COORDINATION COMPOUNDS
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Class 12th
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The compounds in which central metal atom or ion is
surrounded by a group of ligands are known as coordination compounds or
complexes.E.g K4 [Fe (CN)6] etc.
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COORDINATION CHEMISTRY:-The
branch of chemistry that deals with the study of coordination compounds is
known as coordination chemistry.
COORDINATION
COMPOUNDS OR COMPLEXES:-The compounds in which central metal atom or ion is
surrounded by a group of ligands are known as coordination compounds or
complexes.E.g K4[Fe(CN)6] etc.
CENRTAL
METAL ATOM OR ION:-The metal
atom or ion to which ligands are attached in a complex is called central metal
atom or ion.e.g.in the complex K4[Fe(CN)6] Fe is the
central metal atom.
LIGANDS:-The ions or neutral molecules which surround
the central metal atom in a complex are called ligands.The ligands are bonded
to the central metal atom by coordinate bond.e.g. in the complex K4[Fe(CN)6]
central metal atom Fe is surrounded by six cyanide ligands.
COORDINATION
NUMBER:- The number of ligands attached to the central
metal atom is known as coordination number.e.g .in the complex K4[Fe(CN)6]
the coordination number of Fe is six.
COORDINATION
SPHERE:-The ligands directly attached
to the central metal atom constitute coordination sphere of the complex.It is
written in square bracket.It behaves as a unit and is insoluble part of the
complex.e.g. in the complex K4[Fe(CN)6],central metal
atom iron and six cyanide ligands constitute coordination sphere.
COUNTER
ION:-The portion of the complex
outside the coordination sphere is known as counter ion.It is ionisable part of
the complex.e.g. in the complex K4[Fe(CN)6] K+ acts as A counter ion.
COMPLEX
ION:- A complex ion is an
electrically charged species which consists of central metal atom and ligands.
e.g. [Fe(CN)6]4-
, [Co(NH3)6]3+ etc.
COORDINATION
COMPOUNDS OR COMPLEXES:-The
compounds in which central metal atom or ion is surrounded by a group of
ligands are known as coordination compounds or complexes.E.g K4[Fe(CN)6] etc.
TYPES
OF COMPLEXES:-Complexex are of thee types on
the basis of presence or absence of charge on the coordination sphere.
1) CATIONIC
COMPLEXES:-If the coordination sphere of a
complex carries net positive charge then it is called cationic complex.e.g.[Cr(H2O)6]Cl3,[Cr(H2O)5Cl]Cl2
etc.
2)ANIONIC
COMPLEXES:- If the coordination sphere of a complex carries net negative
charge then it is called anionic complex.e.g. K2[NiCl4],K[Ag(CN)2] etc.
3)NEUTRAL
COMPLEXES:-If the coordination sphere of a complex does not carry any
charge then it is known as neutral complex.e.g.[Ni(CO)4],[Fe(CO)5]
etc.
OXIDATION
NUMBER:-It is the charge present on the
central metal ion of the complex.e.g. in the complex K4[Fe(CN)6]
the oxidation number of Iron is +2.
LIGANDS:-The ions or neutral molecules which
surround the central metal atom in a complex are called ligands.The ligands are
bonded to the central metal atom by coordinate bond.e.g. in the complex K4[Fe(CN)6]
central metal atom Fe is surrounded by six cyanide ligands.
DONOR
ATOM:-The atom in the ligand which
donates the electron pair is called donor atom.e.g. in cyanide ligand (CN-)
the donar atom is carbon.
DENTICITY:-The number of donor atoms present in a
ligand is known as its denticity.e.g. ethylenediamine is a didentate ligand due
to the presence of two donor atoms and EDTA is a hexadentate ligand due to the
presence of six donor atoms.
TYPES OF LIGANDS:-On the basis of number of donar atoms
ligands are of thee types.
1)UNIDENTATE
OR MONODENTATE LIGANDS:-The ligands which are bonded to the central metal
atom through one donor atom only are called unidentate ligands.e.g.NH3,CN-,NO3-etc.
2)AMBIDENTATE
LIGANDS:-The unidentate ligand containing more than one donor atoms are called ambidentate
ligands.This ligand can be linked to the central metal atom through either of
the two donor atoms.e.g.
2) DIDENTATE
LIGANDS:-The ligands which are bonded to the central metal atom through two
donor atoms are known as didentate ligands.e.g.ethane-1,2-diamine(NH2CH2CH2NH2),oxalate
ion(C2O4)2-
3)POLYDENTATE
OR MULTIDENTATE LIGANDS:-The ligands which are bonded to the central metal
atom through more than two donor atoms are called polydentate ligands. They may
be tridentate, tetradentate, pentadentate or hexadetnate ligands.e.g. Ethylenediaminetetraacetate
ion is an important hexadentate ligand.It is abbreviated as (EDTA).
CHELATE,CHELATING
LIGAND AND CHELATION
If a didentate or polydentate ligand is attached
to the central metal atom forming a ring like complex then it is known as
chelating ligand and the ring like
complex is called chelate whereas the process of formation of chelate is known
as chelation.e.g.ethylenediamine(en) is a chelating ligand which forms a
chelate complex with central metal atom copper as follow:
TYPES
OF COMPLEXES ON THE BASIS OF TYPES OF LIGANDS:-Two
types
1) HOMOLEPTIC
COMPLEXES:-Complexes in which central metal atom is linked to only one type
of ligands are called homoletptic complexes.e.g.K4[Fe(CN)6],[Co(NH3)6]Cl3
etc.
2)
HETEROLETPTIC COMPLEXES:-The complexes in which central metal atom is linked to more than one
kind of ligands are called heteroleptic complexes.e.g.[Co(NH3)4Cl2]Cl, [Co(NH3)4Cl(NO)2]NO3
etc.
HOMONULCEAR COMPLEXES AND POLYNUCLEAR
COMPLEXES:-Complexes in which only one metal atom is present are known as
homonuclear complexes.e.g. [Co(NH3)4Cl2]Cl
etc.
COMPLEXES in which more than one metal atom
is present are known as polynuclear complexes.E.g.[(NH3)5Cr
– OH—Cr(NH3)5]Cl5 etc.
DOUBLE
SALT:- It is an addition compound which
is formed by the combination of two different
salts.e.g. Potash alum, Mohr’s salt etc.They are hydrated crystalline
compounds.They are very stable in the solid state but split into ions in their aqueous
solutions.
Difference
between Double salts and coordination compounds
DOUBLE SALT
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COMPLEXES
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1)It is formed by the combination of two
different salts.
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1)It
is formed when ligands surround the central metal atom.
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2)They are ionic compounds.
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2)They
may or may not be ionic.
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3)They
do not contain any coordinate bond.
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3)They
always contain coordinate bonds.
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4)They
exist only in the solid state.
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4)They
exist in the solid state as well as in aqueous solutions.
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5)They
lose their identity in solutions
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5)
They retain their identity in solutions
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6)
Metal ions show normal valency.
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6)Metal
ion shows two types of valencies that is primary valency and secondary
valency.
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NOMENCLATRUE OF COORDINATION COMPOUNDS
I) Name of
the complex starts with the small letter.
II) Cationic
part of the complex is written first and then the anionic part.
III) A
little space must be given between cation and anion.
IV) Non
ionic complexes should be given one word name without any space.
V) Ligands
are named first followed by the central metal ion without any space.
VI) Name of
the anionic ligands ends in “O”.
VII) Name of
the cationic ligands ends in “IUM”.
VIII)Neutral
ligands are named as such except following:
IX)In case
of heteroleptic complexes ligands are named in alphabetical order.
X) Numerical
prefixes are used to show the number of ligands of one kind.
XI) We use
bis,tris,tetrakis etc. for two three and four if name of the ligand includes numerical
prefixes.
XII) For
the anionic complexes name of central
metal atom ends in “ATE”.
XIII) Oxidation
number of central metal atom is written in Roman numerals after its name without
any space.
XIV) In case
of ambidentate ligands mention the symbol of the donor atom after
the name of ligand.
XV) In case
of geometrical isomerism cis and trans terms are used before the name of
complex.
XVI) In case
of optical isomers symbols “d”or (+) and”l” or (-) are used for dextrorotatory
and laevorotatory respectively.
THEORIES OF BONDING IN COMPLEXES
1)WERNER’S COORDINATION
THEORY
2)VALENCE
BOND THEORY(VBT)
3)CRYSTAL
FIELD THEORY(CFT)
4)MOLECULAR
ORBITAL THEORY(MOT)
5)LIGAND
FIELD THEORY(LFT)
WERNER’S COORDINATION THEORY
Alfred
Werner was the first inorganic chemist to propose theory of bonding in
complexes.His theory is known as Werner’s coordination theory.The main
postulates of this theory are:
1)In all the
complexes central metal atom exhibits two types of valencies.
a)Primary
valency.
b)Secondary
valency.
