The document discusses various factors that affect the stability of metal complexes. It explains that complexes formed with ligands having higher charge and smaller size are generally more stable. It also discusses the Irving-Williams order of stability and the factors of charge to radius ratio, electronegativity, and basicity of ligands. The chelate effect is described as an important ligand effect where multidentate ligands form more stable complexes due to entropy gains. Kinetic and thermodynamic stability are distinguished from reactivity concepts of labile and inert complexes.

1. V.SANTHANAM
DEPARTMENT OF CHEMISTRY
SCSVMV

2. DEFINING STABILITY
 The statement that a complex is stable is rather loose
and misleading very often.
 It means that a complex exists and 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
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3. Thermodynamic stability
• As for as complexes in solutions are
concerned there are two kinds of stabilities
• Thermodynamic stability – Measure of the
extent to which the complex will be formed
or will be transformed into another species,
when the system has reached equilibrium
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4. Kinetic stability
• Kinetic stability – refers to the speed with
which the transformations leading to
equilibrium will occur.
• Under this , the rates of substitutions,
racemisations and their mechanisms.
• The factors which are affecting the rates of
the reactions are also studied
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5. Labile and Inert complexes
• The complexes which rapidly exchange their
ligands with other species are called labile.
• If the ligand exchange reaction rate is slow
then they are called inert complexes.
• But the reactive nature should not be
confused with the stability.
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6. Stability constant / Formation constant
• According to Bjerrum formation of a complex in
aqueous solution proceeds through a stepwise
fashion with corresponding equilibrium constants
M + L ML K1 = [ML] / [M] [L]
ML + L ML2 K2 = [ML2] / [ML] [L]
ML2 + L ML3 K3 = [ML2] / [ML2] [L]
…………..……………………………….
………….………………………………..
MLn-1 + L MLn Kn = [MLn] / [MLn-1] [L]
These K1,K2 K3 … Kn are called stepwise formation constants
K1
Kn
K3
K2
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7. Overall stability constant
• If the complex formation is considered as a
single step process
M + nL MLn
= [MLn] / [M] [L]ᵝn
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8. Trends in stability constants
[Cu(OH2)4]2+
+ NH3  [Cu(OH2)3(NH3)]2+
+ H2O log K1 = 4.22
[Cu(OH2)3(NH3)]2+
+ NH3  [Cu(OH2)2(NH3)2]2+
+ H2O log K2 = 3.50
[Cu(OH2)2(NH3)2]2+
+ NH3  [Cu(OH2)(NH3)3]2+
+ H2O log K3 = 2.92
[Cu(OH2)(NH3)3]2+
+ NH3  [Cu(NH3)4]2+
+ H2O log K4 = 2.18
• Generally the stepwise stability constant values decrease with
successive replacement by the ligands
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9. Statistical effect
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10. Statistical effect explanation
• When more ligands are entering into the
coordination sphere the number of aqua
ligand decreases.
• This reduces the probability of substitution of
aqua ligand with the new ligand.
• Reflected as decreasing stepwise formation
constants
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11. Relationship between Kn and ᵝn
• Let us consider
ᵝ3 = [ML3] / [M] [L]3
= [ML3] . [ML2] . [ML]
[M] [L]3
. [ML2] . [ML]
= [ML] . [ML2] . [ML3]
[M] [L] [ML] [L] [ML2] [L]
= K1 . K2 . K3
In general
ᵝn = K .K .K . ….. K
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12. Kinetic Vs Thermodynamic stability
• The terms labile and inert refer to the
reactivity of a complex only.
• Not to be confused with its stability.
• An inert complex may be stable or unstable.
• Similarly a labile complex may be stable or
unstable
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13. Exemplification
• The above said fact is clearly shown by the
complex [Hg(CN)4]2-
.
Hg2+
+ 4CN-
[Hg(CN)4]2-
ᵝ ≈ 10 42
• The over all formation constant is having very
high value which means that equilibrium is
lying far too right.
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14. • But when this complex exchanges its CN-
ligands with 14
C labeled CN-
solution very high
rate showing that the complex is labile.
• So the thermodynamic stability is not
connected to the lability or inertness of a
complex.
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15. Explanation of lability and inertness according to VBT
• VBT classifies octahedral complexes into two
types.
• Inner orbital complexes – d2
sp3
• Outer orbital complex – sp3
d2
• The two d-orbitals involved in the hybridization
are the egset of orbitals.
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16. Outer orbital complexes
• The complexes having sp3
d2
hybridization are
called outer orbital complexes.
• In terms of VBT these bonds are weaker.
• They are generally labile.
• Mn(II), Fe(II),Fe(III),Co(II),Ni(II),Cu(II) and Cr(II)
are labile.
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17. Inner orbital complexes
• These complexes generally have d2
sp3
hybridization.
• The hybrid orbitals are filled with the ligand
electrons.
• The t2g orbitals of metal accommodate the d
electrons of the metal.
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18. • If the t2g levels are left vacant then the
complex can associate with an incoming
ligand and the complex is labile
• If all the t2g levels are occupied then the
complex becomes inert.
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19. Labile and inert complexes on the basis of CFT
• According to CFT the ligand field splits the d-
orbitals.
• This splitting leads to a decrease in energy of
the system whose magnitude depends on the
number of d electrons present.
• if the CFSE value increases by association or
dissociation of a ligand then the complex is
labile.
• On the other hand it is inert when there is a
loss in CFSE value. 19santhanam SCSVMV

