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COORDINATION COMPOUNDS
COORDINATION COMPOUNDS
Chapter Outline:
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Prerequisites
Learning Objectives
Introduction
Werner’s theory
Important terms related to co-ordination compounds
Nomenclature of co-ordination compounds
Naming of the complex ion
Isomerism in co-ordination compounds
Geometrical Isomerism
Optical isomerism
Bonding in co-ordination compounds and valence bond theory
Magnetic Properties
Crystal field theory
Limiations of Crystal Field Theory
Bonding in metal carbonyls
Stability of co-ordination compounds
Applications of co-ordination compounds
Summary
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COORDINATION COMPOUNDS
PREREQUISITES
w Transition elements form complex ions because of their smaller radii. The compound containing
complex ion are called co-ordination compounds.
w A complex ion is an electrically charged radical which is formed by the combination of a simple
cation with one or more neutral molecule or one or more simple anions or in some cases positive
groups also.
w Every complex ion contains a central metal atom or ion to which ligands are attached. Ligands
are ions or molecules that surround metal ion in a complex ion. These can be neutral, positive or
negative possessing at least one lone pair.
w Ligands are attached to metal atom or ion by means of a co-ordinate covalent bond.Ligands can be
monodentate (donating lone pair from one donar atom) bidentate or polydentate, two or more atoms
can donate lone pairs.
w Polydentate ligands whose structure permits the attachment of two or more donar sites
at the terminals of a chain to the same neutral ion simultaneously thus closing one or
more rings called chelating ligands and the complex called co-ordination chelate compound.
LEARNING OBJECTIVES
w In this topic we are going to discuss about Werner’s theory of co-ordination compounds.
w About the different terms used in co-ordination chemistry.
wRule for writing the Nomenclature of co-ordination compounds and their formulas.Isomerism
exhibited by co-ordination compounds.
w Bonding in co-ordination compounds with respect to valence bond theory and crystal field theory.
w Stabilities of co-ordination compounds.
w Importance and applications of co-ordination compounds.
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COORDINATION COMPOUNDS
INTRODUCTION
w All transition elements exhibit a characteristic property of complex ion formation, because of their
smaller radii.
Transition elements
w The complex which contain complex ions are called co-ordination compounds and the branch of
chemistry under these compounds is called co-ordination chemistry.
Co-ordinate compounds
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COORDINATION COMPOUNDS
w The development in the field of co-ordination compound or complex ion and organometallics
has contributed a lot in the following ways.
a) It contributed to the growth of chemical industry, such as metallurgy, catalysis etc.
Chemical industry
Metallurgy
b) It provided insight into the structures and functioning of biological systems.
Biological system
c) It contributed in the development of new concepts of chemical bonding and molecular
structures.
Molecular or Addition compounds
w When solutions containing two or more stable salts in simple molecular proportions are
evaporated, crystals of new compound may separate out.
w These new compounds are called molecular or addition compounds.
Addition Compounds
Double salts
Complex compounds
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COORDINATION COMPOUNDS
i) Double salts:
w Those which loose their identity in solution. In aqueous solutions these break down into
simpler ions thus exhibit the properties of these constituent ions.
Eg:
Carnalite
(KCl.MgCl2.6H2O)
Mohr’s Salt
FeSO4.(NH4)2SO4.6H2O
Potash alum
K2SO4.Al2(SO4)3.24H2O
ii) Complex compounds:
w Those which retain their identity in solution. In aqueous solution these do not furnish all simple ions
but instead give more complex ion having complicated structure.
Potassium ferrocyanide
K4[Fe (CN)6]
Hexaammine cobalt (III)
chloride [Co(NH3)6] Cl3
Double Salts
Complex Compounds
They exist only in crystalline state.
They exist both in crystalline and in
aqueous solution.
In aqueous solution, they yield at least one
complex ion.
In aqueous solution, they yield simple ions.
They are made of two simple salts combined in They are made of two simple salts, which
equimolecular proportions.
may or may not be combined in equimolecular
proportions.
They lose their identity in solution.
They retain their identity in solution.
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COORDINATION COMPOUNDS
WERNER’S THEOARY
OF
COORDINATION COMPOUNDS
w Werner proposed a theory called Werner’s coordination theory for which he was awarded
noble prize in 1913. The main postulates of his theory are
Werner
Metals posses
Primary or (principal) linkage
Secondary linkage
w Primary linkages (valencies) are ionisable and are exhibited by a metal in the formation of its simple
salt. Now a days primary valency is referred to as oxidation state of the metal ion.
w Secondary linkages (valancies) are non-ionisable and are exhibited by a metal in the formation of its
complex ion. Secondary valency is also referred as co-ordination number of the metal ion.
H 3N
Cl-
NH3
NH3
Co3+
H 3N
Cl-
Cl- NH3 NH3
w Primary valency is satisfied by negative ions while secondary valency can be satisfied by neutral
molecules, negative ion or positive ion (ligands).
w Every metal atom or ion has a fixed number of secondary linkages i.e., co-ordination number of the
metal atom is fixed.
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COORDINATION COMPOUNDS
w Every metal has a tendency to satisfy both its primary and secondary valencies. The ligands
satisfying secondary valencies are always directed towards fixed positions in space about the
central metal atom or ion.
H 3N
NH3
Cl-
Co+3
Cl-
Cl-
NH3 NH3
Octahedral
w Thus the co-ordination compounds have a definite geometry.
IMPORTANT TERMS RELATED
TO
CO-ORDINATION COMPOUNDS
i) Complex ion:
w A complex ion may be defined as an electrically charged radical which is formed by the combination
of a simple cation with one or more neutral molecules or one or more simple anions or in some
cases positive groups also. It exists as a single entity and is usually indicated with in square
brackets.
w [Cu(NH3)4]2+ is complex ion which is formed by the combination of four neutral molecules of ammonia
with a simple Cu+2 cation.
wThe complex ion carrying a positive charge is called cationic complex.