2)Primary
valency represents oxidation state of central metal atom.
3)Secondary
valency represents coordination number of central metal atom.
4)Primary
valency is ionisable part of the complex.
5)Secondary
valency is non-ionisable part of the complex.
6)Central
metal atom tends to satisfy both of its valenices.
7)Primary
valency is satisfied by the negative ions.
8)In some
cases negative ions may satisfy both types of valencies.
9) Secondary
Valency is satisfied by the ligands.
10) Primary Valency
is non-directional in nature.
11)
Secondary Valency is directional in nature hence it determines structure of the
complex.
12) Primary valency is represented
dotted line (….) whereas secondary valency is represented by solid line (--).
E.g.
Structure of complexes of cobalt on the basis of Werner’s theory.
1) [Co(NH3)6]Cl3
Here all the ammine molecules satisfy
secondary valency and is represented by solid lines whereas three chloride ions
satisfy primary valency which is represented by dotted lines.
[Co(NH3)6]Cl3 [Co(NH3)6]3+ + 3Cl -
Cobalt complex
non-ionisable
ionisable
2) [Co(NH3)4Cl2]Cl
[Co(NH3)4Cl2]Cl [Co(NH3)4Cl2]+ + Cl -
Cobalt complex non-ionisable ionisable
Some other examples are :- [Co(NH3)5Cl]Cl2 and [Co(NH3)3Cl3]
VALENCE BOND THEORY
This theory was given by Pauling in 1931.The
main postulates of this theory are:
1)The central metal ion contains a number of empty
orbitals.
2)The number of empty orbitals are equal to
the coordination number of central metal atom.
3)The appropriate atomic orbitals of the
central metal atom undergo hybridization and form hybrid orbitals.
4)The d-orbitals involved in the hybridization
may be inner (n-1)d orbitals or outer(n)d orbitals.
5)The complexes formed from inner (n-1)d
orbitals are called inner orbital complexes or low spin complexes.
6) The complexes formed from outer (n)d
orbitals are called outer orbital complexes or high spin complexes.
7) The arrangement of hybrid orbitals determines
geometry of the complex.
8)Each ligand contains at least one orbital
with a lone pair of electrons.
9)The empty hybrid orbitals of central metal
atom make coordination bonds with filled orbitals of ligands.
10) The complexes with unpaired electrons are
paramagnetic in nature whereas those with paired electrons are diamagnetic in
nature.
APPLICATION OF VALENCE BOND
THEORY
This theory is very useful in explaining the behavior
of complexes like hybridization, geometry, type of complexes and magnetic character.
While explaining a complex on the basis of VBT following
Points should be mentioned:
I)Atomic number of central metal atom.
II)Electronic configuration of central metal
atom and its ion.
III)Types of hybridization.
IV) Type of complex that is inner orbital or
outer orbital complex.
V) Magnetic character.
VI)Geometry of the complex.
TYPE OF
HYBRIDIZATION
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GEOMETRY
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1) sp
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Linear
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2) sp2
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Trigonal planar
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3) sp3
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Tetrahedral
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4) sp3d
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Trigonal bipyramidal
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5) sp3d2 or d2sp3
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Octahedral
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6) sp3d3
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Pentagonal bipyramidal
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7) dsp2
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Square planar
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(I)IMPORTANCE OF COORDINATION COMPOUNDS IN BIOLOGICAL PROCESSES
1)CHLOROPHYLL:-It is a complex of
magnesium.It is essential for photosynthesis in plants.
2)HAEMOGLOBIN:-It is a complex of iron.It is
a respiratory pigment which carries oxygen in blood.
3)VITAMIN B12:- It is a complex of
cobalt.It is used in medicines.
(II)IMPORTANCE OF COORDINATION COMPOUNDS IN
EXTRACTION OF METALS (Metallurgy)
1)EXTRACTION OF SILVER:-Silver is extracted
from its ore by forming a complex with sodium cyanide.This is called cyanide
process.