20. Factors affecting lability of complexes
• Charge of the central ion: Highly charged ions form
complexes which react slowly i.e. inert
• Radii of the ion: the reactivity decreases with
decreasing ionic radii.
• Charge to radius ratio: if all the factors are similar, the
ion with largest z/r value reacts with the least rate.
• Geometry of the complex: Generally four coordinated
complexes are more labile
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21. FACTORS AFFECTING
STABILITY OF THE COMPLEXES

22. Properties of the metal ion
• Charge and size
• Natural order (or) Irving –William order of
stability
• Class a and Class b metals
• Electronegativity of the metal ion
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23. Charge and size of the ion
• In general metal ions with higher charge and
small size form stable complexes.
• A small cation with high charge attracts the
ligands more closely leading to stable
complexes.
• The following tables explain the facts that if
z/r ratio (polarizing power) of the metal ion is
high then stability of the complex is also high
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24. Effect of ionic radius
Complex ion Charge on the
ion
Ionic radii (Aₒ
)
Value of ᵝ stability
[BeII
(OH)] +
+2 0.31 107
[MgII
(OH)] +
+2 0.65 120
[CaII
(OH)] +
+2 0.99 30
[BaII
(OH)] +
+2 1.35 4
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25. Effect of charge
Complex ion Charge on the
ion
Ionic radii (Aₒ
)
Value of log ᵝ stability
[FeIII
(CN)6] 3-
+3 31.0
[FeIII
(CN)6] 4-
+2 8.3
CoIII
complex +3 high
CoII
complex +2 low
Almost
same
Almost
same
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26. Irving – William order of stability
• Stabilities of the high spin complexes of the 3d
metals from Mn2+
to Zn 2+
with a common ligand
is usually
MnMn2+2+
< Fe< Fe2+2+
< Co< Co2+2+
< Ni< Ni2+2+
< Cu< Cu2+2+
> Zn> Zn 2+2+
• This is attributed to the CFSE values of the
complexes and called natural order of
stability.
• There is a discrepancy with Cu which is due to
Jahn – Teller distortion
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27. CFSE as a function of no of d-
electrons
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 1 2 3 4 5 6 7 8 9 10 11
no of d-electrons
CFSEinmultiplesofΔ.
Crystal Field Stabilization Energy (CFSE) of
d0
to d10
M(II) ions:
Ca2+
Mn2+
Zn2+
double-
humped
curve
Ni2+
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28. log K1(EDTA) as a function of no of d-
electrons
10
12
14
16
18
20
0 1 2 3 4 5 6 7 8 9 10 11
no of d-electrons
logK1(EDTA).
Log K1(EDTA) of d0
to d10
M(II) ions:
Ca2+
Mn2+
Zn2+
double-
humped
curve
= CFSE
rising baseline
due to ionic
contraction
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29. log K1(en) as a function of no of d-
electrons
0
2
4
6
8
10
12
0 1 2 3 4 5 6 7 8 9 10 11
no of d-electrons
logK1(en).
Log K1(en) of d0
to d10
M(II) ions:
double-
humped
curve
Ca2+
Mn2+
Zn2+
rising baseline
due to ionic
contraction
= CFSE
29santhanam SCSVMV

