1.
Factors Affecting The Stability of
Metal Complexes
Presented by
LOKESH JANGID
M.Sc. Chemistry Semester 2nd
Department of Chemistry
S.P.C. Government College Ajmer

2.
Content:-
•Introduction-
•Factors affecting stability of metal complexes-:
1.Nature of the central metal ion
2.Nature of the coordinating group or ligand
3.Presence of ring structure
4. other factors-
a. Forced configuration
b. solvent effect
d. steric effect

3.
What is stability-
The use of term stability without any qualification
Only means that the complex exists under suitable condition and it is
Possible to store the complex for an appreciable time.
two kinds of stability of complexes are generally considered-
1.Thermodynamic stability-
it is a measure of the extent of formation
or transformation of a complex under a given set of condition at
equilibrium.
2. Kinetic stability-
The Kinetic stability of a species refers to the
speed with which transformation leading to the attainment of
equilibrium will occure.

4.
We can quantitatively determine the stability of a metal complex
With the help of following two constant-
1. Instability constant-
when a complex ion dissociates into its
Components, a constant similar to ionisation constant.𝐾𝑖 is called
dissociation constant or less frequently as instability constant
because it is a measure of extent of dissociation.
2. Stability constant-
The reciprocal of dissociation constant which
is a measure of extent of association is called Stability constant.
K= 1/𝐾𝑖. Higher the value of stability constant greater is the
stability of the complexes.

5.
Factors affecting stability of metal complexes
1. Nature of central
metal ion
a. Size and charge
b. Ionization energies
c. Class a and class b
metals
2. Nature of
coordinating group
or ligand
a. size and charge
b. base strength
c. ligand concentration
3. Presence of ring
structure
a. Chelate effect
b. size of chelate ring
c. Number of chelate
ring
d. Entropy effect
4. Other factors
a. forced configuration
b solvent effect
c. Steric effect

6.
Factors affecting stability of metal complexes-
1. Nature of the central metal ion-
a. Size and charge:
the relative stabilities of many complexes can be
explained on the basis of a simple electrostatic model. more stable
complexes expected to be formed by the combination of oppositely
charged ions. moreover, greater the charge and smaller the ions ,
greater the stability of the complexes. Smaller ions are favoured
because their centers can be closer. the following hydroxo
complexes which show a gradual increase in their stability with
the increase in the charge of the metal ion.
KLiOH =2 KMgOH
+2 = 102 KYOH
+2 = 107 KThOH
+3 =1010
As the value of stability constant increases it indicates that the
complexes become more and more stable.

7.
b. Ionization energy:
the electronegativity , covalent nature and
ionic radii can all be related to the ionization energies of atoms.
it is found that the stability constant for the metal complexes with a
ligand increases with the ionization energies of the metallic species.
c. The ions with high polarizability gives complexes with higher
stability constant and also ions with high electronegativity gives
stabler complexes.
d. Class a and class b metal-
metals can be divided into two classes
as class a (hard acid) and class b (soft acid).

8.
Class b metals
.less electropositive metals like Pt, Pd, Hg, Pb and
Rh belong to class b metals.
Class b metals prefer ligands in which the donor
atom is less electronegative i.e.one of the heavier
element of group VA ,VIAand VIIA.
. Class b metal complexes is attributed to the
covalent metal ligand bonds and to the transfer
of electron density from the metal to the ligand
through 𝜋 bonding by making use of d- electrons
present in the metal atoms.
. The most stable complexes of class b metals are
formed with ligand such as P𝑀𝑒3 , 𝑆2− , 𝐼− which
have vacant d- orbitals or the ligand such as CO ,
CNˉ Which have vacant molecular orbital of low
energy.
Class a metals-
. More electropositive metals like Na, Ca,
lanthanides, Ti and Fe belong to class a.
.class a metals form the most stable complexes
with ligands Having more electronegative donor
atoms like N,O or F.
. The stability of class a metal complexes is
attributed to the ionic bonding.
Class a and class b metals:

9.
2. Nature Of the coordinating group or ligand-
Nature of the ligand or important characteristics of ligand
affects to determine the stability of the compounds is
explained in the following manner-
a. Size and Charge:
Ligands with less charge and large size are less stable and
form less stable coordination compounds. Ligands with higher
charge have small size and form more stable compounds.
large size →Less Stable ←Less Charge
Small size → More stable ← More Charge

10.
b. Base strength-
Calvin and Wilson suggested that the higher basic character or
strength of the ligand, higher will be the stability of coordination compounds. It is
defined that a strong base or higher basic strength of the ligand means it forms
more stable compounds or its donating tendency of electron to central metal ion
is higher.
e.g.- Aromatic diamines form unstable coordination compounds while
aliphatic diamines form stable coordination compounds. Ligands like NH3, CN- etc
have more basic character that means they form more stable compounds.

11.
c. Ligand Concentration:
Some coordination complexes exist in aqueous solution only in presence of
higher concentration of coordination group. In some cases aqueous molecules
show greater coordinating tendency than the coordinating group which is
originally present.
e.g. in presence of highly concentrated solution of SCN- (thiocynate ion), the Co2+
metal ion
forms a stable blue colored coordination complex but on dilution in aqueous medium the blue
complex is destroyed and a pink aqua complex [Co(H2O)6]2+ is formed and then by further
addition of ligand (SCN-) pink colour disappears.
[Co(SCN)4]2−
+ H2O → [Co(H2O)6]2+
+ + 4SCNˉ
Blue Pink
The colour change indicates that there is a competition between H2O/SCN- in
formation of complex with Co(II) ion.