Eg: [Cu(NH3)4]2+
w The complex ion carrying a negative charge is called anionic complex.
Eg: [Fe(CN)6]4-
w The complex with no charge is called neutral complex.
Eg: [PtCl2 (NH3)2]
wComplex ions do not give tests of their constituent ions.
ii) Central metal atom or Ion:
wEvery complex ion contains a metal atom or ion to which one or more neutral molecules or ions are
attached. This is known as the central metal atom or ion.
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COORDINATION COMPOUNDS
K4[Fe(CN)6]
w It is sometimes known as the nuclear atom, in a complex the central metal atom or ion behaves as
the electron pair acceptor or lewis acid.
iii) Ligands:
w The central metal ion is surrounded by number of the anions or the neutral molecules or sometimes
positively charged ions possessing at least one lone pair. These surrounded ions are called ligands.
w The ligands are attached to the central metal ion or metal through coordinate bonds or dative
linkages.
K4[Fe(CN)6]
w Ligands act as electron pair donors or Lewis bases. Ligands invariably contain one or more atoms
having lone pairs of electrons.
w Depending upon the number of donor atoms (or sites). The ligands may be classified into
various categories.
They are:
Unidentate (or) Monodentate ligands:
w These ligands contain only one donor atom (or site) which is capable of donating an electron pair
and thus attach to the central metal ion only at one point.
- Eg: CN , F , Cl-, Br-, OH-, H2O, NH3, NO2-, NO+, NO2+, NH2NH3+, etc...
Polydentate (or) Multidentate ligands:
w Ligands which contain two or more such atoms which can simultaneously serve as donor atoms are
called polydentate or multidentate ligands. Ligands with two donor sites called bidentate, and with
three, four, five and six donor sites called tri, tetra, penta and hexa dentate ligands respectively.
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COORDINATION COMPOUNDS
Eg:
COOH2NCH2CH2NH2,H2NCH2COO ,
, SO4-2, EDTA, Pn, tn etc.
COO-
H 2C
N
H 2C
N
CH2COOCH2COOCH2COOCH2COO-
w Some ligands like SO4-2 act as both monodentate as well as bidentate ligands.Certain polydentate
ligands have flexi character i.e., they do not use all the donor atoms to form a complex.
NH3
H 3N
S
O
O
NH3
CO
NH3
O
S
Br
O
H 3N
O
O
CO
H 3N
O
H 3N
NH3
NH3
O
SO4-2 as bidentate ligands
SO4-2 as monodentate
w Eg: EDTA (Ethylen ediamine tetraacetate ion) is a hexadentate ligand but it can also act as a
pentadentate or a tetradentate ligand.
H 2C
H 2C
N
N
CH2COOCH2COOCH2COOCH2COO-
Chelating ligands
w Polydentate ligands, the structures of which permits the attachment of two or more donor sites at
the terminals of a chain to the same metal ion simultaneously, thus, closing one or more rings are
called chelating ligands and the compounds are called chelate compounds.
w Chelate complexes are more stable than ordinary complexes in which the ligand is monodentate.
This increased stability of the compound due to chelation is called the chelate effect. The complex
compound having maximum number of rings formed with the ligands is most stable.
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COORDINATION COMPOUNDS
2+
CH2
H 2N
NH2
CH2
NH2
CH2
Cu
CH2
H 2N
a) Ambidentate ligands:
wThere are certain ligands which have two or more donor atoms but in forming complexes only one
donor atom is attached to metal ion such ligands are called ambidentate ligands.
M
NO2 (Nitro)
M
ONO (Nitrito)
M
CN (Cyano)
M
NC (Isocyano)
M
SCN (Thiocyano)
M
NCS (Isothiocyano)
w Phenoxide ion also serves as ambidentate ligand due to resonance, Ortho- and Para - positions of
the benzene ring in the phenoxide ion also become electron donors.
Odd-
dd-
Phenoxide ion
iv) Co-ordination sphere:
w The central atom and ligands which are directly attached to it are enclosed in square brackets called
the co-ordination sphere. The ligands and the metal atom shown inside the square bracket actually
behave as a single constituent unit.
w The bonding between the metal atom or ion and ligands inside the co-ordination sphere is
non-ionisable.
Co-ordination number:
w The total number of ligands attached to the central metal ion through co-ordination bond is called
the co-ordination number (CN) of the metal ion.
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COORDINATION COMPOUNDS
In [Cu(NH3)4]2+ the co-ordination number of Cu+2 is 4
In [Fe(CN)6]-4 the co-ordination number of Fe+2 is 6
w The co-ordination number of most common metals are 4 and 6.
Ag+, Pt+2, Cr+3, Fe+2, Co+3, and Pt+4 show only one co-ordination number
Ag+ show co-ordination number = 2,
Pt+2 show co-ordination number = 4,
Cr+3, Fe+2, Fe+3, Co+3, Pt+4, show co-ordination number = 6.
Oxidation number or oxidation state
w It is a number that represents an electric charge in which a metal atom or ion has or appear to have
when combined with ligands.
In [Cu(NH3)4]2+ the oxidation number of Cu is +2.
In [Fe(CN)6]4- the oxidation number of Fe is +2.
Charge on complex ion
w The charge carried by a complex ion is the algebraic sum of the charges carried by central metal
ion and the ligands co-ordinated to the central metal ion.
[Cu(NH3)4]2+
w Eg: In [Cu(NH3)4]2+ the charge on Cu ion is +2, NH3 is neutral so the net charge on
the complex is +2.
w Complex ion is represented as shown.
Effective Atomic Number (EAN):
w Sidgwick proposed effective atomic number (EAN) in order to explain the stability of the effective
complex. It is defined as “The resultant number of electrons with the metal atom or ion after gaining
electrons from the donor atoms of the ligand”. Generally EAN coincides with the atomic number of
the next inert gas expect in some cases.
w EAN = Z (atomic number of the metal) – number of e- lost in the ion formation + number electrons
gained from the donor atoms of the ligands.
K4
Rn
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COORDINATION COMPOUNDS
NOMENCLATURE OF CO-ORDINATION
COMPOUNDS
w Nomenclature is important in co-ordination chemistry because of the need to have an
unambiguous method of describing formulas and writing systematic names particularly when
dealing with isomers. The formulas and names adopted for co-ordination entities are based on the
recomendations of the IUPAC (International Union of Pure and Applied Chemistry).
Order of naming ligands
w If the complex compound is ionic, the positive ion whether simple or complex is named first
followed by the negative ion.
Eg:
K[BF4]
[Ag(NH3)2]Cl
K+[BF4]-
(Potassium tetrafluoro borate(III))
[Ag(NH3)2]+Cl-
(Diammine silver(I)chloride)
w But one word name is given to non-ionic and molecular complexes.
Eg:
[Ni(CO)4]
Tetracarbonylnickel(0)
Naming of Ligands
w The number of each kind of ligand is specified using the Greek Prefixes like di, tri, tetra, penta
and hexa. If the name of the ligand is itself complex (i.e., it includes a numerical prefix like
dipyridyl or ethylene diamine) then the term bis (for two), tris (for three), tetrakis (for four), pentakis
(for five) and hexakis (for six) are used followed by the name of ligand.
Eg:
1. [Cu(NH3)4]2+
Tetraammine copper(II)ion
2. [Co(en)3]3+
Tri(ethylene diamine) cobalt(III) ion
1) Negative Lignads
w The name of negative ligands normally ends with ‘O’. Examples are mentioned in the tables.
1 Mono dentate ligands with one unit negative charge
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COORDINATION COMPOUNDS
Name
Halide ion
Hydroxide ion
Cyanide ion
Isocyanide
Nitro ion
Nitrito ion
Formula
(X = Cl-, Br-, I-)
HO-C N
N+ CNO 2
ONO-
Thioisocyanate ion
SCNNCS-
Hydride ion
H
Thiocyanate ion
Amide ion
Acetate ion
Nitrate ion
Cyanate
Isocyanate
NH2-
CH3COONO 3
ONCNCO-
Charge
Name of the ligand
in the complex
Donor atom
-1
Halo
X
-1
-1
Hydroxo
Cyano
O
C
-1
Isocyano
N
-1
Nitro
N
-1
Nitrito
O
-1
Thiocyanato
S
-1
Isothiocyanato
N
-1
Hydrido
H
-1
Amido
N
-1
Acetato
O
-1
Nitrato
O
-1
Cyanato
O
-1
Isocyanato
N
Monodentate Ligands with two Negative Charges
Name
Formula
Charge
Name of the ligand
in the complex
Donor atom
Sulphate ion
SO4-2
-2
Sulphato
O
Oxide ion
Peroxide ion
Carbonate ion
O-2
O2-2
CO3-2
-2
-2
-2
Oxo
Peroxo
Carbonato
O
O
O
-2
-2
-2
-2
Sulphido
Sulphito
Thiosulphato
Imido
O
O
S
N
Sulphide ion
Sulphite ion
Thiosulphate ion
Imide ion
S-2
SO3-2
S2O3-2
NH-2
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COORDINATION COMPOUNDS
Bidentate Ligands with one or two Negative Charges
Name(symbol)
Formula
Charge
Oxalate ion (OX)
COOCOO-
-2
Acetyl acetonate CH3- C - CH = C - CH3
Oxalato
Donor atom
Two-O-atoms
Acetyl acetenato
Two-O-atoms
Glycinate
One-N-atom
&
One-O-atom
Name of the ligand
Donor atom
-O
O
CH2 - COO-
Glycinate ion
-1
Name of the ligand
-1
NH2
Polydentate Ligands having Negative Charges
Name(symbol)
Formula
Ethylene diamine
CH2
triacetate ion
(pentadentate)
CH2
Ethylene diamine
tetraacetate ion
(Hexadentate)
H 2C
H 2C
N
N
N
N
Charge
CH2COOCH2COOH
-3
EDTA3-
-4
EDTA4-
Two-N-atoms
Three-O-atoms
CH2COOCH2COOCH2COOCH2COO-
Two-N-atoms
Four-O-atoms
CH2COO-
2. Neutral ligands
w Neutral ligands have no special endings
Neutral Monodentate Ligands
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COORDINATION COMPOUNDS
Name(symbol)
Formula
Charge
Ammonia
NH3
Zero
Water
H2O
Zero
Phosphine
PH3
Zero
Phospine
P
Nitrogen oxide
NO
Zero
Nitrosyl
N
CO
C5H5N
Zero
Zero
Carbonyl
Pyridine
C
N
Carbon monoxide
Pyridine(Py)
S
Thiourea(tu)
Triphenyl phosphine
N
NH2-C-NH2
(C6H5)3P
Thiocarbonyl
CS
Zero
Zero
Zero
Name of the ligand
Ammine
Aqua(old
Donor atom
N
Aquo)
Thio - urea
Triphenyl phosphine
Tricarbonyl
O
S
P
S
Neutral Bi, tri and Polydentate Ligands
Name(symbol)
Ethylene diamine
(Bidentate)
2,2_dipyridyl(dipy)
(Bidentate)
Diethylene triamine
(tridentate)
Triethylene
tetramine
(tetradentate)
Formula
NH2-CH2-CH2-NH2
Charge
Name of the ligand
Donor atom
Zero
Ethylene diamine
Two-N-atoms
Zero
N
..