Ag2S +
4NaCN 2Na[Ag (CN)2] + Na2S
(ore) soluble
complex
The solution of silver complex is treated with zinc dust to get the
precipitates of silver metal.
2Na[Ag (CN)2] +
Zn 2Ag
+ Na2[Zn (CN)4]
2) EXCTRACTION OF GOLD:-Similarly cyanide
process is used for the extraction of gold.Then gold is extracted from its
complex by treating with zinc dust.
2K[Au(CN)2] + Zn K2[Zn(CN)4] + 2Au
USES OR IMPORTANCE APPLICATIONS
OF COMPELXES
1) Estimation
of hardness of water:- The complexometric titrations are performed with
EDTA to detect the presence of calcium ions and magnesium ions in water. These
ions are responsible for hardness of water. These ions form stable complexes
with EDTA.
2) In
Electroplating:-The coordination compounds are used for electroplating the
articles with silver,gold and copper for good finishing results.
3) Removal
of hardness of water:-The complex compounds are used to remove calcium ions
and magnesium ions from hard water.e.g. sodium metapolyphosphate.
4)As
catalysts:-The complexes are used as catalysts for many industrial
processes:
a)The complex [RhCl(Ph3P)] named
chlorotris(triphenyl phosphine)rhodium(I) is used for selective hydrogenation
of alkenes.This complex is commonly known as Wilkinson catalyst.
5)In
medicines:-a) A platinum complex [PtCl2(NH3)2]
known as cis-platin is used to cure cancer.
b)EDTA is used to treatment of lead
poisoning.
Q:- (III) State
two applications of complexes in qualitative analysis of cations.
Ans:-6)In
qualitative analysis:-a)Ni2+ ion
is detected and estimated by forming pinkish red ppt of nickel dimethyl glyoxime
complex using dimethyl glyoxime(DMG) reagent in the presence of ammonium hydroxide.
b)Silver
radical is detected by separating from the ppt of AgCl, Hg2Cl2
and PbCl2 by dissolving in aqueous solution of ammonia to form
soluble complex[Ag(NH3)2]Cl. Hg2Cl2
and PbCl2 do not form complexes and therefore do not dissolve.
AgCl + 2NH3 [Ag(NH3)2]Cl
7)In
quantitative analysis:-The complexometric titrations are performed with
EDTA to detect the presence of calcium ions and magnesium ions in water.These
ions are responsible for hardness of water.These ions form stable complexes
with EDTA.
Q:-Explain the complex ion [Cr(NH3)6]3+
on the basis of VBT.
Ans:-1)Central metal atom:-Chromium(Cr)
2)Atomic number:-24
3)) Electronic configuration of Cr:-[Ar]183d54s1
Electronic configuration of Cr3+ :-[Ar]183d3
4)In this complex chromium atom undergoes d2sp3 hybridization.
5)This is an inner orbital complex because
(n-1)d orbitals are involved .
6)This
complex is paramagnetic in nature due to the presence of three unpaired
electrons.
7)This complex has octahedral geometry.
APPLICATIONS OF CHELATES
1)In qualitative analysis for the detection
of metal ions
2)In quantitative analysis for the estimation
of Ni2+,Mg2+,and Cu2+ ions.
3)In softening of hard water.
4)In the separation of lanthanides and
actinides.
Q:-Name the central metal atom present in
haemoglobin and chlorophyll?
Q:-What is the shape or geometry of [Ni(CN)4]2-?
Q:-Write IUPAC name of [Ni(CO)4].
Q:-Give few examples of positively charged
ligands.
Ans:-Nitrosonium ion(NO+),Nitronium ion(NO2+),
Q:-Ammonia is a good complexing agent why?
OR
Q:-How is ammonia molecule a good ligand?
Ans:- Ans:-Due to the presence of lone pair
of electrons on the N-atom ,ammonia acts as good complexing agent and forms
complexes with electron deficient species.E.g.
Q:-Explain the following complex ions on the
basis of VBT.(inner orbital complexes)
[Fe(CN)6]3-, [Fe(CN)6]4-.
Ans:-
Q”-Explain the following complex ions on the
basis of VBT.(outer orbital complexes)
[CoF6]3-,[Fe(H2O)6]3+,
Ans:- [CoF6]3-
1)Central metal atom:-Cobalt(Co)
Atomic number:-27
2) Electronic configuration of cobalt:-[Ar]183d74s2
Electronic configuration of Co3+ ion :- :-[Ar]183d6
3) In this complex central metal atom cobalt
is sp3d2 hybridized.