12.
3. Presence of ring structure (chelation)-
a. Chelate Effect:
The process of forming metal chelate by the attachment of multidentate ligand
with central metal ion in which ligand act as chelating agent is known as
chelation. Chelation is expressed by the following unidentate and bidentate
ligand reaction.
M + 2L→ ML2 K=[ML2]/[M][𝐿]2
M + L-L → M-L-L or K=
[𝑀−𝐿−𝐿]
𝑀 [𝐿−𝐿]
Multidentate ligands form more stable coordination compounds than
monodentate ligands. Following factors are of great importance in chelate
formation.

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b. Size of chelate Rings :-
The stability of chelate is depending on the size of chelate ring. The stability
of coordination complex increases with number of chelate ring. It is found that
4-membered rings are unstable and rare than 5-membered rings which are
common and stable. For chelate (saturated chelate) rings the following is the
decreasing order of stability with increasing ring size-
Five membered > Six membered > Seven membered
The stability of metal chelates decrease by increasing the chelate size.
the higher membered rings are uncommon due to-
(i) strain set up in the heterocyclic ring.
(ii) possibility of a long chain multidentate ligand bonding to more
than one atom, giving the formation of the polynuclear complexes
rather than the chelate complexes.

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in general it has been observed that saturated ligand form 5
membered rings give the most stable product. e.g. complexes with
ethylenediamine and C2O2
2-. Fig

15.
But unsaturated ligands (those with double bonds) form very stable
metal complexes containing six membered rings. This is because
in such complexes 𝜋 electron density is delocalised and spread
over the ring which is thus stabilised by resonance. Examples are
acetyl acetone complexes with metals fig. 2

16.
c. Number of chelate Rings:
Increase in the number of rings increase the stability of
compounds. Stability constant of complexes with metal(II) ions
No. of rings
formed
Ligand
0 NH3 3.7 5.3 7.8 12.6
1 en 7.7 10.9 14.5 20.2
2 trien 7.8 11.0 14.1 20.2
3 tren 8.8 12.8 14.o 18.3
5 penen 11.2 15.8 19.3 22.4
stability constants for complexes with
Fe(II) Co(II) Ni(II) Cu(II)
trien : triethylene tetraamine NH2CH2CH2NHCH2CH2NHCH2CH2NH2
tren : triaminoethylamine (H2NCH2CH2)3N
penen : tetrakis (aminoethyl) ethylenediamine (H2NCH2CH2)2NCH2CH2N(CH2CH2NH2)2

17.
d. Entropy Effect –
the chief factor responsible for the stability of the chelate
ring is the entropy change. Considering the electronic effect of the
donor atom to be the same in the monodentate and the bidentate
ligands, it can be seen that the dissociation of a monodentate from
the complex will be higher than that in the chelating bidentate.
The dissociation of the M-L bond in monodentate will release
The ligand completely from the coordination sphere of the metal,
So that it can be easily swept off by the solvent. but the dissociation
Of one M-L bond for the bidentate ligand does not release the ligand
Completely (for which simultaneous dissociation at both ends is
required).hence the stability constant for metal chelate must be
Higher.
[Co(NH3)6]3++ 3en → [𝐶𝑜 𝑒𝑛 3]2++ 6NH3

18.
4. Other factors-
a. Forced configuration-
ligands like porphyrin and phthalcyanine
Which have completely fused planar ring system form extraordinarily
Stable complexes with metal ions that tend to give planar complexes
e.g. Cu2+ complex with phthalocyanine is very stable. Similarly trien
Forms very stable complexes with Ni(II) and Cu(II).
These ligands impose planar configurations
Even on metal ions that have no tendency
to form planar complexes with unidentate
Ligands. for e.g. 𝐵𝑒2+and 𝑍𝑛2+ion normally
Form tetrahedral complexes but when they
Combine with these polydentate ligands they
Are forced to assume planner configuration.
Therefore these complexes are less stable.
Copper(II) Pthalocynine complex

19.
b. Nature of the solvent-
solvent with low dielectric constant and low dipole
moment are expected to increase the stability and the stability
constant.
further, a donor solvent will tend to form its own complexes
with the metal ions, so that the incoming ligand will face a competition
with the solvent molecules to get attached with the metal ions. Hence
strongly donor solvents decrease the stability constants of the metal
complexes.

20.
c. Steric effect-
In some cases the clashing of groups on two coordinated
ligands will result in distortion of bond angles and a decrease in stability is
the phenomenon of F-strain, described by Brown [27], as applied to
coordination compounds.
As steric effect is decreasing, the stability of a complex is increasing.
Due to steric effect in Ni(II) complexes with 2-methyl 8- hydroxy quinoline
are less stable than complexes with 8-hydroxy quinoline because of the
steric hindrance caused by the methyl
group adjacent to the site of coordination.
Similarly complexes of ethylene diamine are more stable than its
tetramethyl derivatives

21.
Reference:-
 Theoretical principles of inorganic chemistry by G.S.MANKU
(Tata McGraw-HILL Publication)
 Coordination chemistry by GURDEEP R. CHATWAL, MAHINDRA S.YADAV,
Mrs. M. Arora
(Campus books publication)

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Thank You…..