Dipyridyl
Two-N-atoms
N
..
NH2-(CH2)2-NH(CH2)2-NH2
Zero
NH2-(CH2)2-NH-(CH2)2
-NH-(CH2)2-NH2
Zero
Diethylene diamine
Triethylene
tetramine
Three-N-atoms
Four-N-atoms
3. Positive Ligands:
These are very few and end with ‘-ium’.
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COORDINATION COMPOUNDS
Name(symbol)
Formula
Charge
Name of the ligand
Donor atom
Nitronium ion
NO2+
+1
Nitronium
N
Nitrosonium ion
NO+
+1
Nitrosonium
N
Hydrazinium ion
NH2NH3+
+1
Hydrazinium
N
5. The order preference of the ligands in the complexes according to old system was as
i) Negative ligands
ii) Neutral ligands
iii) Positive ligands
w There should be no hyphen in between. In case there are more than one negative or positive
ligands they are listed alphabetically.
w IUPAC rules 1991 recommend that all ligands whether anions, neutral or positive be arranged
alphabetically without any preference order.
6. Certain ligands like NO can act as a neutral as well as unipositive ligand. In such case it is
very difficult to find the oxidation state and the charge on the ligand from the given formula
of the complex.
NAMING OF THE COMPLEX ION
w In naming the complex ion, number and name of the ligands are given first, then the central metal
atom followed by its oxidation state indicated by roman numerical in parenthesis.
[ Pt(NH3)2Cl4 ]
Diammine tetrachloroplatinum(IV)
w If the complex ion is a cation, the name of the central metal ion is given as such followed by its
oxidation state indicated by numerals (such as II, III, IV) in the parenthesis at the end of the name
of the complex without a space between the two. If the complex ion is an anion, the name of the
central atom is made to end in ‘ate’ followed by the oxidation number in brackets without any
space between them. For an acid we have a characteristic ending (ic).
Eg: [Pt(NH3)2Cl4]
Diammine tetrachloroplatinum(IV)
Na3[ AlF6 ]
Sodium hexafluoroaluminate(III)
H4[Pt (CN)6]
Hexa cyanoplatinic(II)acid
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COORDINATION COMPOUNDS
w If the complex contains two or more metal ion it is turned polynuclear complex. The ligands
which link the two metal atoms are called bridge groups and are separated from the rest complex
by hyphens and denoted by the prefix ‘µ’ placed before their names.
Eg:
[(en)2 Co
NH2
OH
Co(en)2](SO4)2
bis(ethylene diamine)cobalt (III) - m - amido - m - hydroxo bis(ethylene diamine)
cobalt(III)sulphate
w Common names are used in place of IUPAC names if
a) Structure is not certain
Eg: Zincate ion [ZnO2]-2
b) They are more convenient than IUPAC names
Eg: Ferro cyanide for [Fe(CN)6]-4
Ferri cyanide for [Fe(CN)6]-3
Formula writing:
w Following rules have to be followed while writing the formula of the complex from IUPAC names.
1. Cation whether simple or complex is written first followed by the anion.
2. The order of formulating a complex ion is reverse to that adopted in naming i.e., the
central metal atom is written first followed by the ligand.
3. When there are more than one type of ligands present, they are arranged in the
reverse order to that adopted in naming the complexes i.e., in the order as anionic
ligands first, then neutral and then cationic ligands in the last, but this is not yet
being fully followed. If there are a number of anionic ligands they are written
alphabecally according to the first symbol of their formulae.Same for neutral and
positive also.
4. Whole of the complex ion is enclosed in a square bracket.
5. The charge on the complex species is equal to the charge on the metal atom i.e., its
oxidation state plus the total charge carried by all the ligands coordinated to the
metal atom i.e., thus the formula of tetraamine copper(II) chloride is [Cu(NH3)4]Cl2
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COORDINATION COMPOUNDS
ISOMERISM
wCoordination compounds show mainly two types of isomerism
(i) Structural isomerism
(ii) Stereo isomerism
i) Structural isomerism
It is of the following types
Ionisation isomerism
w Ionisation isomers yield different ions in solution although they have same composition. This type
of isomerism is due to the exchange of groups between the complex ion and the ions outside it.
Eg:
1. [Co(NH3)5Br]SO4 is red-violet and in solution gives a precipitate of BaSO4 with
BaCl2 confirming the presence of SO4-2 on the other hand [Co(NH3)5SO4]Br is red
and does not give test for sulphate ion in the solution, but instead gives a
precipitate of AgBr with AgNO3.
2. [ Pt(NH3)4(OH)2SO4] and [Pt(NH3)4SO4](OH)2
3. [Pt (NH3)4Cl2]Br2 and [Pt(NH3)4Br2]Cl2
Hydrate isomerism
wIsomers in which they differ in the number of
water molecules in the coordination sphere are called
hydrate isomers.
Eg: [Cr(H2O)6]Cl3 – violet ; [Cr(H2O)5Cl]Cl2.H2O – Bluish green;
[Cr(H2O)4Cl2]Cl2.H2O – Dark green
[Co(NH3)4H2O Cl]Cl2 and [Co(NH3)4Cl2]Cl.H2O
[Co(NH3)4H2O Cl]Br2 and [Co(NH3)4Cl.Br]Br.H2O
Linkage Isomerism
w This type of isomerism is seen in compounds where monodentate ligands are having two or more
donor atoms.
Eg:
H 3N
H 3N
NH3
Co
NH3
2+
H 3N
ONO
NH3
H 3N
Penta ammine nitrito cobalt (III) ion
NH3
Co
NH3
2+
NO2
NH3
Penta ammine nitro cobalt (III) ion
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COORDINATION COMPOUNDS
Coordination position isomerism:
w This type is due to the difference in the distribution of ligands in two coordination centers. Generally
bridged complexes involving different ligands show this isomerism.
