Chapter-09

Coordination Compounds

Marks – 5 with option - 7

 Prof. Sheetalkumar S Bhong
M.Sc. (Org.Chem), M.Ed., SET,M.Phil.,M.B.A.(Mktg.)DSM,CRCC,Ph.D(Appeared)
Member of Board of Studies Balbharti, Pune and Chemistry subject expert H.S.C
Board Maharashtra State
Sinhgad college of Arts,Science & commerce(Jr.) Ambegaon
bk.Pune-41

Contact- 9763520755
shitalbhong@gmail.com
 (d and f block elements)

Explain sidwick’s electronic theory with suitable example. (Oct.2015) 2m

Explain cationic and anionic complexes of co-ordination compounds(oct.2015) 2m.
Introduction
The branch of chemistry which deals with the study of coordination
compounds or complex compounds is known as coordination chemistry.

Coordination compounds are widely distributed in minerals,plants and

animals play important biological functions.
Chlorophyll—coordination compound of magnesium Hemoglobin red pigment in blood —coordination
compound of Iron(Fe)
coordination compound:

It is a compound in which, central metal atom or ion is attached to a group of

surrounding molecules or ions by coordinate covalent bonds

Example: A chemotherapy drug, cisplatin, [Pt(NH3)2Cl2], is a coordination compound

in which two NH3 molecules and two Cl- ions use their lone pairs of electrons to bond
with central Pt(II) ion.

Lewis acid –base interaction:

Formation of complex is a Lewis acid –base interaction.The ligands are directly

linked to central metal ion through coordinate covalent bonds.In the formation of these
bonds both electrons are shared by ligand donor atoms because the metal ion is usually
an electron deficient species.
Thus ,ligands are electron pair donors and act as Lewis bases.
The central metal ion is an electron pair acceptor serves as Lewis acid.
Ligands: Imp.1 Mark

The molecules or ions that surround the central metal atom or ion
in a coordination compound are called Ligands or donor groups.

Example:
1. Cisplatin, [Pt(NH3)2Cl2] two NH3 molecules and two Cl- ions are ligands
2. K4[Fe(CN)6] six cyanide ions are the Ligands.

Ligand donor atoms:

The atoms of ligand that are attached directly to the metal are called ligand
donor atoms.
Example: In cisplatin ‘N’ and ‘Cl’ are ligand donor atoms. Each ligand donor
atom contributes the share of one electron pair to form coordinate bond with
metal ion , as shown in cisplatin.
Types of Ligands:
Ligands are classified as monodentate and polydentate ligands depending upon the
number of donor atoms they have.

1.Monodentate ligands:
A monodentate ligand is the one where a single ligand donor atom shares an
electron pair to form a coordinate bond with the central metal ion.

Example:
The ligands Cl -, OH -or CN- attached to metal have electron pair on Cl, O and N,
respectively which are donor atoms :

NH3 molecule has only one donor atom i.e. nitrogen atom.
Hence it can be linked to the metal of complex only through ‘N’ atom.
2.Polydentate ligands:
A polydentate ligand has two or more ligand donor atoms linked to the central
metal ion.
Polydentate Ligands are further classified into bi,tri…………..hexa dentate depending
upon the number of ligand donor groups linked with the ligand.

i.Ethylenediamine binds to metal using electron pair on each of its two nitrogens.
It is a bidentate ligand

Similarly oxalate ion (C2O4)2- is also bidentate ligand. It bonds to metal ion using
electron pair of two ‘O’ donor atoms.
ii. Ethylenediaminetetraacetate ion (EDTA)4- binds to metal ion by electron pairs of
four oxygen and two nitrogen atoms. It is a hexadentate ligand.

Common mono and polydentate ligands

3. Ambidentate ligand: Imp.1 Mark

A ligand that has two donor atoms but uses the electron pair of only
one donor atom to bond with central metal ion to form a coordinate bond is
called ambidentate ligand.

