The document discusses factors that affect the stability of metal complexes. It outlines three main factors: (1) the nature of the central metal ion, (2) the nature of the ligand, and (3) surrounding conditions. For the nature of the ligand, it discusses several sub-factors that can influence stability, including size and charge of the ligand, its basic character, concentration, covalent character, and ability to form chelate complexes or use resonance. The document provides examples to illustrate each factor and sub-factor. It also discusses applications of chelate complexes.
1. Dr. Geeta Tewari
Department of Chemistry
D. S. B. Campus
Kumaun University, Nainital
Email: geeta_k@rediffmail.com
Factors affecting stability of complexes
2. Factors
(1) Nature of the central metal Ion
(2) Nature of the ligand
(3) Surrounding conditions
3. (2) Nature of ligand
(i) Size and charge of ligand
(ii) Basic character
(iii) Ligand concentration
(iv) Covalent character of ligand
(v) Dipole moment
(vi) π-bonding capacity of ligands
(vii) Steric hindrance
(viii) Chelate effect
(a) Ring size (b) Number of rings (c) Charge on the ligand
(ix) Macrocyclic effect
(x) Resonance effect
(3) Surrounding conditions
4. A smaller ligand approaches a metal cation more closely as
compared to the larger ligand.
In the similar manner, the ligand with high charge can form
strong bond with metal ion.
Stability of the complexes formed by the ligands with smaller
size and higher charge is more than the complexes formed by the
ligands with larger size and smaller charge.
The order of ligands (halides) to form stable complexes with the
same metal ion is:
F->Cl->Br->I-
(i) Size and charge of ligand
5. • High Basic character- ease to donate lone pair of electron.
Calvin and Wilson suggested that more the basic character of
a ligand, greater will be the stability of complex formed.
NH3 > H2O > F–
• Thus, NH3 forms more stable complex as compared to H2O
and H2O more stable complex than F-.
• Aliphatic diamines form stable complex compounds while
aromatic diamines form unstable complex compounds.
(ii) Basic character
6. Sometimes coordination complexes exist in
aqueous solution only due to the presence of
higher concentration of ligands.
Sometimes water molecules show greater
coordination tendency than the other ligand
which is originally present.
(iii) Ligand concentration
7. (iii) Ligand concentration
When SCN- (thiocynate ion) ligand is present in the solution
in high amount, the Co2+ metal ion forms a stable blue
colored complex [Co(SCN)4]2- with SCN- while on dilution,
the blue coloured complex destroyed and a pink complex
[Co(H2O)6]2+ is formed.
Further addition of SCN- ligand again regenerates blue
coloured complex and destroyed pink colored complex.
Competition between the two i.e. H2O and SCN- for
coordination with the cobalt ion is indicated by the colour
change.
[Co(SCN)4]2- + 6H2O [Co(H2O)6]2+ + 4SCN-
Blue Pink
8. During synthesis of tetra amine cupric sulphate complex
[Cu(NH3)4SO4.H2O], at lower concentration of ammonia, copper
hydroxide is formed, while at higher concentration of ammonia,
tetra amine cupric sulphate complex is formed.
CuSO4 + NH4OH (low concentration) → Cu(OH)2
CuSO4 + NH4OH (high concentration) → [Cu(NH3)4SO4.H2O]
(iii) Ligand concentration
9. (iv) Covalent character of ligand
Higher the covalent character of ligand, higher will be the
stability of the complex. This is true for soft acids.
Decreasing order of covalent character and stability of complex
-
10. (v) Dipole moment
In case of neutral ligands, higher the magnitude of dipole
moment, higher will be the stability of the complex.
The order of stability of complexes formed by some
neutral ligands is given below:
NH3 > C2H5NH2 > (C2H5)2NH > (C2H5)3N
11. The ligands such as CO, CN–, alkene, R3P, R2S etc.
which have vacant *molecular orbitals form stable
complexes.
(vi) -bonding capacity of ligands
12. (vii) Steric hindrance
The presence of bulky groups present near the donor atom in a
ligand is responsible for repulsion between donor atom and
metal ion.
This repulsion causes weakening of metal ligand bonding and
therefore, the complex become unstable.
8-hydroxy quinoline 2-methyl-8-hydroxy
quinoline
13. (viii) The Chelate effect
The term chelate is given by Morgan and Drew (1920)
Bidentate or multidentate ligands form complexes having
ring or cyclic structure.
These complexes are called chelate and these ligands are
called chelating ligands.
These complexes are more stable than non chelate ligands.
This greater stability of chelates is called chelate effect.
14. (a)Ring size
The chelates having 5 or 6 membered rings are more stable than
the other chelates (form less than 5 membered ring).
Four membered ring Seven membered ring
High strain and thus unstable Large repulsion between
bulky groups and hence, less stable
Factors affecting chelate effect
S
O:
O:
:O
O:
M
H 2
C
H 2
C
C H 2
H 2 C
H 2 N N H 2
M
15. 6-membered chelates of ligands showing conjugation
or of heterocyclic ligands, are more stable than the 5-
membered chelates.
Factors affecting chelate effect
C O
M
HC
C O
H3C
CH3
C
O
M CH
C
O
CH3
H3C
C H O
M
H C
C H O
H 3 C
C H 3
16. (b) Number of rings
The chelates in which the polydentate ligand
form more number of cyclic or ring structure is
more stable than the chelate in which a
polydentate ligand form lower number of ring
structures.