30. log K1(tpen) as a function of no of d-
electrons
0
5
10
15
20
0 1 2 3 4 5 6 7 8 9 10 11
no of d-electrons
logK1(tpen).
Log K1(tpen) of d0
to d10
M(II) ions:
Ca2+
Mn2+
Zn2+
double-
humped
curve
N N NN
N Ntpen
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31. Class a and Class b metals
• Chatt and Ahrland classified metals into three
types.
• Class a , Class b and border line.
• Class a : H, alkali and alkaline earth metals, Sc -> Cr,
Al -> Cl, Zn -> Br , In, Sn , Sb , I, lathanides and
actinides
• Class b: Rh ,Pd , Ag , Ir , Pt , Au and Hg
• Border line: Mn -> Cu , Tl -> Po, Mo , Te , Ru , W ,
Re , Os and Cd
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32. • Class a metals form more stable complexes with
ligands in which coordination atoms are from
second period. ( N , O , F)
• Class b metals form more stable complexes with
ligands having third period elements as ligating
atoms. (P , S , Cl)
• Class b metals are having capacity to form pi bonds
with the ligand atoms. The expansion is possible only
from the third period donor atoms.
• Border line metals do not show any noticeable trend.
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33. Electronegativity of the metal
atom
• The bond between metal and ligand atom is,
to some extent due to the donation of
electron pair to the metal.
• If the metal is having a tendency attract the
electron pair (Higher electronegativity) then
more stable complexes are formed .
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34. Properties of ligand
• Size and charge
• Basic character
• Chelate effect
• Size of the chelate ring
• Steric effect
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35. Size and charge of the ligand
• To some extent we can say that if the ligand is
smaller in size and bearing higher charge it
will form more stable complexes.
• For example usually F-
forms more stable
complexes that Cl-
• In the case of neutral mono dentate ligands,
high dipole moment and small size favour
more stable complexes.
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36. Basic character of ligands
• If the ligand is more basic then it will donate
the electron pair more easily.
• So with increased basic character more stable
complexes can be expected.
• Usually the ligands which bind strongly with H+
form more stable complexes.
• This is observed for IA, IIA, 3d, 4f and 5f
elements
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37. The chelate effect or chelation is one of the most important ligand effects inThe chelate effect or chelation is one of the most important ligand effects in
transition metal coordination chemistry.transition metal coordination chemistry.
"The adjective chelate, derived from the great claw or chela (chely - Greek)
of the lobster, is suggested for the groups which function as two units
and fasten to the central atom so as to produce heterocyclic rings."
J. Chem. Soc., 1920, 117, 1456
Ni2+
Chelate
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38. What are the implications of the following results?
NiCl2 + 6H2O → [Ni(H2O)6]+2
[Ni(H2O)6]+2
+ 6NH3 → [Ni(NH3)6]2+
+ 6H2O log β = 8.6
[Ni(NH3)6]2+
+ 3 NH2CH2CH2NH2 (en)
[Ni(en)3]2+
+ 6NH3
log β = 9.7
[Ni(H2O)6]+2
+ 3 NH2CH2CH2NH2 (en)
[Ni(en)3]2+
+ 6H2O
log β = 18.3
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39.  NH3 is a stronger (better) ligand than
H2O
 ∆O NH3 > ∆O H2O
 [Ni(NH3)6]2+
is more stable
 ∆G = ∆H - T∆S (∆H -ve, ∆S≈ 0)
 ∆G for the reaction is negative
Complex Formation: Major Factors
[Ni(H2O)6] + 6NH3
→[Ni(NH3)6]2+
+ 6H2O
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40. Chelate Formation: Major Factors
 en and NH3 have similar N-donor environment
 but en is bidentate and chelating ligand
 rxn proceeds towards right, ∆G negative
 ∆G = ∆H - T∆S (∆H -ve, ∆S ++ve)
 rxn proceeds due to entropy gain
 ∆S ++ve is the major factor behind chelate
effect
[Ni(NH3)6]2+
+ 3 NH2CH2CH2NH2 (en)
[Ni(en)3]2+
+ 6NH3
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41. Cd2+
+ 4 NH3
↔ [Cd(NH3
)4
]2+
Cd2+
+ 2 en ↔ [Cd(en)2
]2+
Chelate Formation: Entropy Gain
Ligands
4 NH3
4 MeNH2
2 en
∆G
kJmol-1
-42.5
-37.2
-60.7
∆H
kJmol-1
- 53.2
-57.3
-56.5
∆S
JK-1
mol-1
- 35.5
- 67.3
+13.8
log β
7.44
6.52
10.62
Cd2+
+ 4 MeNH2
↔ [Cd(MeNH2
)4
]2+
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42. Reaction of ammonia and en with Cu2+
[Cu(H2O)6]2+
+ en → [Cu(en)(H2O)4]2+
+ 2 H2O
Log K1 = 10.6 ∆H = -54 kJ/mol ∆S = 23 J/K/mol
[Cu(H2O)6]2+
+ 2NH3 → [Cu(NH3)2(H2O)2]2+
+ 2 H2O
Log β2 = 7.7 ∆H = -46 kJ/mol ∆S = -8.4 J/K/mol
Chelate Formation: Entropy Gain
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43. Chelate effect
• The stability of the complex of a metal ion with a
bidentate ligand such as en is invariably significantly
greater than the complex of the same ion with two
monodentate ligands of comparable donor ability,
i.e., for example two ammonia molecule.
• The attainment of extra stability by formation of
ring structures , by bi or poly dentate ligands which
include the metal is termed as chelate effect.
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44. Why chelates are more stable?
Suppose we have a metal ion in solution,
and we attach to it a monodentate ligand,
followed by a second monodentate ligand.
These two processes are completely
independent of each other.
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45. Why chelates are more stable?
• But suppose we have a metal ion and we attach to it
one end of a chelating ligand
• Attachment of the second end of the chelate is now
no longer an independent process once one end is
attached, the other end, rather than floating around
freely in solution, is anchored by the linking group in
reasonably close proximity to the metal ion.
• Therefore more likely to join onto it than a
comparable monodentate ligand would be.
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46. HO
OH
O
OSH
SH
(R,S)-2,3-dimercaptosuccinic acid
As, Cu, Pb, Hg
HS OH
SH
M+
S
S
OH
M
Dimercaprol
As
Hg
Au
Pb
D-Penicillamine
Zn
As
Hg
Au
Pb
46santhanam SCSVMV