Example:
1.NO2- group has donor atoms(N and O) out of the two only one donor atom is
linked to the metal as M-ONO or M-NO2.

2.SCN- ions has two donor atoms (S and N) but only one can be linked to the
metal such as M ← SCN or M← NCS.
 Terms used in coordination chemistry:

Coordination sphere :
The central metal ion and the ligands attached to it are always enclosed in a square
bracket. This is called a coordination sphere, which is a discrete structural unit.

When the coordination sphere comprising central metal ion and the surrounding
ligands together carry a net charge, it is called the complex ion.
The ionisable groups shown outside the square bracket are the counter ions.

Example: Compound K4[Fe(CN)6] has [Fe(CN)6]4- coordination sphere

with the ionisable K⊕ ions representing counter ions. The compound ionizes as :
K4[Fe(CN)6] 4K⊕ + [Fe(CN)6]4-
 Charge number of complex ion and oxidation state of metal ion :

The net charge residing on the complex ion is its charge number. It is algebraic sum
of the charges carried by the metal ion and the ligands. The charge carried by the metal
ion is its oxidation state (O.S.).

Example: The complex[Fe(CN)6]4- has charge number of -4. It can be utilized to

calculate O.S. of Fe. Thus,
Charge number of complex = net charge carried by complex ion
= oxidation state of metal ion + total charges of ligands.
charge number of complex = -4
-4 = (Oxidation state of Fe ion + charge of ligands)
= (Oxidation state of Fe ion + 6 × charge of CN ion)
= (Oxidation state of Fe + 6 × (-1))

Oxidation state of Fe = -4 + 6 = +2. Fe(II) or Fe2+

 Coordination number (C.N.) of central metal ion :
The coordination number of central metal ion attached to monodentate ligands is equal
to number of ligands attached to it.
Example: [Co(NH3)4Cl2]⊕.Here four ammonia molecules and two chloride ions, that
is, total six ligands are attached to the cobalt ion. All these are monodentate since each
has only one donor atom. There are six donor atoms in the complex. Therefore, the
coordination number of Co3⊕ ion in the complex is six.
Consider the bidentate ligand C2O42- or ethylenediamine (en). The complexes,
[Fe(C2O4)3]3 -and [Co(en)3]3⊕, have three bidentate ligands each. The total donor atoms in three of
ligands is six and the C.N. of Fe3⊕and Co3⊕ in these complexes is six.
Coordination number of central metal ion in a complex is the number of
ligand donor atoms directly attached to it or the number of electron pairs
involved in the coordinate bond.
The shape of complex depends on coordination number of metal ion
C.N 2 4 4 6

Shape of Complex Linear Tetrahedral Square planer Octahedral

 Double salt and coordination complex
Combination of two or more stable compounds in stoichiometric ratio can give two
types of substances, namely, double salt and coordination complexes.

Double salt : A double salt dissociates in water completely into simple ions.
For example (i)Mohr's salt, FeSO4(NH4)2SO4.6H2O dissociates as :

Imp.1 Mark
ii. Carnalite KCl.MgCl2.6H2O dissociates as:

Coordination complex : A coordination complex dissociates in water with at least

one complex ion.
For example: K4[Fe(CN)6] dissociates as the complex ion and counter ion.
Werner theory of coordination complexes : Imp.2 Mark
The first attempt to explain nature of bonding in coordination compounds was
put forth by Werner. The postulates of Werner theory are as follows.