EDTA (Ethylene diamine tetraacetic acid)
complex having five chelate rings, is more
stable than complex with four monodentate
carboxylate ligands and two monodentate
nitrogen donor ligands.
Factors affecting chelate effect
[Ca(EDTA)]2-
17. • The complex [Ni(dien)2)]2+ is more stable (4 rings) than the
complex [Ni(en)3)]2+ (3 rings).
• Both the complexes are octahedral with six nitrogen atoms
surrounding the nickel ion.
• dien (diethylenetriamine, 1,4,7-triazaheptane) is
a tridentate ligand and en is a bidentate ligand.
Factors affecting chelate effect
H 2 N
N
H
N H 2
N i
N H 2
H 2 C
H 2 C
N H
H 2 C
C H 2
N H 2
H N C H 2
C H 2
H 2 N
C H 2
C
H 2
H 2 N
2 +
N
i
H
2
C
C
H
2
H
2
N
H
2
N
C
H
2
C
H
2
N
H
2
N
H
2
H
2
C
H
2
C
N
H
2
N
H
2
2
+
18. (c) Charge on the ligand
The charged ligands such as glycinate and oxalate form less
stable chelates than uncharged ligands like ethylene diamine
(en) and triamino triethylamine (trien).
{(COO-)2}3M < (NH2CH2COO-)3M < (NH2CH2CH2NH2)3M
Factors affecting chelate effect
N
i
2
+
O
H
2
O
H
2
H
2
N
C
H
2
C
H
2
N
H
2
H
2
N
H
2
C
H
2
C
N
H
2
N
i
2
+
O
H
2
O
H
2
O
-
C
C
H
2
N
H
2
O
-
C
H
2
C
N
H
2
O O
Log β = 14.5 Log β = 10.4
19. (d) Basicity of ligand
The stability of chelates increases with increase in
basic nature of the ligand.
Factors affecting chelate effect
20. Stability of chelates can be explained on the
basis of standard free energy change (∆Gº)
and standard entropy change (∆Sº).
Consider formation of coordination
compound of monodentate and bidentate
ligands.
Thermodynamic origin of chelate effect
22. ∆Gº = (∆Hº) - (T∆Sº)
A product will be stable if value of ∆Gº is negative (-ve) and value of
∆Sº is positive (+ve).
During the formation of a nonchelated compound with monodentate
ligands (i), the number of molecules on both side is equal (5 molecules
each side), hence, ∆Sº will be zero and the value of ∆Gº will be negative.
With bidentate and tridentate ligands (ii and iii), number of product
molecules (5 molecules) is greater than the number of reactant molecules
(3 and 2 molecules).
Hence, entropy of products is greater than reactants and thus, ∆Sº will be
positive and value of ∆Gº will become more negative.
Thus, it may be concluded that ∆Gº has more negative value for chelates
which are formed by the reaction of bidentate or other polydentate
ligands and therefore, are more stable as compared to the nonchelated
compounds.
23. Used in water softening.
Used in food industry to preserve fruits, fruit juices, food stuffs.
Used in agriculture as common components of fertilizers in agriculture
and to detoxify poisonous metal such as mercury, arsenic
and lead present in polluted water.
Used in medical used as supplements of ions, to decrease metal
poisoning and in the treatment for autism.
Have chemical applications such as homogeneous catalyst and used for
the removal of rust from iron and steel. Metal chelates are also used in
dying industry.
Used in physiological chemistry (in human body) such as
haemoglobin, vitamin B12, chrorophyll, cytochrome and plastocyanine
are physiologically important chelates.
Uses of chelates
24. (ix) Macrocyclic effect
Macrocyclic ligands are nine or more membered cyclic molecules
with three or more donor atoms that can form a large ring and
surround the central atom or ion partially or fully and bond to it.
The central atom or ion resides at the centre (cavity) of the large
ring. The donor atoms in macrocyclic ligands may be N, O, S or
P.
Some macrocyclic ligands have conjugated systems. This
complex formed is more rigid and inert as compared to the
chelate compound and is known as macrocyclic complex.
Stability of complex formed by macrocyclic ligand is several
times greater than complexes formed by open ended multidentate
ligands.
This greater stability of macrocyclic complex as compared to
chelates is called as macrocyclic effect.
26. The ligands that show resonance, form more stable
complexes as compared to the non-resonating ligands.
For example, acetylacetonate ligand shows resonance,
hence, forms stable chelated complex.
H3C C C
H
C CH3
O O-
H3C C C
H
C CH3
O-
O
Resonance structures of acetylacetonate
(x) Resonance effect
W. U. Malik, G. D. Tuli and R. D. Madan. Selected topics in Inorganic Chemistry. 8th Edition. 2014. S Chand Publishing.
27. Many complexes that are stable under specific
conditions may not be stable under some other set of
conditions. [Co(NH3)6]3+ complex is stable in
aqueous solution under neutral conditions while
unstable in an acidic solution.
[Co(NH3)6]3+ + 6H3O+ → [Co(H2O)6]3+ + 6NH4
+
Hence, stability of a complex should be discussed
with the surrounding conditions such as heat, light,
acidity or basicity.
(3) Surrounding conditions