47. CH2N
CH2
CH2
C
C
CH2 N
CH2
CH2 C
C
O
O
O
O
O O
OO
EDTA
*
* *
*
**
Important Chelating Ligands
47santhanam SCSVMV

48. Anticoagulant
Ca2+
EDTA: another view
48santhanam SCSVMV

49. Macrocylic Ligands
Important Chelating Ligands
49
santhanam SCSVMV

50. [Cu(OH2)4]2+
+ en [Cu(OH2)2(en)]2+
+ 2 H2O
log K1 = 10.6 ΔH = -54 kJ mol-1
ΔS = 23 J K-1
mol-1
[Cu(OH2)4]2+
+ 2 NH3 [Cu(OH2)2(NH3)2]2+
+ 2H2O
log β2 = 7.7 ΔH = -46 kJ mol-1
ΔS = -8.4 J K-1
mol-1
50santhanam SCSVMV

51. number of chelate rings
Metal
complex
No. of
rings
Values of log ᵝ
Mn (II) Fe (II) Co (II) Ni (II) Cu (II) Zn (II) Cd (II)
M (NH3)4 0 - 23.7 5.31 7.79 12.59 9.06 6.92
M (en)2 2 4.9 7.7 10.9 14.5 20.2 11.2 10.3
M (trien) 3 4.9 7.8 11.0 14.1 20.5 12.1 10.0
M (tren) 3 2.8 8.8 12.8 14.0 18.8 14.6 12.3
M (dien)2 4 7.0 10.4 14.1 18.9 21.3 14.4 13.8
M (penten) 5 9.4 11.2 15.8 19.3 22.4 16.2 16.2
51santhanam SCSVMV

52. Chelate ring size - i
In chelates ertain ring sizes are more
preferable than others.
Here are some data for cadmium complexes
of bidentate amines of the type
H2N(CH2)nNH2, where n = 1-4, i.e ring sizes
4-7.
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53. Chelate ring size - ii
• When n = 1, the resulting four-membered ring is too
strained at the sp3
-hybridized carbon which wants to
try to have bond angles of 109°.
• It is worth pointing out, however, that there are lots
of perfectly stable four-membered chelate rings that
contain an sp2
-hybridized carbon in that position,
such as carboxylates (O2CR), dithiocarbamate
(S2CNR2), xanthate (S2COR) and so on
53santhanam SCSVMV

54. Chelate ring size - iii
• When n = 2, the resulting five-membered ring is
obviously the most stable one available, though n = 3
(six-membered ring) isn't bad either.
• When n = 4, the stability of the seven-membered
ring is starting to drop again. This is because in order
to accommodate the longer hydrocarbon chain, the
two nitrogens are being forced too far apart
54santhanam SCSVMV

55. Chelate ring size - iv
• The angle occupied by a chelate ligand, in this case
the N-Cd-N angle, is called the bite angle.
• In an octahedral complex, it's going to be happiest at
90°.
• If we try to force the nitrogens too far apart so that
they have a much bigger bite angle, eventually
something will have to give, and one end of the
ligand will dissociate. Hence the lower stability
constant.
55santhanam SCSVMV

56. Steric factors
• when bulky groups are present near or on the
ligating atom, the steric forces come into play.
• Presence of bulkier groups near coordination
sites reduce the chances of ligand getting
closer to the metal.
• Even when complex is formed, to get relieved
from the steric hindrance the bond may
dissociate. This reduces the stability of
complex
56santhanam SCSVMV

57. EXPERIMENTAL DETERMINATIONEXPERIMENTAL DETERMINATION
OFOF
STABILITY CONSTANTSSTABILITY CONSTANTS
57santhanam SCSVMV

58. Spectrophotometric method
• While formation of a complex a striking colour change
also occurs.
• The absorption obeys Beer – Lambert’s law
– A = ε . C. l
• A can be measured by using a spectrophotometer
• If ε and l are known then C can be calculated.
• Considering the following reaction,
M2+
+ L ML2+
K = [ML2+
] / [M2+
] [L]
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59. It is known that ,
CM = [M2+
] + [ML2+
]
CL = [L] + [ML2+
]
A = ε [ML2+]. C[ML2+] . l
C[ML2+] = A / ε [ML2+].l
So
[M2+
] = CM - (A / ε [ML2+].l)
[L] = CL - (A / ε [ML2+].l) 59santhanam SCSVMV

60. • A series of solutions containing varying ratios
of metal and ligand are taken.
• The absorption of the solution at wavelength
maximum is measured.
• From the absorbance and C,l values K is
calculated.
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61. Potentiometric method
• Also known as Bjerrum method
• When ligand is a weak base or acid, there is
competition between hydrogen ions and
metal ions for the ligand .
L + H+
 HL+
Ka = [HL+
] / [L] [H+
]
L + M+
 ML+
KF = [ML+
] / [L] [M+
]
• If CH,CM and CL are the molar concentrations
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62. CH = [H+
] + [HL+
]
CL = [L] + [ML+
] + [HL+
]
CM = [M+
] + [ML+
]
• Solving the equations by using the association
constant of the ligand
[ML+
] = CL-CH+[H+] – CH-[H+
] / Ka [H+
]
[M+
] = CM – [ML+
]
[L] = CH – [H+
] / Ka [H+
]
santhanam SCSVMV 62

63. • Except [H+
] all the other parameters are
known , hence the stability constant can be
calculated after measuring the pH of the
solution by using a pH meter
• In order to get precise results the ligand must
be a moderately weak base or acid.
• KF value should be within 105
times of the
association constant
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64. santhanam SCSVMV 64