Postulate (i) Metal in a complex has two types of valencies: primary (ionizable)
valency and secondary (nonionizable) valency.
Postulate (ii) Primary valencies are ionizable and are satisfied by anions.In modern
terms,it is called oxidation state of metal
Postulate (iii) Secondary valencies are nonionizable and are satisfied by neutral
molecules or negative ions.In modern terms,we call it coordinatation number of metal.
Postulate (iv) Primary valencies are non rigid and non directional.Secondoray valencies
have directional properties therefore its number and positon determines geometry of the
complex
Postulate (v) In a complex, there are two types of spheres.
i) Coordination or inner sphere: In this sphere the groups present cannot be easily
separated as they are firmly attached to meta
ii) Ionisation sphere or outer sphere : The groups present in this sphere are loosely
bound and can be easily separated.
Experimental Observations:
1) a. Werner treated 1 mole of a purple coloured complex CoCl3 5NH3 with excess of
AgNO3 produces 3 moles AgCl Precipitate. This shows that 3 Cl- ions present in
ionization sphere.
b. When complex was heated with HCl, ammonia is not removed. This shows that it is
strongly bound to cobalt in coordination sphere. Therefore formula of the complex
is [Co(NH3)5 ]Cl3.

Problem: 1 mole of a green coloured complex of CoCl3 4NH3 on treatment with excess
of AgNO3 produces 1 mole of AgCl.What is the formula of the complex.
[Co(NH3)4Cl2]Cl
 Classification of complexes:

Classification on the basis of types of ligands

Homoleptic complexes: Imp.1 Mark

Complexes in which a metal ion is attached to only one type of ligands are
homoleptic
Example: [Co(NH3)6]3+ .Here only one type(ammonia) of ligands are
attached to Co3+ ion
Heteroleptic complexes: Imp.1 Mark

 Complexes in which a metal is bound to more than one type ligands are
heteroleptic.
Example: [Co(NH3)4Cl2]+
There are two types of ligands 4 ammonia and two Cl- are attached to CO3+ ion
 Classification on the basis of charge on the complex

i.Cationic sphere complexes : Imp.1 Mark

A positively charged coordination sphere or a coordination compound having a

positively charged coordination sphere is called cationic sphere complex.

Example: the cation [Zn(NH3)4]2⊕ and[Co(NH3)5Cl]SO4 are cationic complexes.

The latter has coordination sphere [Co(NH3)5Cl]2⊕;the anion SO42- makes it electrically
neutral.

ii. Anionic sphere complexes : Imp.1 Mark

A negatively charged coordination sphere or a coordination compound having negatively

charged coordination sphere is called anionic sphere complex.

Example: [Ni(CN)4]2- and K3[Fe(CN)6] have anionic coordination sphere;

[Fe(CN)6]3- and three K⊕ ions make the latter electrically neutral.
 iii. Neutral sphere complexes : Imp.1 Mark

A coordination complex does not possess cationic or anionic sphere are called neutral
sphere complexes.

Example:[Pt(NH3)2Cl2] or [Ni(CO)4] have neither cation nor anion but are neutral
sphere complexes.
 9.5 IUPAC nomenclature of coordination compounds :

Rules for naming coordination compounds recommended by IUPAC are as follows:

1. In naming the complex ion or neutral molecule, name the ligand first and then the
metal.
2. The names of anionic ligands are obtained by changing the ending -ide to -o and -
ate to-ato.
3. The name of a complex is one single word. There must not be any space between
different ligand names as well as between ligand name and the name of the metal.

4. After the name of the metal, write its oxidation state in Roman number which
appears in parentheses without any space between metal name and parentheses.
5. If complex has more than one ligand of the same type, the number is indicated with
prefixes, di-, tri-, tetra-, penta-, hexa- and so on.

6. For the complex having more than one type of ligands, they are written in an
alphabetical order. Suppose two ligands with prefixes are tetraqua and dichloro.While in
alphabetical order,tetraqua is first and then dicholro.
7. If the name of ligand itself contains numerical prefix then display number by prefixes
with bis for 2, tris for 3,tetrakis for 4 and so forth. Put the ligand name in parentheses.
For example,(ethylenediamine)3 or (en3) would appear as tris (ethylenediamine) or
tris (ethane-1,2-diamine).

8. The metal in cationic or neutral complex is specified by its usual name while in the
anionic complex the name of metal ends with 'ate'.
IUPAC names of anionic and neutral ligands
IUPAC names of metals in anionic complexes
 IUPAC names of some complexes

ii) Compounds containing complex anions and simple cations

Any two names, each 1 Mark

 iv) Compounds containing complex cations and simple anion

Write the representation of

• Tricarbonatocobaltate(III) ion.
• Sodium hexacyanoferrate(III).
• Potassium hexacyanoferrate (II)
• Aquachlorobis (ethylenediamine) cobalt(III).
• Tetraaquadichlorochromium(III) chloride.
• Diamminedichloroplatinum(II).
Write the formulas for the following coordination compounds:
(a) Tetraammineaquachloridocobalt(III) chloride
(b) Potassium tetrahydroxidozincate(II)
(c) Potassium trioxalatoaluminate(III)
(d) Dichloridobis(ethane-1,2-diamine)cobalt(III)
(e) Tetracarbonylnickel(0)

Solution: (a) [Co(NH3)4(H2O)Cl]Cl2 (b) K2[Zn(OH)4] (c) K3[Al(C2O4)3]

(d) [CoCl2(en)2]+ (e) [Ni(CO)4]

Write the IUPAC names of the following coordination compounds:

(a) [Pt(NH3)2Cl(NO2)] (b) K3[Cr(C2O4)3] (c) [CoCl2(en)2]Cl
(d) [Co(NH3)5(CO3)]Cl (e) Hg[Co(SCN)4]
Solution:
(a) Diamminechloridonitrito-N-platinum(II) (b) Potassium trioxalatochromate(III)
(c) Dichloridobis(ethane-1,2-diamine)cobalt(III) chloride
(d) Pentaamminecarbonatocobalt(III) chloride
(e) Mercury (I) tetrathiocyanato-S-cobaltate(III)
Effective Atomic Number(EAN) : Imp.1 Mark

It is the total number of electrons around the central metal ion
present in a complex
Calculated as the sum of the electrons on the metal ion and the number of
electrons donated by the ligands.
It can be calculated using the formula.

EAN= Z-X+Y
Where
Z= Atomic number of the metal
X= Number of electrons lost during the formation of the metal
ion from its atom.
Y= number of electrons donated by the ligands.
EAN Rule: Imp.1 Mark
“A metal ion continues to accept electron pairs from ligands till the total
number electrons present around the metal ion in the complex becomes
equal to the atomic number of the next noble gas atom.”
Thus if the EAN is equal to 18 (Ar),36 (Kr),54 (Xe), or 86 (Rn) then the EAN rule is obeyed.

EAN of few metal ions

Example: Imp.1 Mark
1.Consider [Co(NH3)6]3⊕

Oxidation state of Cobalt is +3, six ligands donate 12 electrons.

Z = 27; X = 3; Y = 12
EAN of Co3⊕ = Z-X+Y = 27 - 3 + 12 = 36.

2. Consider [Fe(CN)6]4-

Oxidation state of Fe is +2 , six ligands donate 12 electrons

Z= 26 , X=2, Y=12
EAN of Fe2+ = Z-X+Y = 26-2+12 = 36

3. Consider [Zn(NH3)4]2+

Oxidation state of Zn is +2 , four ligands donate 8 electrons

Z= 30 , X=2, Y=8
EAN of Zn2+ = Z-X+Y = 30-2+8 = 36
Certain other coordination compounds however do not obey the EAN rule.
Example: [Fe(CN)6]3- and Cu[NH3]42⊕ have EAN 35 and [ Pt (NH3)4]2+ have EAN 84

Exceptions for the EAN rule

 ISOMERISM IN COORDINATION COMPOUNDS

Isomers

STRUCTURAL
STEREO ISOMERS ISOMERS

OPTICAL IONISATION LINKAGE

GEOMETRIC Or
ISOMERISM/
Distereomers
Enantiomers

COORDINATION SOLVATE

TRANS ISOMERS CIS ISOMERS

Stereoisomers :

Isomers have the same connections among constituent atoms but a different
arrangements of the atoms in space are called stereoisomers
There are two kinds of stereoisomers in coordination compounds:
(a) geometric isomers or distereoisomers and (b) enantiomers or optical isomers.

a. Geometric isomers or distereoisomers :

These are non mirror image stereoisomers. These are possible in heteroleptic
complexes. In these isomers, there are cis and trans types of arrangements of ligands.

Cis-isomers : Identical ligands occupy adjacent positions.

Trans-isomers : Identical ligands occupy the opposite positions.

Cis and trans isomers have different properties. Cis and trans isomerism is
observed in square planar and octahedral complexes.
i.Cis and trans isomers in square planar complexes :

The square planar complexes of MA2B2and MA2BC type exist as cis and trans isomers,
where A, B and C are monodentate ligands, M is metal.
Example :[Pt(NH3)2Cl2],(MA2B2type) [Pt(NH3)(H2O)Cl2] (MA2BC type)

Here the cis isomer is more soluble in water than the trans isomer. The cis isomer
named cisplatin is an anticancer drug while the trans isomer is physiologically
inactive. The cis isomer is polar with non-zero dipole moment. The trans isomer
has zero dipole moment as a result of the two opposite Pt – Cl and two Pt-NH3 bond
moments, which cancel each other.
ii. Cis and trans isomers in octahedral complexes :

The octahedral complexes of the type MA4B2, MA4BC and M(AA)2B2 exist as cis and
trans isomers. (AA) is a bidentate ligand. [Co(NH3)4Cl2]⊕,(MA4B2 type)
b. Optical isomers (Enantiomers) :

The complex molecules or ions that are non superimposable mirror images of each
other are enantiomers. The non superimposable mirror images are chiral.

Enantiomers have identical properties however differ in their response to the plane-
polarized light. The enantiomer that rotates the plane of plane-polarized light to right
(clockwise) is called the dextro (d) isomer, while the other that rotates the plane to
left (anticlockwise) is called laevo (l) isomer.

i. Optical isomers in octahedral complexes :

ii. Octahedral complexes existing as both geometric and optical isomers:
In this type of complex, only the cis isomer exists as pair of enantiomers

Square planar complexes do not show enantiomers since they have mirror
plane and axis of symmetry.
Structural isomers (Constitutional isomers) :

Structural isomers possess different linkages among their constituent atoms but they
have same chemical formulae.
They can be classified as linkage isomers, ionization isomers, coordination
isomers and solvate isomers.
Imp.1 Mark
a.Linkage isomers :
These isomers are formed when the ligand has two different donor atoms.
It coordinates to the metal via different donor atoms.
Thus the nitrite ion NO2- having two donor atoms show isomers as :

[Co(NH3)5(NO2)]2⊕ and [Co(NH3)5(ONO)]2⊕

The nitro complex has Co-N bond and the nitrito complex is linked through
Co-O bond. These are linkage isomers.
b. Ionization isomers : Imp.1 Mark
Ionization isomers involve exchange of ligands between coordination and ionization
spheres.
Example: [Co(NH3)5SO4]Br and [Co(NH3)5Br]SO4
(I) (II)
In compound I, anion SO42-, bonded to Co is in the coordination sphere while Br- is in
the ionization sphere.
In compound II, anion Br -is in the coordination sphere linked to Co while SO42- is in the
Ionisation sphere. These complexes in solution ionize to give different ions.
c. Coordination isomers : Imp.1 Mark
Coordination isomers show interchange of ligands between cationic and anionic spheres
of different metal ions.

Example:

In isomer I, cobalt is linked to ammine ligand and chromium to cyanide ligand.

In isomer II the ligands coordinating to metals are interchanged.
Cobalt coordinates with cyanide ligand and chromium to NH3 ligand.
I and II are examples of coordination isomers.

d. Solvate isomers (Hydrate isomers when water is solvent) : Imp.1 Mark

These are similar to ionization isomers. Look at the complexes.

In compound I, the solvent water is directly bonded to Cr. In compound II,H2O

appears as the free solvent molecule. I and II represent solvate (hydrate) isomers.
 Stability of the coordination compounds:
The stability of coordination compounds can be explained by knowing their stability
constants. The stability is governed by metal- ligand interactions.
In this the metal serves as Lewis acid electron-pair acceptor while the ligand as Lewis
base (since it is electron donor).
The metal-ligand interaction can be realized as the Lewis acid-Lewis base interaction.
Stronger the interaction greater is stability of the complex.
Consider the equilibrium for the metal ligand interaction :

where a, x, [a⊕ + nx- ] denote the charge on the metal, ligand and the complex,
respectively. Now, the equilibrium constant K is given by
Stability of the complex can be explained in terms of K.
Higher the value of K larger is the thermodynamic stability of the complex.
The equilibria for the complex formation with the corresponding K values are given
below.

From the above data, [Co(NH3)6]3⊕ is more stable than [Ag(CN)2]- and [Cu(CN)4]2- .
Factors which govern stability of the complex :
Stability of a complex is governed by
(a) charge to size ratio of the metal ion and (b) nature of the ligand.
a. charge to size ratio of the metal ion
Higher the ratio greater is the stability. For the divalent metal ion complexes their
stability shows the trend : Cu2⊕ > Ni2⊕ > Co2⊕ > Fe2⊕ > Mn2⊕ > Cd2⊕.
The above stability order is called Irving-William order.
In the above list both Cu and Cd have the charge +2, however, the ionic radius of Cu2⊕ is
69 pm and that of Cd2⊕ is 97 pm.
The charge to size ratio of Cu2⊕ is greater than that of Cd2⊕ . Therefore the Cu2⊕ forms
stable complexes than Cd2⊕.
b. Nature of the ligand.
A second factor that governs stability of the complexes is related to how easily the
ligand can donate its lone pair of electrons to the central metal ion that is, the basicity
of the ligand. The ligands those are stronger bases tend to form more stable complexes.
Example: CN- ion is more basic than NH3.Hence cyano complexes are more stable
than ammine complexes.
Valence bond theory (VBT) Assumptions of VBT

i. Central metal ion provides vacant d orbitals for formation of coordinate bonds with
ligands.
ii. The vacant d orbitals along with s and p orbitals of the metal ion take part in
hybridisation.
iii. The number of vacant hybrid orbitals formed is equal to the number of ligand
donor atoms surrounding the metal ion which equals the coordination number of metal.
iv. Overlap between the vacant hybrid orbitals of the metal and filled orbitals of
the ligand leads to formation of the metal- ligand coordinate bonds.
v. The hybrid orbitals used by the metal ion, point in the direction of the ligand.
vi. The (n-1)d or nd orbitals used in hybridisation allow the complexes to be
classified as (a) outer orbital and (b) inner orbital complexes.
 Number of Orbitals and Types of Hybridisation

Type of hybridisation decides the structure of the complex.

For example when the hybridisation is d2sp3 the structure is octahedral.
Steps to understand the metal-ligand bonding include :

i. Find oxidation state of central metal ion

ii. Write valence shell electronic configuration of metal ion.
iii. See whether the complex is low spin or high spin. (applicable only for
octahedral complexes with d4 to d8 electronic configurations).

iv. From the number of ligands find the number of metal ion orbitals required for
bonding.
v. Identify the orbitals of metal ion available for hybridisation and the type of
hybridisation involved.
vi. Write the electronic configuration after hybridisation.
vii Show filling of orbitals after complex formation.
viii. Determine the number of unpaired electrons and predict magnetic
behaviour of the complex.
Octahedral complexes Imp.3 Mark
a. [Co(NH3)6]3⊕ [low spin or Inner orbital or spin paired complex]
i.Oxidation state of central metal Cobalt(Z=27 4s23d7) is +3
ii.Valence shell electronic configuration of Co3⊕ is represented in box diagram as
shown below :

iii. Number of ammine ligands is 6, number of vacant metal ion orbitals required for
bonding with ligands must be six.
iv. Complex is low spin, so pairing of electrons will take place prior to
hybridisation.
v. Electronic configuration after pairing would be

vi. Six orbitals available for hybridisation are two 3d, one 4s, three 4p orbitals
The orbitals for hybridization are decided from the number of ammine ligands which
is six. Here (n-1)d orbitals participate in hybridization since it is the low spin complex.

vii. Electronic configuration after complex formation.

viii. As all electrons are paired the complex is diamagnetic.

b. [CoF6]3- [High spin or Outer orbital or spin free complex]
i.Oxidation state of central metal Co (Z=27 4s23d7) is +3
ii.Valence shell electronic configuration of Co3⊕ is Imp.3 Mark

iii. Six fluoride F- ligands, thus the number of vacant metal ion orbitals required for
bonding with ligands would be six.
iv. Complex is high spin, that means pairing of electrons will not take place
prior to hybridisation. Electronic configuration would remain the same as in the free
state shown above.
v. Six orbitals available for the hybridisation. Those are one 4s, three 4p, two of 4d
orbitals

Six metal orbitals after bonding with six F- ligands led to the sp3d2 hybridization. The d
orbitals participating in hybridisation for this complex are nd.
vi. Six vacant sp3d2 hybrid orbital of Co3+ overlap with six orbitals of fluoride forming
Co - F coordinate bonds.
vii. Configuration after complex formation.

viii. The complex is octahedral and has four unpaired electrons and hence, is
paramagnetic.
Tetrahedral complex
[Ni(Cl)4]2-
i. Oxidation state of nickel is (Z=28 4s23d8)+2
ii. Valence shell electronic configuration of Ni2+

iii. number of Cl ligands is 4. Therefore number of vacant metal ion orbitals required for
bonding with ligands must be four.
iv. Four orbitals on metal available for hybridisation are one 4s, three 4p. The complex is
tetrahedral.

The four metal ion orbitals for bonding with Cl ligands are derived from the sp3
hybridization.
vi. Four vacant sp3 hybrid orbital of Ni2⊕overlap with four orbitals of Cl- ions.
vii. Configuration after complex formation would be.
viii.The complex has two unpaired electrons and hence, paramagnetic.
Square planar complex

[Ni(CN)4]2-
i. Oxidation state of nickel is(Z=27 4s23d8) +2
ii. Valence shell electronic configuration of Ni2+

iii. Number of CN ligands is 4, so number of vacant metal ion orbitals required for
bonding with ligands would be four.
iv. Complex is square planar so Ni2⊕ ion uses dsp2 hybrid orbitals.
v. 3d electrons are paired prior to the hybridisation and electronic configuration of Ni2⊕
becomes :
vi. Orbitals available for hybridisation are one 3d, one 4s and two 4p which give dsp2
hybridization.
vii. Four vacant dsp2 hybrid orbitals of Ni2⊕ overlap with four orbitals of CN ions to
form Ni - CN coordinate bonds.
vii. Configuration after the complex formation becomes.

viii. The complex has no unpaired electrons and hence, diamagnetic.

 Limitations of VBT

1. VBT fails to explain the colours exhibited by coordination compounds.

2. VBT fails to explain quantitative interpretation of magnetic data.
3. It doesn't distinguish between weak field and strong field ligands.
4. It cannot predict exactly the tetrahedral and square planar structure of 4
coordinate complexes.
5. It does not give a quantitative interpretation of the thermodynamic or
kinetic stabilities of coordination compounds.
 Crystal Field theory (CFT)