4)This is an outer orbital complex as ns-d
orbitals are involved in hybridization.
5)This complex is paramagnetic in nature due
to the presence of four unpaired electrons.
6)This complex has regular octahedral
geometry.
Q:-Give three examples of outer orbital
complexes and inner orbital complexes each.
Q:-Explain the following complex ions with
coordination number four on the basis of VBT.
[Ni(CN)4]2-,[NiCl4]2-,[FeCl4]2-,[Zn(NH3)4]2+and [Ni(CO)4]
Ans:- The complexes with coordination number four
undergo two types of hybridization .
1)sp3
hybridization:-The complexes with
sp3 hybridization have
tetrahedral geometry. e.g [NiCl4]2-.
2)dsp2
hybridization:-The complexes with
dsp2 hybridization have square planar geometry. eg[Ni(CN)4]2-
[Ni(CN)4]2-
1)Central metal
atom:-Ni
Atomic no:-28
2)Electronic
configuration:-[Ar]18 3d8 4s2
Electronic configuration of Ni2+ [Ar]183d8
3)In this complex ion Nickel undergoes dsp2 hybridization
4)This is an inner orbital complex or low
spin complex.
5)This complex is diamagnetic in nature as
all the electrons are paired.Cyanide ion is a strong field ligand and hence it
forces the electrons of the d-orbitals to get them paired up.
6)It has square planar geometry.
2)[NiCl4]2-
1)Central metal
atom:-Ni
Atomic no:-28
2)Electronic configuration:-[Ar]18 3d8 4s2
Electronic configuration of Ni2+ [Ar]183d8
3)In this complex the central metal ion
nickel has sp3 hybridization.
4)It is an outer orbital complex.
5)It is paramagnetic in nature due to the
presence of two unpaired electrons.
Chloride ion is a weak field ligand and hence it does not force the electrons
of the d-orbitals to get paired up.
6)This complex has tetrahedral geometry.
Q:-what is chelate effect:-
Ans:-The
formation of chelate rings increases the stability of the complexes. This
stabilization of complexes due to the process of chelation is known as chelate
effect.
Q:-Name
the compound used for measuring the hardness of water?
Ans:-EDTA
CRYSTAL FILED THEORY(CFT)
This theory was given to explain bonding and
properties of the complexes. The main points of this theory are:
1)The interactions between central metal atom
and ligands are purely electrostatic in nature.
2)All the five d-orbitals of free metal ion are degenerate in nature that
is they have equal energies.
3)Under the influence of ligand’s field these
five degenerate d-orbitals split into two parts.
a)Triply degenerate d-orbitals with lower
energy are called t2g orbitals(dxy,dyz and dzx).
b)Doubly degenerate d-orbitals with higher
energy are called eg .
4)The splitting of five degenerate d-orbitals into different sets of orbitals in
the presence of ligands is called crystal field splitting.
5)The energy difference between the two sets
of d-orbitals that is t2g and
eg is
called crystal field splitting energy. It is denoted as
O.
6)Crystal field splitting energy measures
field strength of ligands. Depending upon the field strength the ligands are of
two types:
a) STRONG FIELD LIGANDS:-The ligands
which cause crystal field splitting to a large extent are called strong filed
ligands.e.g.CN-,CO etc.
b)WEAK FIELD LIGANDS:-The ligands
which cause crystal field splitting to a smaller extent are called weak field
ligands.e.g.Br-,Cl-,OH-,H2O, etc.
7)Let us consider the splitting of octahedral
and tetrahedral complexes under the influence of ligand’s field.
Q:- Explain the splitting of degenerate
d-orbitals in octahedral complexes on the basis of CFT.
a) SPLITTING OF D-ORBITALS FOR OCTAHEDRAL
COMPLEXES:-
b) SPLITTING OF D-ORBITALS FOR TETRAHEDRAL
COMPLEXES:-
Q:-Explain the splitting of degenerate
d-orbitals of transition metal ion on the basis of CFT.
SPECTROCHEMICAL SERIES
The arrangement of ligands in the increasing
order of their crystal field splitting energy values is known as
spectrochemical series.e.g.Iodide ligand (I-) is present
at the beginning of the series because it has lowest value of CFSE and carbonyl
ligand (CO) is present at the end of the series because it has highest value of
CFSE.
