This document discusses the stability of metal complexes. There are two types of stability: thermodynamic stability and kinetic stability. Thermodynamic stability is determined by equilibrium constants and Gibbs free energy, while kinetic stability refers to the reactivity of ligand substitution reactions and is determined by reaction rates and activation energies. The factors that influence the stability include the nature of the metal and ligand, such as their size, charge, and electronegativity. The stability tends to increase with higher charges, smaller sizes, and more basic ligands. Chelates are generally more stable than open structures.

1. Stability Of Complexes
Manahil Azam (860138)
Department of Chemistry

2. 2
 Stability means that a complex exist under suitable
and required conditions it can be stored for a long
time.
But this cannot be generalized to all complexes.
 One particular complex may be stable towards a
reagent and highly reactive towards another reagent.
Defining Stability

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Types Of Stability
There are two types of Stability:
Thermodynamic Stability
Kinetic Stability

4. Thermodynamic Stability
It is define as the measure of tendency of a metal ion
to selectively form a specific metal complex and is
directly related to the metal – ligand bond energies.
It is also define by two factors:
• Equilibrium constant
(High value then complex is stable)
(Low value then complex is unstable)
• Gib’s free energy (∆𝐺)
(+ve value then complex is stable)
(-ve value then complex is unstable)

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Kinetic Stability
It is refers to the reactivity or ability of metal complex
to undergo Ligand substitution reaction.
It is also define by two factors:
• Rate of reaction
(Slow then kinetically stable)
(Fast then kinetically unstable)
• Energy of activation
(High value then complex is stable)
(Low value then complex is unstable)

6. Types of complexes
There are two types:
• Labile – the complex which rapidly exchange their
ligand with other specie.
• Inert – the complex which have ligand exchange
reaction rate is low.

7. Thermodynamic Stability
Thermodynamic stability of complexes explain on the
basis of two factors:
• Stepwise stability constant (kn)
• Overall stability constant (𝜷)

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Stepwise Stability Constant (kn)
Formation of a complex in aqueous solution proceed through
a stepwise fashion with corresponding equilibrium constants.
M + L ML K1 = 𝑀𝐿 / 𝑀 𝐿
ML + L ML2 K2 = 𝑀𝐿2 / 𝑀 𝐿
ML2 + L ML3 K3 = 𝑀𝐿3 / 𝑀 𝐿
MLn-1 + L MLn Kn = 𝑀𝐿 𝑛 / 𝑀𝐿 𝑛−1 𝐿
These K1, K2, K3, …….. Kn are called stepwise stability constants.

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Overall Stability Constant (𝜷)
If the complex formation is considered as a single step
process:
M + nL MLn
𝜷 = 𝑀𝐿 𝑛 / 𝑀 𝐿 n
• If the value of 𝜷 will become high then complex is
thermodynamically stable.
• If its value become low then complex is thermodynamically
unstable.

10. Relationship between Kn & 𝜷
To find out the value of overall stability constant, multiply the
equilibrium constant values of stepwise stability constant as
follow:
𝜷= K1 × K2 × K3 × ………….. × Kn
Trend in stability constant:
K1 > K2 > K3 > …………. > Kn
• Generally the stepwise stability constant value decrease with
successive replacement by the ligand, because crowding
across complex increase step by step.

11. Factor affecting on Thermodynamic Stability
Nature of Metal
1. Charge:
• Low charge (Fe+2)/ Unstable
• High charge (Fe+3)/ Stable
2. Size:
• Small size/ Stable complex
• Large size/ Unstable complex
3. Irving – William order of stability:
• If different metals are bonded with same ligands then they show the
Stability order as below
Mn+2 < Fe+2 < Co+2 < Ni+2 < Cu+2 > Zn+2
• Stability of complexes increase with the increase in atomic number of
metals, but Cu+2 does not follow this trend because it shows Jahn
teller distortion. This overall trend is known as Irving – William order
of stability.

12. 4. Class a & b:
 According to Chatt & Ahrland
• Class a – include lighter metals (IA, IIA, Sc, Ti, V)if bonded with (N,
O, F) then the complex will be stable.
• Class b – include heavier metals (Hg, Au, Ag) if bonded with (Cl, P,
S) then the complex will be stable.
5. Electronegativity of Metal:
 High value – if a metal receives electron pair more fastly from ligand
then the forming complex will be more stable.
 Low value – if a metal does not receives electron pair fastly from the
ligand then the forming complex will be unstable.

13.  Nature of Ligand
1. Charge:
• High charge/ stable complex
• Low charge/ unstable complex
2. size:
• Large size/ unstable complex
• Small size/ stable complex
3. Basic Nature:
• High value – if a ligand has greater ability to donate the electron pair
to the metal then the forming complex will be sable.
• Low value – if a ligand has smaller ability to donate electron pair to
the metal then the forming complex will be unstable.
4. Chelation effect:
 Chelates are more stable than open chain structure or complexes.
• No. of rings – more rings more stable complex
• Size of ring – 5 cornered ring will be more stable
• Steric hindrance – bulky or large ligands will form unstable complex
while lighter or small ligands will form stable complexs

14. Kinetic Stability
Kinetic stability (inertness & lability) of complexes explained
on the basis of two theories:
 According to:
• Valance Bond Theory (VBT)
• Cristal Field Theory (CFT)

15. According to VBT
VBT classifies octahedral complexes into two types:
• Outer orbital complexes – d2sp3
• Inner orbital complexes – sp3d2
• The two d-orbitals are involved in hybridization
are the eg set of orbitals.

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Outer orbital complexes
The complexes having sp3d2 hybridization are called
outer orbital complexes.
In term of VBT these bonds are weaker. It can easily
breakable so, new incoming ligand will replace the old
ligand fastly.
These complexes are generally labile.

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Inner orbital complexes
These complexes generally have d2sp3 hybridization.
The hybrid orbitals are filled with the ligand electrons.
The t2g orbitals of metal accommodate the d electrons of
the metal.
If the t2g level are left vacant then the complex can
associate with an incoming ligand and the complex is
labile.
If the t2g levels are fully occupied then the complex will be
inert.

18. According to CFT
According to CFT ligand field splits d – orbitals.
This splitting leads to a decrease in energy of the system
whose magnitude depends on the no. of d – electrons
present.
If the CFSE value increase by association or dissociation of a
ligand then complex is labile.
On the other hand the complex will be inert if CFSE value
decrease.

19. Factors affecting on Kinetic Stability
1. Charge on Metal:
• High / Inert complex
• Low / Labile complex
2. Size of Metal:
• Small / Inert complex
• Large / Labile complex
3. Coordination no. of complex:
• CN = 04 (Labile complex)
• CN = 06 (Inert complex)
 Following are the factors affect on the Kinetic stability: