni is also in + 2 oxidation state and having 3d8
But in presence of strong ligand CN
up. Thus, there is no unpaired electron present. Hence, it is colourless.
Answer.21
In both the complexes, Fe is in + 2 oxidation state with d6 configuration. This means
that it has four unpaired electrons.Both CN
ligands occupy different relative positions in the spectrochemical series. They differ in
crystal field splitting energy (∆
to different wavelengths/frequencies fr
the transmitted colours are also different. This means that the complexes have
different colours in solutions.
, Ni is in + 2 oxidation state and having 3d8 electronic configuration, in
which there are two unpaired electrons which do not pair in the presence of the wea
0 ligand. Hence, it is coloured. The d-d transition absorbs red light and the
complementary light emitted is green.
Ni is also in + 2 oxidation state and having 3d8 electronic configuration.
But in presence of strong ligand CN– the two unpaired electrons in the 3d orbitals pair
up. Thus, there is no unpaired electron present. Hence, it is colourless.
In both the complexes, Fe is in + 2 oxidation state with d6 configuration. This means
that it has four unpaired electrons.Both CN– ion and H2O molecules which act as
ligands occupy different relative positions in the spectrochemical series. They differ in
∆0). Quite obviously, they absorb radiations corresponding
to different wavelengths/frequencies from the visible region of light. (VIBGYOR) and
the transmitted colours are also different. This means that the complexes have
.
electronic configuration, in
which there are two unpaired electrons which do not pair in the presence of the weak
d transition absorbs red light and the
electronic configuration.
npaired electrons in the 3d orbitals pair
up. Thus, there is no unpaired electron present. Hence, it is colourless.
In both the complexes, Fe is in + 2 oxidation state with d6 configuration. This means
O molecules which act as
ligands occupy different relative positions in the spectrochemical series. They differ in
). Quite obviously, they absorb radiations corresponding
om the visible region of light. (VIBGYOR) and
the transmitted colours are also different. This means that the complexes have
Answer.22
In metal carbonyl, the metal carbon bond (M
character. The bond are formed by overlap of atomic orbital of metal with that of C
atom of carbon monoxide in following sequence:
(a)σ -bond is first formed between metal and carbon when a vacant d
atom overlaps with an orbital containing
monoxide (: C = O:)
(b)In addition to σ -bond in metal carbonyl, the electrons from filled d
transition metal atom/ ion are back donated into anti bonding π
monoxide. This stabilises the metal ligand bonding. The above two concepts are shown
in following figure:
Answer.23
(i) K3[Co(C2O4)3] =>[CO(C204)3]
number is also 6 as C2042- is didentate. Co+3 is a case in which all elec
(ii) cis – [Cr(en)2Cl2]+ Cl–
x + 0—2 =+1
Oxidation state, x =+3
Coordination number is 6 as ‘en’ is didentate. Cr
(iii) (NH4)2[COF4] = (NH4)22+[COF4]
x —4 =—2.
Oxidation state, x = + 2
Coordination number=4.
Co2+ is a d5 case, paramagnetic
(iv)[Mn(H20)6]2+S042-
x+0f+2
.•. Oxidation state, x- + 2
Coordination number is 6.
Mn+2 is a d5 case, paramagnetic
Answer.24
(i) K[Cr(H20)2(C204)2|-3H20
In metal carbonyl, the metal carbon bond (M – C) possess both the σ and π
character. The bond are formed by overlap of atomic orbital of metal with that of C
atom of carbon monoxide in following sequence:
bond is first formed between metal and carbon when a vacant d
atom overlaps with an orbital containing lone pair of electrons on C
bond in metal carbonyl, the electrons from filled d
transition metal atom/ ion are back donated into anti bonding π-orbitals of carbon
ses the metal ligand bonding. The above two concepts are shown
)3]3-. x + 3 (-2) = -3. Oxidation state, x=+3 Coordination
is didentate. Co+3 is a case in which all elec
Coordination number is 6 as ‘en’ is didentate. Cr3+ is a cfi case, paramagnetic.
] = (NH4)22+[COF4]2-
case, paramagnetic
case, paramagnetic
3H20
C) possess both the σ and π -bond
character. The bond are formed by overlap of atomic orbital of metal with that of C-
bond is first formed between metal and carbon when a vacant d-orbital of metal
lone pair of electrons on C-atom of carbon
bond in metal carbonyl, the electrons from filled d-orbitals of a
orbitals of carbon
ses the metal ligand bonding. The above two concepts are shown
3. Oxidation state, x=+3 Coordination
is didentate. Co+3 is a case in which all electrons are paired
is a cfi case, paramagnetic.
IUPAC name is potassiumdiaquadioxalatochromate (III) hydrate.
Coordination number = 6
Oxidation state of Cr: x + 0 + 2 (-2) = – 1
.‘. x = + 3
Shape is octahedral Electronic configuration of Cr3+ = 3d3=t32ge
°g .
Magnetic moment,
μ=√n(n+2=√3×5=√15BM
= 3-87 BM
(ii) [Co(NH3)5CIlCl2IUPAC name is pentaamminechloridocobalt (III) chloride
Coordination number of Co = 6 Shape is octahedral.
Oxidation state of Co, x + 0 -1 = + 2 .’. x = + 3
Electronic configuration of Co3+ = 3d6 = t62ge°g n=0, μ =0 .
(iii) CrCI3(Py)3. IUPAC name istrichloridotripyridine chromium (III).
Answer.25
Formation of a complex in solution is an equilibrium reaction. It may be represented as
M+4L⇌ML4
The equilibrium constant of this reaction is the measure of stability of the complex.
Hence the equilibrium constant is also called as stability constant or Instability
constant may be defined as equilibrium constant for reverse reaction. The formation
of above complex may also be written in successive steps:
Stability constant is written as
β4=K1K2K3K4.
Greater the stability constant, stronger is the metal-ligand bond.
The stability of complex will depend on
(a)nature of metal
(b)Oxidation state of metal
(c)Nature of ligand e g. chelating ligand form stabler complexes
(d)Greater the basic strength of the ligand, more will be the stability.
Answeer.26
When a didentate or a polydentate ligand contains donor atoms positioned in such a
way that when they coordinate with the central metal ion, a five or a six membered
ring is formed, the effect is called chelate effect. For example,
Answer.28
Coordination number of cobalt = 6
Hence, the complex is [Co (NH3)6] Cl2. It ionizes in the solution as follows :
Thus, three ions are produced. Hence, the correct option is (iii)
Answer.29
The oxidation states are: Cr (III), Fe (II) and Zn (II).
Electronic configuration of Cr3+ = 3d3, unpaired electron = 3
Electronic configuration of Fe2+ = 3d6, unpaired electron = 4
Electronic configuration ofZn2+ = 3d10, unpaired electrons = 0
μ=√n(n+2)
where V is number of unpaired electrons Hence, (ii) has highest value of magnetic
moment.
Answer.30
Page No. 260
Answer.31
In each of the given complex, Fe is in + 3 oxidation state. As C2042-is didentate chelating
ligand, it forms chelate rings and hence (iii) out of complexes given above is the most
stable complex.
Answer.32
As metal ion is fixed, the increasing field strengths, i.e., the CFSE values of the ligands
from the Spectro-chemical series are in the order: H20<NH3< NO2–;
Thus, the energies absorbed for excitation will be in the order: