Dr. s. H. Burungale

1. STABILITY OF
COORDINATION
COMPOUNDS
Dr. S. H. BURUNGALE

2. LABILITY and INERTNESS
Complexes in which exchange of one or more
ligands are rapidly exchanged are called labile
complexes.
If the rate of ligand exchange is slow then
the complex is said to be inert.
Lability is not related to the thermodynamic
stability of a complex.
A stable complex may be labile or inert , so
as the unstable complex

3. TYPES of REACTION of
COMPLEXES
Substitution of ligands
Solvolysis
Anation
Reactions of coordinated ligands
Racemization
Electron transfer reactions
Photo chemical reactions

4. Ligand displacements are nucleophilic
substitution reactions.
• Rate is governed by ligand
nucleophilicity
The rate of attack on a complex by a given
ligand relative to the rate of attack by a
reference base.

5. Three types of ligands are present – Entering
Ligand: Y – Leaving Ligand: X – Spectator Ligand
• Species that neither enters nor leaves •
Particularly important when located in a Trans
position, designated T
Dissociative: One of the ligands dissociates from
the reactant, to form a reaction intermediate with
lower coordination number than reactants or
products • Octahedral complexes and smaller metal
centers • Rates depend on leaving group

7. Dissociative: One of the ligands dissociates
from the reactant, to form a reaction
intermediate with lower coordination number
than reactants or products • Octahedral
complexes and smaller metal centers • Rates
depend on leaving group

13. [Cu(NH3)4(H2O)2]2+ is labile. Its aqueous solution
is blue in color. When concentrated hydrochloric
acid is added to this solution, the blue solution
immediately turns green ,giving [CuCl4]2- . But
when the complex is kept as such it remains as such
with out any decomposition (i.e stable)

14. Associative: reaction intermediate is formed
by including the incoming ligand in the
coordination sphere and has higher
coordination number than reactants or products
• Lower coordination number complexes •
Rates depend on the entering group

17. Interchange Mechanism
It is a continuous single step process
Two types exist
Interchange associative (IA ) – Bond
making more important
Interchange dissociative (ID) – Bond
breaking more important

20. INERT AND UNSTABLE COMPLEX
[Co(NH3)6]3+ reacts slowly.
When this complex is treated with
concentrated HCl, no reaction takes
place. Only when it is heated with 6M
HCl for many hours, one NH3 is
substituted by Cl- .
[Co(NH3)6]3+ + HCl
[Co(NH3)5Cl]2+ + NH4 +

21. Factors Affecting Lability
complexes
Size of the central metal ion Smaller the size of the
metal ion, greater will be the inertness because the
ligands are held tightly by the metal ion. Charge
on the central metal ion Greater the charge on the
metal ion, greater will be the inertness of the
complex. Since the M-L bonds are stronger.

22. d-electron configuration If electrons are
present in the antibonding eg * orbitals, the
complex will be labile -the ligands will be
weakly bonded to the metal and hence can be
substituted easily. Complexes with empty t2g
orbitals, will be labile because ligands can
approach easily without much repulsion. In
short, if the complex contains less than three d-
electrons, it will be labile. Or, if one or more eg
* electrons are present, it will be labile

23. No. of d electrons & electron configuration Nature Example d0
Labile [CaEDTA]2- d1 ; t2g 1 eg 0 Labile [Ti(H2O)6]3+ d2 ;
t2g 2 eg 0 Labile [V(phen)3]3+ d3 ; t2g 3 eg 0 Inert
[V(H2O)6] 3+ d4 (high-spin); t2g 3 eg 1 Labile [Cr(H2O)6]3+
d4 (low-spin); t2g 4 eg 0 Inert [Cr(CN)6]4- d5 (high-spin); t2g
3 eg 2 Labile [Mn(H2O)6]2+ d5 (low-spin); t2g 5 eg 0 Inert
[Mn(CN)6]4- d6 (high-spin); t2g 4 eg 2 Inert [Mn(H2O)6]2+
d6 (low-spin); t2g 6 eg 0 Inert [Fe(CN)6]4- d7 , d8 , d9 , d10
Labile

29. 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

30. Stability of coordination compounds:
Thermodynamic equilibrium constant.
stability depend upon the interaction between
metal and ligand.
If interaction strong thermodynamic stability
strong.
Reaction between metal ion and ligand is
based on lewis acid and lewis base.
The greater the value of stability constant
more stable is the complex.

