Unit-1.4.
BY
Dr. K. Agilandeswari
Assistant Professor
Department of Chemistry
PSGCAS
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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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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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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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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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Stability constant / Formation constant
According to Bjerrum formation of a complex in
•aqueous solution proceeds through a stepwise fashion
with corresponding equilibrium constants
K1 = [ML] / [M] [L]
M+L K1 ML
K2= [ML2] / [ML] [L]
ML + L K2 ML2
K3= [ML2] / [ML2] [L]
ML2 + L K3 ML3
…………..……………………………….
………….………………………………..
K
MLn-1 + L MLn n
Kn= [MLn] / [MLn-1] [L]
These K1,K2 K3 … Kn are constants Called formation
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constant 6
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Overall stability constant
• If the complex formation is considered as a single
step process
M + nL MLn
ᵝ n = [ML ] / [M] [L]
n
n
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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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STABILITY
• Discussed under two main types:-
1. Thermodynamic Stability
2. Kinetic Stability
Generally we talk about THERMODYNAMIC
STABILITY, unless KINETIC or RATE OF
FORMATION word is mentioned.
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POINTS OF DIFFERENCE
• Thermodynamic Stability
Relates to the time period of
existence of a species in a particular form.
• Kinetic Stability
Refers to the faster rate of formation of the
particular species.
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STABILITY CONSTANTS
• Higher the value of equilibrium constant for a
reaction, more stable is the product formed.
TYPES:
1. Stepwise stability constant
2. Overall stability constant
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FORMATION OF MLn COMPLEX
• Two different ways:
1. Stepwise formation
M + L------> ML ; K1 = [ML]/[M][L]
ML + L ----> ML2 ; K2 = [ML2]/[ML][L]
.
.
MLn-1 + L -----> MLn ; Kn = [MLn]/ [MLn-1][L]
• K1, K2, Kn are known as stepwise stability constants.
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2. Overall formation:
M + L------> ML ; β1 = [ML]/[M][L]
M + 2L ----> ML2 ; β2 = [ML2]/[M][L]2
.
.
M + nL -----> MLn ; βn = [MLn]/[M][L]n
• β1, β2, βn are known as overall stability
constants.
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• Proceeding with rate constants, we get,
β1 = K 1
β 2 = K 1 . K2
βn = K1 . K2 . K3 ………. Kn
Taking log, we get,
log βn = log K1 + log K2 + log K3 +……. log Kn.
• Complex is stable if log β >= 8.
https://www.youtube.com/watch?v=9K2ldhQYmsA
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Relationship between Kn and ᵝ n
• 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
1 2 3
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n
11
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INERTNESS & LABILITY OF COMPLEXES
• Lability refers to reactivity and Inertness refers
to non- reactivity of the complex.
• In other words, complex with ability to
exchange its ligand(s) present inside co-
ordination sphere is labile otherwise inert.
• Inert complexes have substitution reaction half
life period larger than a minute & opposite for
labile complexes.
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Factors affecting stability 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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FACTORS AFFECTING STABILITY OF
COMPLEXES
Nature of the central metal ion
Nature of the ligand
Chelating effect
Macrocyclic effect
Resonance effect
Steric effect or steric hindrance
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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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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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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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Effect of charge
Complex ion Charge on the
ion
Ionic radii (Aₒ)
Value of log ᵝ stability
[FeIII(CN)6] 3- +3 31.0
Almost
[FeIII(CN)6] 4- +2 same 8.3
CoIII complex +3 high
Almost
CoII complex +2 same low
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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
Mn2+< Fe2+< Co2+< Ni2+< Cu2+> Zn 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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Crystal Field Stabilization Energy (CFSE) of d0 to d10
M(II) ions:
CFSE as a function of no of d-
electrons
1.4
CFSE in multiples of
1.2 Ni2+
1 double-
0.8 humped
curve
0.6
0.4
0.2
0
0 1Ca2+2 3 4 5 6Mn2+7 8 Z9n2+ 10 11
no of d-electrons
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Log K1(EDTA) of d0 to d10 M(II) ions:
log K1(EDTA) as a function of no of d-
electrons
= CFSE
20
double-
18
logK1(EDTA.)
humped
curve
16
Zn2+
14
Mn2+ rising baseline
12
due to ionic
10 Ca2+ contraction
0 1 2 3 4 5 6 7 8 9 10 11
no of d-electrons
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Log K1(en) of d0 to d10 M(II) ions:
log K1(en) as a function of no of d-
electrons
= CFSE
12
10 double-
humped
logK1(en).
8 curve
6
Zn2+
4
rising baseline
2 Ca2+ Mn 2+
due to ionic
contraction
0
0 1 2 3 4 5 6 7 8 9 10 11
no of d-electrons
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➢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. Dr.KA
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 Dr.KA
Electronegativity of the metal atom
• The bond between metal and ligand atom, 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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FACTORS AFFECTING STABILITY OF
COMPLEXES
1. Nature of Metal Ion:
• +ve Charge on metal ion ↑, stability of complex↑
• Size of metal ion ↓, stability of complex ↑
• Charge:size ratio ↑, stability of complex ↑
• more electronegative metal ions form complexes
with ligand having donor atom of high
electronegativity & vice versa.
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2. Nature of ligand:
• -ve charge on ligand ↑, stability of complex ↑
• Size of ligand ↓, stability of complex ↑
• Basic strength of ligand ↑, stability of complex↑
• Presence of chelating ligand makes the complex
more stable.
• Ligand bulkiness↓ , stability of complex↑,
because of less steric hindrance.
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Properties of ligand
• Size and charge
• Basic character
• Chelate effect
• Size of the chelate ring
• Steric effect
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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. Dr.KA
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
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Chelate
"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+
The chelate effect or chelation is one of the most important ligand effects in
transition metal coordination chemistry.
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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 = 8.6
[Ni(H2O)6]+2 + 3 NH2CH2CH2NH2 (en) log = 18.3
[Ni(en)3]2+ + 6H2O
[Ni(NH3)6]2+ + 3 NH2CH2CH2NH2 (en) log = 9.7
[Ni(en)3]2+ + 6NH3
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Complex Formation: Major Factors
[Ni(H2O)6] + 6NH3
→[Ni(NH3)6]2+ + 6H2O
NH3 is a stronger (better) ligand than H2O
O NH3 > O H2O
[Ni(NH3)6]2+ is more stable
G = H - TS (H -ve, S 0)
G for the negative
Chelate Formation: Major Factors
[Ni(NH3)6]2+ + 3 NH2CH2CH2NH2 (en)
[Ni(en)3]2+ + 6NH3
en and NH3 have similar N-donor environment
but en is bidentate and chelating ligand
rxn proceeds towards right, G negative
G = H - TS (H -ve, S ++ve)
rxn proceeds due to entropy gain
S ++ve is the major factor behind chelate
effect
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Chelate Formation: Entropy Gain
Cd2+ + 4 NH3 [Cd(NH3)4]2+ Cd2+ + 4 MeNH2 [Cd(MeNH2)4]2+
Cd2+ + 2 en [Cd(en)2]2+
Ligands log G H S
kJmol-1 kJmol-1 JK-1mol-1
4 NH3 7.44 -42.5 - 53.2 - 35.5
4 MeNH2 6.52 -37.2 -57.3 - 67.3
2 en 10.62 -60.7 -56.5 +13.8
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Chelate Formation: Entropy Gain
Reaction of ammonia and en with Cu2+
[Cu(H2O)6]2+ + 2NH3 → [Cu(NH3)2(H2O)2]2+ + 2 H2O
Log 2 = 7.7 H = -46 kJ/mol S = -8.4 J/K/mol
[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
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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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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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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.
SH O
HO
OH
O SH Zn
As
(R,S)-2,3-dimercaptosuccinic acid D-Penicillamine Hg
Au
As, Cu, Pb, Hg Pb
SH S
M+
As
M Hg
OH Au
HS OH S Pb
Dimercaprol
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Important Chelating Ligands
O EDTA O
*O C CH2 CH2 C O*
N* CH2 CH2 N*
*O C CH2 CH2 C O*
O O
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EDTA: another view
Ca2+
Anticoagulant
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Number of chelate rings
Values of log ᵝ
Metal No. of
complex rings
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
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Chelate ring size - I
➢ In chelates certain 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.
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Triethylenetetramine- Trien
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Important Chelating Ligands
Macrocylic Ligands
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The macrocyclic effect follows the same principle as the
chelate effect, but the effect is further enhanced by the cyclic
conformation of the ligand.
Macrocyclic ligands are not only multi-dentate, but because
they are covalently constrained to their cyclic form, they allow
less conformational freedom. The ligand is said to be "pre-
organized" for binding, and there is little entropy penalty for
wrapping it around the metal ion.
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For example heme b is a tetradentate cyclic ligand which is
strongly complexes transition metal ions, including (in
biological systems) Fe+2.
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Crown ethers such as 18-crown-6 (below, center right) are cyclic hard bases that
can complex alkali metal cations. Crowns can selectively bind Li+, Na+, or
K+ depending on the number of ethylene oxide units in the ring.
The chelating properties of crown ethers are mimetic of the natural
antibiotic valinomycin (below right), which selectively transports K+ ions across
bacterial cell membranes, killing the bacterium by dissipating its membrane potential.
Like crown ethers, valinomycin is a cyclic hard base.
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https://www.youtube.com/watch?v=73dw6w0zNXA
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https://www.youtube.com/watch?v=IIu16dy3ThI
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https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_General_Che
mistry_(Petrucci_et_al.)/24%3A_Complex_Ions_and_Coordination_Compounds/24.0
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7%3A_Color_and_the_Colors_of_Complexes
https://www.youtube.com/watch?v=vwY-xWMam7o
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https://www.youtube.com/watch?v=WhyMOVvAu3s&t=460s
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https://qd-uki.co.uk/squid-enables-highly-accurate-study-of-magnetic-materials/
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THANK YOU
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