OH
(NH3)4Co
OH
2+
&
Co(NH3)2Cl2
OH
Cl(NH3)3Co
Unsymmetrical
OH
2+
Co(NH3)3Cl
Symmetrical
Ligand Isomerism
w This arises in those complexes in which the two ligands are isomers themselves. This type is very
rare.
CH2 _ CH _ CH3
CH2 _ CH2 _ CH3
NH2 NH2
NH2
1,2- diamino propane
NH2
1,3- diamino propane
Coordinate isomerism
w This type is possible when both positive and negative ions of a salt are complex ions.
The two isomers differ in the distribution of ligands in the cation and the anion.
Eg:
[Co(NH3)6] [Cr(CN)6] and [Cr(NH3)6] [Co(CN)6]
[Cr(NH3)6] [Co(C2O4)3] and [Co(NH3)6] [Cr(C2O4)3]
GEOMETRICAL ISOMERISM
w Geometrical isomers differ in the spatial distribution of atoms or groups about the central atom or
atom in polynuclear compounds. Those complexes in which the two same ligands occupy adjacent
positions to each other or opposite to each other are able to show this isomerism. cis (or) trans
respectively
Cl
Cl
Pt
cis
NH3
Cl
NH3
NH3
Pt
NH3
Cl
trans
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COORDINATION COMPOUNDS
w This isomerism cannot be exhibited by coordination compounds having 2 (or) 3 coordination
numbers as it is not possible to have more than a single arrangement of ligands in space around
the central ion.
+
Cl
NH3
Co
NH3
NH3
Cl
NH3
NH3
NH3
Cl
Co
Cl
cis
+
NH3
NH3
trans
w Geometrical isomerism is not found among tetrahedral complexes of the type MA4 (or) MA3B (or)
MA2B2 because all the four ligands are equidistant from each other.
w Only square planar and octahedral complexes show this type of isomerism.
Square Planar Complexes
w Square planar complexes of the type [M(AA)2], MA4, MA3B and MAB3 do not show this type of
isomerism.
w Square planar complexes of the type MA2B2, MA2BC, and MABCD show this type of isomerism i.e.,
cis-trans isomerism.
Eg: (1) MA2B2 ---- Dichloro diammine platinum(II)
Cl
Cl
Pt
cis
NH3
Cl
NH3
H 3N
NH3
Pt
Cl
trans
Eg: (2) MA2BC ---diammine chloro nitro platinum(II)
O2N
O2N
Pt
NH3
O2N
NH3
H 3N
cis
Pt
NH3
Cl
trans
Eg: (3) MABCD ---ammine bromo chloro pyridine platinum(II)
H 3N
Cl
Pt
Py
H 3N
Br
Br
Pt
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Py
H 3N
Cl
Cl
Pt
Br
Py
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COORDINATION COMPOUNDS
This type of complex show three geometrical isomers
w Geometrical isomerism can also occur in square planar complexes if the chelate group is not
symmetrical.
Eg: [Pt(gly)2] Diglycinato platinum(II)
CH2
NH2
Pt
O
Cov
NH2
CH2
CH2
O
Co
Co
NH2
Pt
O
Cis
Trans
O
Co
NH2
CH2
w Geometrical isomerism is also shown by bridged binuclear planar complexes of the type
M2A4X2 Eg: [PtCl2 PEt3]2
Cl
Et3P
Pt
Cl
Cl
Cl
Pt
PEt3
Cl
Et3P
Pt
cis
Cl
Pt
Cl
PEt3
Cl
trans
Octahedral complexes
wOctahedral complexes of [MA5B] or M(AA)3 [ where AA = Symmetrical tridentate ligand ]
do not exhibit geometrical isomerism.
______
Eg: MA4B2
> [Co(NH3)3Cl3] tetraammine dichloro cobalt(III) ion
Cl
H 3N
H 3N
Co
Cl
Cl
H 3N
NH3
H 3N
Co
NH3
NH3
Cl
NH3
cis
trans
Eg: MA3B3 ____> [Co(NH3)Cl3] triammine trichloro cobalt (III)
NH3
NH3
O2N
O2N
Co
NH3
H 3N
NH3
O2N
Co
NO2
NO2
NH3
NO2
cis
trans
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COORDINATION COMPOUNDS
Eg: Another type is facial (fac) and meridional(mer) isomers.
Facial (fac)
Meridional (mer)
w M(AA)2B2 where A-A can be any symmetrical tridentate like ethylene diamine(en) and
B can be any anionic ligand such as Cl-, CN- ,NO2- etc...
Eg: [Pt(en)2cl2] bis(ethylene diamine)dichloro platinum( II )
en
Cl
Cl
Pt
en
Pt
en
Cl
Cl
en
w Complexes having six different ligands[M(ABCDEF)] shall exhibit geometrical isomerisms
theoretically fifteen different isomers should be possible. In practice, three isomers of the complex.
[Pt(C5H5N)(NH3)(NO2)(Cl)(Br)(I)] have actually been isolated.
w As in square planar complexes unsymmetrical bidentate ligands also give rise to geometrical
isomerism in octahedral complexes of the type [M(AB)3].
OPTICAL ISOMERISM
w This isomerism arises when a compound can be represented by two asymmetrical structures
(called optical isomers).
w The two isomers are structurally the mirror images of each other and non-super imposable
and do not possess the plane of symmetry.
w Optical isomerism is common in octahedral complexes involving 2 (or) 3 symmetrical
bidentate groups of the type [M(AA)2X2 ], and [M(AA)3 ] and [M(AA)B2X2]
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COORDINATION COMPOUNDS
Eg: Cis-dichloro bis(ethylene diamine) Cobalt (III) cis-form is optically active (d and l forms) where
as trans - forms is optically inactive due to plane of symmetry.