31. Stability of complex:
There are two types of stability of complex
1. Thermodynamic stability
2 kinetic stability
THERMODYNAMIC STABILITY: •
Thermodynamic stability so called stability of
the complex.
• Thermodynamically complexes divided into two
types 1. Stable complex. 2. Unstable complex.

32. 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

33. 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

34. 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.

37. 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

39. 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

41. 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.
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.
• 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.

42. 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.

43. 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

44. FACTORS AFFECTING STABILITY OF THE
COMPLEXES
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

45. 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

53. 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

54. 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.

55. 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

56. Properties of ligand • Size and charge
• Basic character • Chelate effect
• Size of the chelate ring • Steric effect
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

57. 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

63. 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.

64. 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

65. Why chelates are more stable?
• 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.

72. 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

74. Determination of Stability
Constants
Three methods were used to determine the
stoichiometry of the complex , the mole ratio,
continuous variation and the slope ratio method.
Mole ratio method :-
The continuous variation method :-
Slope - ratio method :-

75. Mole ratio method
The method was described by Yoe and Jones ( ' and it is applied
as fojlows:-
In a series of 100 cm3 separatory funnels, 5 cm3 aliquot of the
metal solution (XM) were completed to 11 cm3 with the addition
of 6 cm3 of pH 8 buffer and then extracted with varying
amounts of oxime solution (XM) in amyl alcohol. The organic
layer were transfered to volumetric flasks (25 cm3) and diluted
to the mark with amyl alcohol.

76. The absorbances were measured against
solution of ligand as a blank in lcm cells at X
nm. and plotted against the mole ratio of
Ligand/Metal. The corresponding point at the
molar ratio axis to intersecting point gives
directly the ligand to metal ion ratio.

77. The continuous variation
method
The modification of the Job's continuous variation
method performed by. Vesburgh and Cooper was
applied to find the stoichiometry and formation
constant, stability constant, of the complex formed
between Ligand and Metal ion.

78. Determination of the formation constant:-
Determination of the formation constant of the complex can be determined
by job,s variation method.
Mn+ + X [MXn]n+
Kf =
[MXn]n+
( Mn+ ) x (L)x

80. The resulting from continuous variation method were
plotted against mole fraction of the metal. A triangular
shaped curve was obtained. The ratio of the
stoichiometry was determined from the mole fraction at
the point of intersection formed by the extrapolation
of the legs of the triangle

88. Method of Continuous Variations
The method of continuous variations, also called
Job’s method, is used to determine the stoichiometry
of a metal-ligand complex. In this method we
prepare a series of solutions such that the total moles
of metal and ligand, ntotal, in each solution is the
same. If (nM)i and (nL)i are, respectively, the moles of
metal and ligand in solution i, then
ntotal = (nM)i + (nL)i

89. The relative amount of ligand and metal in each
solution is expressed as the mole fraction of ligand,
(XL)i, and the mole fraction of metal, (XM)i,
XL)i = (nL)i/ntotal
(XM)i = 1 – (nL)i/ntotal = (nM)i/ntotal
The concentration of the metal–ligand complex in any
solution is determined by the limiting reagent, with the
greatest concentration occurring when the metal and the
ligand are mixed stoichiometrically.

90. If we monitor the complexation reaction at a wavelength
where the metal–ligand complex absorbs only, a graph of
absorbance versus the mole fraction of ligand will have
two linear branches—one when the ligand is the limiting
reagent and a second when the metal is the limiting
reagent. The intersection of these two branches
represents a stoichiometric mixing of the metal and the
ligand. We can use the mole fraction of ligand at the
intersection to determine the value of y for the metal–
ligand complex MLy.
y = (nL/nM) = (XL/XM) = (XL/1 –XM)
The illustration below shows a continuous variations plot
for the metal–ligand complex between Fe2+ and o-
phenanthroline. As shown here, the metal and ligand
form the 1:3 complex Fe(o-phenanthroline)3
2+.

91. THANK
YOU