Q:-What are low spin complexes and high spin
complexes?
LOW SPIN COMPLEXES:-The complexes that contain strong field
ligands and have high value of CFSE are called low spin complexes. Such complexes
have very few number of unpaired electrons .e.g.[Fe(CN)6]3-,[Co(NH3)3]3+etc.
HIGH SPIN COMPLEXES:-The complexes that contain weak field
ligands and have low value of CFSE are called high spin complexes. Such
complexes have maximum number of unpaired electrons ..e.g.[Fe(H2O)6]3+,[CoF6]3-
etc.
CALCULATION OF CRYSTAL FIELD
SPLITTING ENERGY
For octahedral complexes the five degenerate
d-orbitals split into two sets one with two
d-orbitals above the average energy level called eg orbitals. And the other with three d-orbitals
below the average energy level called t2g
orbitals. The average energy
level is known as bary center.
Each electron occupying t2g orbitals results in lowering of energy by
-0.40
O.
Similarly, each electron occupying eg orbitals results in rising of energy by +0.60
O.
If x
is the number of electrons in t2g orbitals and y is the number of electrons in eg orbitals
then
CFSE =
(-0.4x + 0.6y)
O.
E.g. CFSE for d1,d2,d3,d4,d5
to d10
CFSE
for d3 = (-0.4 ¿ 3) = -1.2
O.
CFSE for d7 :- There are two
possibilities for low spin complex and high spin complex.
CFSE for low spin complex that is in the
strong ligand field =(-0.4¿6 + 0.6¿1) = -1.8
O.
CFSE for high spin complex that is in the
weak ligand field =(-0.4¿5 + 0.6¿2) = -0.8
O.
EXPLANATION OF MAGNETIC PROPERTIES ON THE
BASIS OF CFT
(1)PARAMAGNETIC SUBSTANCES:-The
substances which are weakly attracted by the magnetic field are called
paramagnetic substances and the phenomenon is known as paramagnetism.E.g. Cu2+,Fe3+,Cr3+
etc.Paramagnetism occurs due to the presence of unpaired electrons.
(2)DIAMAGNETIC SUBSTANCES: - The
substances which are weakly repelled by the magnetic field are called
diamagnetic substances and the phenomenon is known as diamagnetism.E.g. NaCl,H2O,
Benzene
etc. Diamagnetism
occurs due to the presence of paired electrons.
Consider the following complexes: .[Fe(CN)6]3-,[Co(NH3)3]3+.[Fe(H2O)6]3+,[CoF6]3-
[Fe(CN)6]3-,:-
1) Central metal atom: Fe
2)Atomic number:26
3)Oxidation number of Fe:+3
4) Electronic configuration
Fe: [Ar]18 3d64s2
Fe3+ : [Ar]18
3d5
5)Cyanide is a strong field ligand hence it
has large value of CFSE.
6)It is weakly paramagnetic due to the
presence of only one unpaired electron.
7)It is a low spin complex due to large value
of CFSE.
[Fe(H2O)6]3+
1) Central metal atom: Fe
2)Atomic number:26
3)Oxidation number of Fe:+3
4) Electronic configuration
Fe: [Ar]18 3d64s2
Fe3+ : [Ar]18
3d5
5) water is a
weak field ligand hence it has low value of CFSE.
6)It is strongly paramagnetic due to the
presence of five unpaired electrons.
7)This is a high spin complex due to small
value of CFSE.
[Co(NH3)6]3+
1)Central metal atom:Co
2)Atomic number:27
3)Oxidation number:+3
4)Electronic configuration:
Co: [Ar]18 3d74s2
Co3+ : [Ar]18
3d6
5)Ammonia is a strong field ligand hence it
has high value of CFSE.
6)This is a diamagnetic complex due to the
absence of unpaired electrons.
7) It is a low spin complex due to large
value of CFSE.
[CoF6]3-
1)Central metal atom:Co
2)Atomic number:27
3)Oxidation number:+3
4)Electronic configuration:
Fe: [Ar]18 3d74s2
Fe3+ : [Ar]18
3d6
5) Fluoride ion is a weak field ligand hence
it has low value of CFSE.
6) It is a paramagnetic complex due to the
presence of four unpaired electrons.
7) It is a high spin complex due to small
value of CFSE.
Q:-Explain the complexes .[Fe(CN)6]4-,[Co(NH3)3]3+.[Fe(H2O)6]2+,
on the basis of CFT.
Q:-Explain the color of coordination
compounds on the basis of CFT.
Ans:-The color of the complexes can be
explained on the basis of crystal field splitting theory. According to this theory
under the influence of ligands, the five degenerate d-orbitals of transition
metal ion split into two sets. If the energy difference that is CFSE between
these two sets of orbitals is small then the electrons present in the lower set
of orbitals can absorb the radiations from the visible region of
electromagnetic spectrum. The energy of these radiations is sufficient to
excite the electrons to the higher set of orbitals. Due to the absorption of
certain radiations from the visible region, the reflected or transmitted light
is colored. The visible light is a mixture of radiations of different
wavelengths which range from blue (400nm) to red (700nm).The color of a solid
complex or its solution is due to the light which is not absorbed but which is reflected
or transmitted. The absorbed and reflected colors are complementary to each
other.
E.g.
The solution of [Ti(H2O)6]3+ is purple in color. This is due to the reason that Ti3+ has one electron in
d-sub shell(3d1) .In the ground state this electron is present in
one of the lower set of orbitals(t2g). When this electron absorbs radiations of
green and yellow wavelengths from the visible region it gets excited to one of
the higher set of orbitals (eg).This is known as d-d transition. The
reflected or transmitted colors are blue and red which are complementary to the
absorbed colors that is green and yellow. The solution of the complex [Ti(H2O)6]3+ therefore, looks purple.
Q:-Explain stability of complexes in the
solution?
Ans:-The salts in their aqueous solutions
form complexes known as aqua complexes. e. g.CuSO4 in water forms
aqua complex of copper as follow:
CuSO4 + 4H2O [Cu(H2O)4]SO4
(aqua complex)
When liquid ammonia is added to the aqueous
solution of copper salt containing its aqua complex the water molecules get
replaced by the ammonia molecules stepwise as follow:
Here K1
is the equilibrium constant and is known as stepwise stability constant. In
this way each water molecule is replaced by the ammine ligands stepwise.
Now the
stability of this complex is calculated from the stepwise stability constant
for each step separately as follow:
Similarly, the values of remaining stepwise
stability constants i.e. K2, K3 and K4 are
calculated .Lastly the stability of a complex depends upon the value of overall
stability constant (K) which is calculated as follow:
K
= K1¿K2¿K3¿K4
Greater the value of overall stability
constant more stable will be the complex.
E.g. K for the above mentioned complex ion
that is [Cu(NH3)4]2+ is =
4.5¿1021
Whereas K for
the Cobalt complex [Co(NH3)6]3+ is =
5.0¿1033
Hence out of above two complexes, [Co(NH3)6]3+ is more stable due to greater value of its overall stability
constant i.e. K.
ISOMERISM
Two or more compounds having same molecular
formula but different physical and chemical properties are called isomers and
the phenomenon is known as isomerism. Complex compounds show two types of
isomerism.
1) Structural isomerism
2) Stereoisomerism or space isomerism
1)
STRUCTURAL ISOMERISM
Two or more compounds having same molecular
formula but different structural arrangement of atoms around the central metal
atom are called structural isomers and the phenomenon is known as structural isomerism.
It is further divisible into four parts.
(I) IONIZATION ISOMERISM
(II) HYDRATE
ISOMERISM
(III)
LINKAGE ISOMERISM
IV)
COORDINATION ISOMERISM
(I) IONIZATION
ISOMERISM
Compounds having same molecular formula but
give different counter ions in solution are called ionization isomers and the
phenomenon is known as ionization isomerism.
E.g.
[CoBr(NH3)5]SO4 and [Co SO4 (NH3)5]Br are ionization isomers because they give
different counter ions in their solutions that is sulphate ion and bromide ion respectively.
[CoBr(NH3)5]SO4
[CoBr(NH3)5]2+ +
SO42-
(violet)
[Co
SO4 (NH3)5]Br [Co SO4 (NH3)5]+ +
Br-
(red)
(II) HYDRATE ISOMERISM OR SOLVATE
ISOMERISM
Compounds having same molecular formula but
differ in the number of water molecules present as ligands are called hydrate
isomers or solvate isomers and the phenomenon is known as hydrate isomerism.