[M (AA)2x2]
[Co(en)2Cl2]
Cl
en
Co
Cl
Cl
Cl
Co
en
en
[M (AA)3]
[Cr (en)3 ]3+
[M (AA)3]
[Cr (C2O4)3 ]3-
Tris ethylenediamine chromium (III) ion
Trioxaloto chromate (III) ion
en
en
en
en
Mirror
Co
en
Co
en
en
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COORDINATION COMPOUNDS
[CoCl2(NH3)2en]3+
[M (AA)B2X2 ]
Diammine dichloro(ethylene diamine) cobalt (III) ion
w Complexes containing hexadentate ligand(EDTA) show optical activity
Eg: [Co(EDTA)] ------ethylene diamine tetra aceto cobaltate(III) ion.
w Complex of the type [MABCEDF] type also exhibit iptical isomersm.
Eg: [Pt(Py)(NH3)(NO2)(Cl)(Br)(I)]
w The optical activity has been observed for chelated tetrahedral and square planar complexes
but only rarely. In tetrahedral complexes only the type which occur in bis chelates with
unsymmetrical ligand has been detected. These have been found in Be(II), B(III), Zn(II) and
Cu(II) complexes.
Eg: Bis (salicyl aldehyde) boron (III) cation has been found to be racemic and resolution
has been accomplished.
H
C=O
O
B
O
O=C
H
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COORDINATION COMPOUNDS
Eg: The two enantiomers of bis (benzoyl acetonato) beryllium (II)
Eg: Square planar complexes are seldom optically active.
BONDING In Co-ordination COMPOUNDS
w The formation of co-ordination compounds can be explained electronically by the number
of theories like
Valence bond theory
w This was given by Linus Pauling.
Linus Pauling
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COORDINATION COMPOUNDS
w In this approach, the basic assumption made is that the metal ligand bonds arise by way of donation
of pairs of electron from ligands to the metal i.e., through forming the co-ordinate covalent bond.
wIn order to accomidate these electrons the metal ion must posses requisite number of vacant
orbital of similar energy.
Orbitals of Co3+ion
3d
4s
4p
4d
w These metal orbitals undergo hybridization to give a set of hybrid orbitals of equal energy
with the approach of the ligands.
d3sp3 hybridised orbitals of Co3+
3d
sp3d3 hybrid
4d
The main postulates are
w The ion of the transition elements have some incompletely filled d-orbital.
w Some or all the electrons in the incompletely filled d-orbital are unpaired.
w Sometimes the unpaired (n-1)d-electrons couple as fully as possible prior to bond formation. In this
process some (n-1)d-orbitals become vacant. Thus central metal atom makes available a number of
empty orbitals equal to co-ordination number of for the formation of co-ordinate bonds with suitable
ligand orbitals.
w Since the energy of (n-1) d-orbital is only slightly less than that of s and p-orbitals of the n-orbit these
vacant orbitals mix together to form new equivalent orbitals called hybrid orbitals.
w Each ligand has atleast one sigma orbital containing a lone pair of electrons.
w The hybrid orbitals formed take part in the formation of hybrid bonds with the ligands by overlapping.
The bond formed takes place by accepting lone pair of electrons by the hybrid orbital of metal from
the ligands. When (n-1)d-orbitals is used for bonding only two d-orbitals are used remaining do not
participate in bonding and the complex called inner orbital complex.
w Some times in place of (n-1)d-orbitals, outer nd-orbitals are used for hybridization. The complex thus
formed is called outer orbital complex.
w If there are unpaired electrons in d-orbital then the complex will be paramagnetic and no unpaired
electrons diamagnetic.
w Inner orbital complex or low spin or spin paired complex. Outer orbital complex or high spin or spin
free complex.
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COORDINATION COMPOUNDS
Drawbacks of valence bond theory
w It describes bonding in co-ordination compounds only qualitatively.
w It does not offer any explanation for the optical absorption spectra of complexes.
w It does not describe the detailed magnetic properties of co-ordination compounds.
MAGNETIC PROPERTIES
w The diamagnetic and paramagnetic behavior of co-ordination compounds can be explained on
the basis of crystal field theory.
Eg:
w Co (III) show diamagnetic properties in [Co(NH3)6]3+ and paramagnetic properties in [CoF6]3complex which can be explained on the basis of crystal field theory.
w D0 is small, the energy required to pairup the 4th and 5th electrons with the lower d-orbitals (t2g) is
higher as compared to that required to place the electron in higher d-orbital (eg) but D0 is higher in
so 4th and 5th electron pairs up.
Colour of complexes
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COORDINATION COMPOUNDS
w The coloured nature of solutions of co-ordination compounds can also be explained on the basis
of crystal field theory, because the energy difference between two sets of d-orbital is usually small
thus excitation of electron from lower energy to higher energy is very easy and can be achieved
even by the absorption of low energy radiations of visible region.
w As a result of the absorption of such selected wave lengths of visible light, the complexes
appeared coloured.
(a) Ruby: this gems stones was found in marble frome Mogo, Myanmar.
(b) Emerald: this gems stones was found in Muzo, Columbia.
Ruby
Emerald
[Ni(H2O)6]2+(aq) + en (aq) = [Ni(H2O)4(en)]2+(aq) + 2H2O
Green
Pale blue
[Ni(H2O)4(en)]2+(aq) + en (aq) = [Ni(H2O)2(en)2]2+(aq) + 2H2O
Blue/Purple
[Ni(H2O)2(en)2]2+(aq) + en (aq) = [Ni(en)3]2+(aq) + 2H2O
Voilet
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COORDINATION COMPOUNDS
CRYSTAL FIELD THEOARY (CFT)
w This was developed by H.Bethe and V.Vleck in 1935.