C.F.T. is based on following assumptions

1. CFT assumes that metal-ligand bonding is entirely ionic .It arises from electrostatic
attraction between positively charged metal ion and negatively charged ligands
2. Five d orbitals, dxy,dyz,dxz,dz2 and dx2-y2 in an isolated gaseous ion or atom are
degenerate that is they have the same energy.
3. When ligands approach metal ion, their electrons are repelled by electrons of metal
ion.As a result of this repulsion,d orbitals of metal ion raised in energy and split
into two groups, designated as t2g and eg. This splitting is called crystal field
splitting. The energy separation between two groups is Δo.
4. The axial orbitals, dz2 and dx2-y2 experience more repulsion and are raised eg level
of higher energy
The orbitals dxy,dyz,dzx that lie between the axes suffer lesser repulsion. They
are ,therefore lowered in energy relative to average energy. They are placed in t2g level
5. Strong field and weak field ligands
i. Strong field ligands:

In these ligands donor atoms are carbon, nitrogen or phosphorus .These ligands
produce relatively large value of Δo. Therefore, Δo > P where P is the pairing energy of
electrons. Thus it is easier to pair up electrons than to place them in either dz2 or dx2-y2
orbitals. This results in low spin complex (Octahedral)
ii. Weak field ligands:
In these ligands donor atoms are halogens, oxygen or sulphur. These ligands produce
relatively small value of Δo.Therefore for metal complex with weak field ligands Δo <P.
It is then easier with weak field ligands Δo <P. It is then easier to place an electron in
either dz2 or dx2-y2 than to pair up electrons. This gives rise to high spin complex
(Octahedral).

6. A choice between high spin and low spin electronic configuration arises only for
d4 - d7 octahedral complexes.
In d1 to d3 complexes, all electrons occupy lower energy d orbitals independent of Δo.
In d8 to d10 complexes, dxy,dyz and dxz orbitals each contain two electrons. dz2 and
dx2-y2 orbitals contain two,three or four electrons,which is again independent of Δo.
7. Spectrochemical series
The arragnement of ligands in order of their increasing field strength is called
spectrochemial series
I- < Br- < Cl- < S2- < F- < OH- < C2O42-<H2O<NCS <EDTA< NH3,<en< CN- < CO.
d orbital diagrams for high spin and low spin d4-d7complexes
 Applications of coordination compounds Imp.2 Mark

Coordination compounds found many applications in the area of analytical

chemistry, metallurgy, biological systems, industry and medicine.
These are described below:
1.In qualitative and quantitative analysis: Coordination compounds find use in
many qualitative and quantitative chemical analysis.

2.To estimate hardness of water: Hardness of water is due to the presence of Ca2+
and Mg2+ ions in it. The ligand EDTA forms stable complexes with Ca2+ and Mg2+ ions .
The selective estimation of these ions can be done due to difference in
the stability constants of calcium and magnesium complexes.

3.Extraction processes of some metals, like those of silver and gold, make use of
complex formation. Gold, for example, combines with cyanide in the presence of oxygen
and water to form the coordination entity [Au(CN)2]– in aqueous solution. Gold can be
separated in metallic form from this solution by the addition of zinc .
4.Purification of metals can be achieved through formation and subsequent
decomposition of their coordination compounds.
For example, impure nickel is converted to [Ni(CO)4], which is decomposed to yield pure
nickel.
5. In biological systems. The pigment responsible for photosynthesis, chlorophyll, is
a coordination compound of magnesium. Haemoglobin, the red pigment of blood which
acts as oxygen carrier is a coordination
compound of iron.
6. Catalysts for many industrial processes. Examples include rhodium complex,
[(Ph3P)3RhCl], a Wilkinson catalyst, is used for the hydrogenation of alkenes.

7.InElectoplating:Articles can be electroplated with silver and gold much more

smoothly and evenly from solutions of the complexes, [Ag(CN)2]– and [Au(CN)2]– than
from a solution of simple metal ions.
8.In black and white photography, the developed film is fixed by washing with hypo
solution which dissolves the undecomposed AgBr to form a complex ion, [Ag(S2O3)2]3–.
9.In medicines: Pt complex cisplatin is used in the treatment of cancer.
EDTA is used for treatment of lead poisoning.