Coordination compounds

Introduction
The Coordination compounds are very important in day to day life, many compounds exist
as coordination compounds as they have special type of linkage that is coordinate bond. It is
formed between electron rich and electron deficient species.

The branch of chemistry that deals with the study of coordination compounds are called
as” coordinate chemistry “
Examples of some coordinate compounds like:
Haemoglobin – coordination compound of iron (Fe)
Chlorophyll – coordination compound of magnesium (Mg)
Vitamin B12- coordination compound of cobalt (Co)

Double salts
Double salt is the association of many substances or we can say they mainly contain two
salts in equimolar concentration.
like Mohr’s salt FeSO4.(NH4)2SO6H2O
Potash alum K2SOAl2 (SO4)3.24H2O etc.

In coordination compound once the compound is formed its ions than the compound as a
whole do not lose its identity but when dissolved in water they show different properties
whereas in case of double salts they don’t show different properties but looses their identity
in water

[Co(NH3)6]Cl3 gives Co(NH3)6 ]+ 3Cl (coordination compound in water )

FeSO4.(NH4)2SO4.6H2O Fe2+ ,NH4+,SO42-,H+,OH- ions (double salt in water )

Coordination compounds exist in solid as well as aqueous state whereas the double salts
exist in solid state and in aqueous state they dissociate into ions .

Terminology involved
central metal atom:
it is the acceptor metal atom present in the coordination compound
It is mostly d block element
It act as an acceptor as it has vacant d orbital
Due to accepting nature they act as Lewis acids
For example in complex: {Co(NH3)6}Cl3 in this Co is central metal atom

Ligands:
They are donor atom may be single atom or group of atoms
They are negatively charged or neutral species with lone pair Br-,Cl- etc or neutral like
NH3,NO,CO etc
They act as Lewis bases
We can also call ligands as nucleophile as they are nucleus loving
For example in complex: {Co(NH3)6}Cl3 in this NH3 is ligand

Nature of ligands: Their nature is determined by the property denticity

Denticity it determines the total number of donor atoms in a molecule.
On the basis of the property denticity ligands are :
Monodentate /unidentate: is that which has one donor atom
Examples : CN-, OH-,Cl-,Br-,NH3:,H2O,CO etc

Di-dentate: if it has two donor atoms Examples

(OXALATE ION) (ethylene diamine )

Polydentate : if it has 3 or more donor atoms in it DIETHYLENE TRIAMINE (tridendate)
NH2-CH2-CH2-NH-CH2-CH2-NH2

Hexadentate: ethylene diamine tetra acetic acid (EDTA)

chelate: when the ligand has two or more donor atoms and they are arranged in such a way
that they give rise to a ring like structure than the effect is called chelating and the ring
formed is chelate .
For example:
uses of chelates:
They are used in softening of hard water
In qualitative analysis for detection of metal
In separation of lanthanoids and actinoids

Ambidentate ligand: that have two donor atoms but doesn’t show chelation. This group
bond at a time by one atom. For example: CN-,NO2 etc

coordination sphere: It is the combination of atom and ligands.

Example: in [ Co (NH3)6 Cl3 ]
[ Co (NH3)6 ] is coordination entity or complex
They are written in square brackets [coordination entity]
This coordination sphere may be positively changed, negatively charged
If it is positively charged than it is called as cationic entity and in case of negative charge it
is anionic entity or complex.
Example: [Co(NH3)6]3+ cationic entity [Ag (CN)6]- anionic entity
counter ion:
The atom or group of atoms written outside bracket is called counter ion.
Example: in [Co(NH3)6].Cl3
Cl3 is a counter ion or the ionizable part when dissolved in water
If coordination sphere is with positive charge than counter ion is with negative charge or vice
versa.

Coordination sphere
The central metal atom and ligands attached to it forms coordination entity and is written
within square brackets
Example :in [Co(NH3)6].Cl3
In this in [Co(NH3)6] is coordination entity

Coordination number: it is the number of ligands bonded with the central metal atom for
example: IN [Ag (CN)2]- “the no. Of ligands attached are 2 therefore coordination no. is 2 “.

Oxidation number: it is the residual charge left on the atom when all other atoms are
removed from it .it is calculated by assigning appropriate charges to ligands and then
equating the sum of the charges on the central atom and the ligands equal to the charge on
the coordination sphere.
For example: [Co (NH3)6] Cl3 in this oxidation state of NH3 is 0 and for Cl is -1, and
ooxidation state of Co is taken as x then
(x-3) x 1=0 or x=+3
3-
another example: [Cu (CN)4] in this oxidation state of Cu is taken as x, CN as -1 so,
x + (-1 X 4) = -3 or x=+1

Homo-leptic and hetero-leptic compounds

Homoleptic: are those which have same kind of ligands attached to central metal atom
Example:in [Co (NH3)6] Cl3 is homoleptic as ligands are same

Heterolyptic: when ligands attached are of different type

Example: in [CoCl3(NH3)3]3+ is heterolytic because ligands attached are of different types.

Nomenclature of coordination compounds

Like for ionic compounds the naming is done by writing first cation name and then naming
anion.
For example: NaCl is ionic and written as sodium chloride that is in name first part is
cationic and other is anionic.
In the same way coordination compounds consist of two parts:Cation and anion. While
naming,the name of cationic part is written first followed by anionic part. There are certain
set of rules that are followed while writing name of cation.
Naming for cationic species: in which coordination sphere is positively charged
Name of cation is written first
Let’s say the coordination sphere is cationic that is with positive charge for example [Co
(NH3)6] Cl3 for the naming of coordination entity is done first and later the counter ion .so in
given complex the name of [Co (NH3)6] is written first than the counter ion Cl
In case of coordination entity: the name of ligands is written first and then the central metal
atom. For example, in this [Co (NH3)6]the name of NH3 is written first than for Co.
If the ligands are homoleptic than you can write in any manner but if they are heteroleptic
than the alphabetical order is followed
For example: in complex [Co (NH3)5CL] the name of ligands is written in alphabetical order
that is for ammine first and then chloro.
Rules for writing the name of the ligand:
If the ligand names end with ‘ate’ or ‘its‘than e is replaced by o
Like for oxalate it is written as oxalate, sulphite as sulphito
If ligand name end with’ ide ‘than ide is replaced by ‘o ‘
Example for chloride it becomes chlorido
Neutral ligands are name as such like for water it is aqua, for ammonia it is ammine
If more than one ligand is present than alphabetical order is followed and di,tri, tetra is
prefixed before the name of ligand.
For example: in complex [Co (NH3)6] ligand name will be hexamine that is hexa for 6
For polydentate ligands they include numerical prefix –like di is replaced by bi, tri is
replaced by tris, tetra is replaced bytetra is so on
For example: [CoCl2(en)2] Cl in this the ligands are dichlorobisethylenediamine
In case of Central metal atom:
Oxidation state of central metal atom is written in numeral after the name of central metal
atom
For example: in complex [Co (NH3)6] Cl3 it is hexamine cobalt (III) that is cobalt (III) is
central metal atom with its Oxidation state in numeral
if complex is cationic than normally the name is used: example is same as given above
if complex is anionic than metal atom name ends with ate
for example: in complex K3[fe(CN)6] in this the name is written as potassium
hexacyanoferrate(III) that is in this example coordination entity is anionic complex and the
name of central metal atom is written with ate followed by Oxidation state in numeral
if complex is neutral than the normal central metal atom name is used
some commonly used ligands:
bromo Br- ammine NH3
-
floro F aqua H2O
-
hydroxo OH nitrosyl NO
-
cyano CN carbonyl CO
2-
carbonato CO3 ethylene diamine H2NCH2CH2NH2
-
acetate CH3COO
some examples of iupac naming:
Diamine argentum(i)dicyanoargentate(i)
[Cr (NH3)(H2O)3]Cl3 [Pt (NH3)2Cl (NO2)]
Triaminetriaquachromium(III)chloride Diaminechloronitroplatinum(III)
[Co(en)3]2(SO4)3 K3[Cr(C2O4)3]
Trisethylenediaminecobalt(III)sulphate Potassiumtrioxalatochromate(III)
[Ag(NH3)2][Ag(CN)2]
Isomerism in Coordination Compounds
Isomerism is the phenomenon in which compounds have same molecular formula but
different structures and these different structures are called as isomers. Isomers are those that
have different physical and chemical properties.
There are two types of isomers: Structural isomerism Stereoisomerism

Structural isomerism: Different types are as follows:-

ionization isomerism
Hydrate isomerism
Coordination isomerism
Linkage isomerism

Stereoisomerism: Different types are as follows:-

geometrical isomerism
Optical isomerism

Structural isomerism:The compounds have same molecular phenomenon but different

structures

Ionization isomerism: they differ in productions of ions in aqueous solution

For example:à [CoBr(NH3)5]SO4 and [Co (NH3)5SO4 ]Br are ionization
isomers when dissolved in water
[CoBr(NH3)5]SO4 à[CoBr(NH3)5]2+ + SO42-
[Co (NH3)5SO4 ]Bràà[CoSO4 (NH3)5]2+ + Br-

Hydrate or solvate isomerism: in this they differ in number of molecules of water of

crystallization
For example: [Cr(H2O)6]Cl3 and [Cr(H2O)5Cl]Cl2H2Oare hydrate isomers
When dissolved in water
[Cr(H2O)6]Cl3à[Cr(H2O)6] + 3Cl- (no water molecule )
[Cr(H2O)5Cl]Cl2H2Oà[Cr(H2O)5Cl]+2Cl+H2O(will give one water molecule )

Coordination isomerism: they differ in coordination entities

For example:[Co(NH3)6] [Cr(CN)6] and :[Co(CN)6] [Cr(NH3)6] are coordination isomers

Linkage isomerism: it is in case of ambidentate ligands and in this they differ in the point of
attachment.
For example, in case of CN(cyano) and in case of NC (isocyano)

Stereoisomerism: in these compounds have same molecular formula but differ in spatial
arrangements of ligands.
It is of two types:
Geometrical isomerism
Optical isomerism
Geometrical isomerism: it is due to difference in the geometrical arrangements of ligands
around central metal atom.
it is of further two types : Cis and Trans
Cis : when the similar ligands are on adjacent position
Trans : when the similar ligands are on opposite positions
Cis and trans isomers are shown below (this is shown by compounds with coordination
number 4 and 6
Example: [Pt (NH3)4Cl2]

Another type of Geometrical isomerism that occurs in octahedral compounds in Ma3b3 like
[Co(NH3)3(NO3)3].
It can be of two types : Facial(fac) and Meridional(mer)

Fac :In this three donor atoms of same ligands occupy adjacent positions at corners of an
octahedral face
mer :In this three donor atoms of same ligands occupy positions around the meridian of an
octahedron.

Optical isomers:they can also show optical isomerism if chiral center is present in them .if
they possess chiral center they can rotate the plane of polarized light .
Example: [Co(en)3]

Werner’s theory
According to the theory the postulates are:
Metal exhibit 2 types of vacancies: primary valency and secondary valency
Primary valency gives the information about oxidation state and secondary valency gives the
information about coordination number
Primary valency is variable whereas secondary valency is always fixed
Secondary valency that is coordination number determines the geometry of molecule or we
can say polyhedral of the molecule.
Metal stabilize all its vacancies
Depending upon this theory various structures of coordination compound was explained :
In CoCl3. (NH3)6 à In this NH3 is secondary In CoCl3.(NH3)5 the ionizable chlorides are
valency and Cl is primary valency only 2

In COCl3.(NH3)4 the ionizable ions are only one chloride ion

Please note the dark lines shows ionizable part and light lines show non ionizable part in all
the figures.
Limitations of Werner’s theory:
He was able to explain many facts about coordination compounds but failed to give any
information about why only certain elements participate in coordinate bond, why the
coordination entity has special geometry
Due to these reasons other theory was proposed that is valence bond theory
Valence bond theory
It was given by Pauling in 1931
He proposed the idea of donating lone pair to central metal atom.
Bonding in coordination compound occur due to overlap of orbital of ligand with vacant
orbital of central metal atom
All the vacant d orbitals have same energy. but the degeneracy of d orbital breaks when
ligand approaches
Hybridization is considered while drawing polyhedral
Metal ions in presence of ligands can use their (n-1)d ns np or ns np Nd.
If the inner d orbital is used than the complex is regarded as inner orbital complex and if
outer d orbital is used than the complex is outer orbital complex.
The ligands decide which orbitals out of these to be used and accordingly the geometry is
decided.
If hybridization
sp3- tetrahedral Sp3d2- octahedral
dsp2-square planar d2sp3-octahedral
Sp3d-trigonal bipyramidal
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
Let us take one example: of example [Co(NH3)6]3+. In this central metal atom Co atomic no.
is 27. The electronic configuration of Co = (Ar)183d74s2
Co3+=(Ar)183d6

Example [Fe(Co)5]: (inner orbital complex and diamagnetic )

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 behavior explained wasn’t satisfactory
This theory couldn’t give the information about color of compounds
This theory failed to distinguish between strong and weak ligand
TO OVERCOME THE SHORT COMINGS A NEW THEORY WAS PROPOSED:
CRYSTAL FIELD THEORY
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 metal atom 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
Organo-metallic compounds
They are those compounds in which metal or metalloid or a non-metal is directly linked to
carbon atom of a hydrocarbon.
For example: (C2H5)2Zn etc
Please note that metal cyanides and metal carbides are not organometallic compounds as in
them carbon atom is not directly joined to metal.
Types of organometallic compounds:
Sigma organometallic compounds
Pi organometallic compounds
Sigma and pi organometallic compounds In detail:
Sigma organo-metallic compounds: these are the compounds obtained by bonding of non-
metal with metalloid elements with carbon. forexample:RMgX, (CH3)3Al etc.
Pi organo- metalliccompound: these compounds are formed mainly by transition elements.
Normal sigma bond is formed through the pi electron cloud of organic molecule. Forexample
: ferrocene ,zeisse’s salt etc
Sigma and pi organo-metaliccompound: these compounds are formed by transition metal
carbonyls .for example : Ni(CO)4,Fe(CO)5 etc
Shapes of these structures are shown below:
For example :
Shape of [Ni(Co)4]
Shape of [Fe(Co)5]

Considering bonding in metal carbonyls: these compounds possess both s and p characters.
The M-C sigma bond is formed by donation of lone pair if electrons on the carbonyl carbon
into a vacant orbital of 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:-

Applications of coordination compounds

They are used in estimation of hardness of water as calcium and, magnesium ions form
complexes with EDTA.
It is used in estimation and detection of metal ions. for example: Ni2+ions is estimated using
dimethyl glyoxime
It is used in Extraction of metals
It is used in medicines like cis platin is used in treatment of cancer
It is used in animal and in plant world like hemoglobin is a complex of iron, chlorophyll is a
complex of magnesium and so on