H.Bethe
V.Vleck
w This theory is based upon the fact that the degenerate d-orbitals of metal ions are splitted energy
wise due to concentration of point charges in certain specific positions.
w This is seen because the energy of the orbitals lying in the direction of point charges increases
more in comparision to orbitals lying in between the point charges as the electrons in former case
experience greater repulsion than in latter case.
w So two groups of orbitals are seen with energy wise i.e., one with higher and other with lower
energy.
w This splitting of 5 degenerate orbitals into two sets is called crystal field splitting.
w In case of octahedral complexes, eg set (dx
2
- y2, dz ) is of higher energy while in case of tetrahedral complexes, t2g set (dxy,dyz,dxz) has higher energy.
2
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COORDINATION COMPOUNDS
Average energy of the
d-orbitals in spherical
crystal field
Splliting of d-orbitals
in octahedral
crystal field
w The difference in energy levels is orbitrarily taken to be D. The set of lower energy stabilizes the
complex ion by 0.4 units and that with higher energy destabilizes the ion by 0.6 units. The gain in
energy achieved by preferential filling of electrons in orbitals is called crystal field stabilization
energy(CFSE).
Average energy of the
d orbitals in spherical
crystal field
Splliting of d orbitals
in tetrahedrel crystal field
d-orbitals free ion
w The tetrahedral crystal field splitting is only 4/9 of the octahedral splitting and there is no known low
spin tetrahedral complex. The number of unpaired electrons in octahedral and tetrahedral complex
having different configuration can be summarized as here one point must always be kept in mind
that crystal field theory considers purely electrostatic attractions between central metal ion and
ligands.
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COORDINATION COMPOUNDS
Factors which affect the magnitude of crystal field splitting are
Nature of ligands:
w The ligand with smaller size, large negative charge with a good s donor and p acceptor properties
will give large crystal field splitting.
w The spectrochemical series of ligands based on their crystal field splitting is
i- < Br- < Cl- < F < OX- ≈ H2O < P4 ≈ NH3 < en < NO2- < CN- < CO.
Oxidation state of metal ion:
w Crystal field splitting will be more in metal ion with higher oxidation states.
w Larger the size of d-orbitals, more crystal field splitting is seen.
w Geometry of the complex-splitting is different for tetrahedral and octahedral complexes.
LIMITATIONS OF CRYSTAL FIELD THEORY
w CFT can explain the spectra of the metal ions and complexes on the assumption that these
arise from the transition of electrons from lower energy d-orbital to higher energy d-orbital.
w However the position and intensities of spectral bands calculated on the basis of CFT do not
always coincide with those determined experimentally.
w Apart from this a pure electrostatic interaction between central metal ion and ligand fails to explain
the relative positions of ligands in spectro chemical series.
w So it is evident that covalent bonding too makes a significant contribution towards the metal
ligand bonding.
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COORDINATION COMPOUNDS
w Based on the above facts CFT was modified so as to include the contribution of covalent
bonding in the metal ligand bond and is called ligand field theory which assumes that the
extent of covalent character in metal ligand bond is generally low as compared to the extent
of the ionic character of the bond that means the conclusions arrived at by the crystal field
theory are still valid to a large extent.
The strength of a complex ion depends upon the following factors
w Higher charge of the central metal ion i.e., greater ionic potential (ionic charge/ionic radius)
and greater is the stability.
w Greater base strength of the ligand, greater will be the stability.
w Ring formation (chelation) in structure of the complexes is the chief factors, which increases the
stability of the complex in solution.
If
w a multidentate ligand happens to be cyclic without any steric effects, a further increase in
stability occurs. This is called as near cocyclic effect.
BONDING IN METAL CARBONYLS
w The homoleptic carbonyls (ligands are only carbonyls) are formed by most of the transition
metals. These carbonyls have simple well defined structures.
w Tetra carbonyl nickel (0) is tetrahedral, penta carbonyl iron (0) is trigonal bipyramidal while hexa
carbonyl chromium(0) is octahedral.
Tetrahedral [Ni(CO)4]
Octahedral [Cr(CO)6]
Trigonal bipyramida [Fe(CO)5]
w Deca carbonyl dimanganese(0) is made up of two square pyramidal Mn(CO)5 units joined by a
Mn-Mn bond. Octacarbonyl dicobalt(0) has a Co-Co bond bridged by two Co groups.
[Mn2(CO)10]
[Co2(CO)8]
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COORDINATION COMPOUNDS
w The metal carbon bond in metal carbonyl possess both s and p character. The metal carbon s bond
is formed by the donation of lone pair of electrons on the carbonyl carbon into a vacant orbital of the
metal.
w The metal carbon p bond is formed by the donation of a pair of electrons from a filled d-orbital of
metal into the vacant antibonding p-orbital of carbon monoxide.
w The metal to ligand bonding creates a synergic effect which strengthens the bond between CO and
the metal.
Synergic bonding
STABILITY OF CO-ORDINATION COMPOUNDS
w The stability of a complex in solution refers to the degree of association between the two species
involved in the state of equilibrium. The magnitude of the equilibrium constant for the association,
quantitatively expresses the stability.
For the reaction
M + 4L
ML4
larger the stability constant, higher the proportion of ML4 that exists in solution.
w Free metal ions rarely exist in solution because generally metal is surrounded by solvent molecules
which will compete with the ligand molecules.
w By ignoring solvent molecules the stability constants of the reactions are
M + L
ML
K1 = [ML]/[M][L]
ML + L
ML2
K2 = [ML2]/[ML][L]
ML2 + L
ML3
K3 = [ML3]/[ML2][L]
ML3 + L
ML4
K4 = [ML4]/[ML3][L]
K1, K2, K3 and K4 are called stepwise stability constants
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COORDINATION COMPOUNDS
w Eg: In the addition of four ammine groups to copper show stability constants as
logK1 = 4.0, logK2 = 3.2, logK3 = 2.7, logK4 = 2.0 and logK = 11.9
w The instability constant or the dissociation constant of coordination compounds is defined as the
reciprocal of the formation constant.