For example:
[Cr(H2O)6]Cl3
and [CrCl2(H2O)4]Cl.2H2O are
hydrate isomers because they differ in the number of water molecules as ligands
that is six and four respectively.
(III) LINKAGE ISOMERISM
Compounds having same molecular formula but
differ with respect to the attachment of donor atom of ambidentate ligand with
the central metal atom are called linkage isomers and the phenomenon is known
as linkage isomerism. For example [Cr(SCN)(H2O)5]3+ and [Cr(NCS)(H2O)5]3+ are linkage isomers because:
In [Cr(SCN)(H2O)5]3+,Sulphur is the donor atom and
In [Cr(NCS)(H2O)5]3+ Nitrogen is the donor atom.
(IV) COORDINATION ISOMERISM
This type of isomerism occurs when both
cation and anion are complexes. The two isomers differ in the distribution of
ligands in the cationic and anionic parts .For example:
a)[Co(NH3)6] [Cr(CN)6]
b) [Cr(CN)6] [Co(NH3)6]
In the above two examples the ammine and
cyanide ligands differ in their distribution in the cationic and anionic parts.
2) STEREOISOMERISM
OR SPACE ISOMERISM
Compounds which differ in the relative spatial arrangement of
ligands around central metal atom are
called stereoisomers and the phenomenon is known as stereoisomerism. It is of
two types:
(I)
GEOMETRICAL ISOMERISM
(II) OPTICAL
ISOMERISM
(I) GEOMETRICAL ISOMERISM
The isomerism in which the ligands occupy different positions around
the central metal atom, is known as geometrical isomerism. Geometrical isomers
are of two types
i) cis-isomers:-the geometrical isomers in which
two identical ligands occupy adjacent positions.
ii) trans-isomers:- The geometrical isomers
in which two identical ligands occupy positions opposite to one another.
Therefore this isomerism is also known as
cis-trans isomerism. The complexes with coordination number four and six show
geometrical isomerism.
(a) COMPLEXES
WITH COORDINATION NUMBER FOUR
Complexes with coordination number four have
two types of geometry.
i) TETRAHEDRAL
GEOMETRY
II) SQUARE PLANAR GEOMETRY
i) TETRAHEDRAL
GEOMETRY:-Complexes with tetrahedral
geometry do not show geometrical isomerism because in them all the positions
are adjacent to one another.
II) SQUARE PLANAR GEOMETRY:- square planar complexes show geometrical
isomerism.
1) MA2B2
E.g. [PtCl2(NH3)2],[Pd(NH3)2(NO2)2],[PtCl2(py)2]
etc.
2) MA2BC:- E.g. [PtCl(NH3)2(NO2)] diamminechloronitroplatinum(II),[Pt(NH3)Cl(py)2]
b) COMPLEXES
WITH COORDINATION NUMBER SIX
Complexes with coordination number six have
octahedral geometry. There are three types of octahedral complexes.
i) MA2B4 type.E.g. [CrCl2(NH3)4]+ , [Fe(CN)4(NH3)2]-
ii) M(AA)2B2.:- Here
(AA) means symmetrical didentate ligand like (en) and (ox).E.g.[Co(en)2Cl2]+, [Co(ox)2Cl2]+:
K[Cr(ox)2(H2O)2]
iii) MA3B3 type:- This
type of complexes show two types of geometrical isomers:
a) FACIAL ISOMER: when three identical ligands are present on
the same triangular face of the octahedron, the geometrical isomer is known as
facial or fac-isomer.
b)
MERIDIONAL ISOMER: When three
identical ligands are present on the same equatorial plane of the octahedron
i.e. around the meridian of the octahedron, the isomer is known as meridional
or mer-isomer.
E.g. [CoCl3(NH3)3]
, [Co(NH3)3(NO2)3] etc.
(II) OPTICAL
ISOMERISM
Compounds which rotate the plane of polarized
light equally but in opposite direction and are mirror images of each other are
called optical isomers and the phenomenon is known as optical isomerism. The
optical isomer, which rotates the plane of polarized light to the right, is
called dextrorotatory. It is designated as “d” or “+”.On the other hand the
isomer which rotates the plane of polarized light to the left is known as
laevorotatory. It is designated as “l” or “-“. Only those compounds exhibit
optical activity which do not have plane of symmetry in their structures. The
optical isomers have identical physical and chemical properties. They differ
only in the direction in which they rotate the plane of polarized light.
For example:
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