APPLICATIONS OF CO-ORDINATION
COMPOUNDS
Estimation of hardness of water:
w EDTA a hexadentate ligand form complex with various metal ions. So it is used in the estimation of
hardness of water by a volumetric method.
w Hardness of water is due to Ca+2 and Mg+2 ions. Since stability constant values of EDTA complex
with Ca+2 and Mg+2 are 107 and 108 respectively, it helps the selective estimation of different ions.
H 2C
H 2C
N
N
CH2COOCH2COOCH2COOCH2COO-
Analytical chemistry or Qualitative and Quantitative Estimation of metal ions:
(a) Nickel detected and estimated as its red dimethyl glyoxime complex which is a chelate
complex.
(b) Mg and Al are estimated as complexes of 8-hydro-oxy quinoline(oxime)
(c) The separation of Ag+ from Hg22+ in the first group of analysis is based on the fact that while
silver chloride soluble in aqueous ammonia, Hg2Cl2 forms a black insoluble material.
AgCl + 2NH4OH
Hg2Cl2 + NH4OH
[Ag(NH3)2]Cl + 2H2O
[Hg(NH2)Cl] + Hg + HgCl + H2O
Black insoluble
(d) The separation of II B group sulphides from II A group sulphides is based on the fact that
sulphides of II B group from complex sulphides with yellow ammonium sulphide which are
soluble while sulphides of II A group do not react.
Ag2S5 + 3(NH4)2S
2(NH4)3[AsS4]
Sb2S5 + 3(NH4)2S
2(NH4)3[SbS4]
SnS2 + (NH4)2S
(NH4)2[SnS3]
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COORDINATION COMPOUNDS
(e) The detection of Cu+2 is based on the formation of a blue tetra ammine copper sulphate
complex which give a deep blue coloured solution upon addition of NH3 to Cu+2 ions.
CuSO4 + 4NH3
[Cu(NH3)4]SO4
w It also forms a chocolate coloured precipitate with potassium ferrocyanide.
2CuSO4 + K4[Fe(CN)6]
Cu[Fe(CN)6] + 2K2SO4
(f) Fe+3 is detected by formation of a blood red coloured complex with KSCN.
Fe+3 + KSCN
[Fe(SCN)]2+ + K+
(g) S-2 is detected by the formation of a violet-coloured complex with sodium nitro prusside.
Na2S + Na2[Fe(Cn)5NO]
Na4[Fe(Cn)5NOS]
Sodium nitro prusside
Violet colour
Animal and plant world:
w Cyclic ligand impact extra stability to co-ordinate compounds. Thus Mg in chlorophyll and iron in
hemoglobin are stable towards dissociation due to the formation of coordination compounds.
w Hemoglobin of the blood gets oxygenated through the binding of di-oxygen, O2 to the ferrous ions in
hemoglobin.
Electroplating of metals:
w Co-ordination compounds are used electroplating industry.
Eg: Gold and silver are electroplated from their coordination compounds such as
K[Ag(CN2)] or K[Au(CN)4]
Extraction of metals:
w The formation of complex compounds is used in the extraction of some metals.
Eg: Ag and Au are extracted.
2K[Ag(Cn)2]+Zn
K2[Zn(Cn)4] + 2Ag
w This is due to more electropositive nature of Zn as compared to Ag and Au.Nickel tetra carbonyl
complex is used for the extraction and purification of nickel.
Ni+4CO
[Ni(CO)4]
D
Ni+ 4CO
In chemotherapy:
w Platinum complex, cis[PtCl2 (NH3)2] called cis-platin is used in cancer chemotherapy. Vitamin B12
is a coordination compound of cobalt and is essential to prevent anaemia.
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COORDINATION COMPOUNDS
As catalysts:
wA coordination complex of Ti and Al is used as a catalyst to convert ethylene and propylene into
polythene. Ziegler-Natta catalyst is TiCl4 + (C2H5)3Al or (CH3)3 Al. Wilkinson Catalyst is used for
hydrogenation of alkenes.
In pigments:
w Many pigments such as phthalocyanines used in paints which are co-ordination compounds.
SUMMARY
w The chemistry of co-ordination compounds is an important and challenging part in
modern inorganic chemistry.
w Developments in this part have given new concepts of bonding, structure in chemical industry and
biological systems.
w The first explanation about formation, bonding, structure of co-ordination compounds was
given structure of co-ordination compounds was given by A.Werner.
w He postulated that metal in co-ordination compound shows two types of linkages
(primary and secondary) or valencies (ionisable and non- ionisable) or (ionic and covalent).
w The valence bond theory (VBT) explains the formation of magnetic behaviour and geometrical
shapes of co-ordination compounds, but it failed to give a quantitative interpretation of magnetic
behaviour and their optical properties.
w The crystal field theory (CFT) is based on the effect, degeneracy of d-orbitals provide different
electronic arrangements in strong and weak crystal fields. This theory quantitatively explained about
orbital separation energies, magnetic moment, spectral and stability parameters.
w The metal carbon bond in metal carbonyls possesses both s and p characters. The ligand to metal
is s bond and metal to ligand is p bond. This unique synergic bonding provides stability to metal
carbonyls.
w The stability of co-ordination compounds is measured in terms of step wise stability constants or
over all stability constant.
w The stability due to chelation is called chelate effect.
w Co-ordination compounds are of great importance. They have extensive applications in
chemotherapy, photography, analytical and medicinal chemistry, metallurgical process, as catalyst,
in electroplating in pigments etc.
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