Thermodynamic and Kinetic aspects of metal complexes.

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 ligandexchange is slow then
the complex is said to be inert.
Labilityis 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. Liganddisplacements are nucleophilic
substitution reactions.
• Rate is governed by ligand
nucleophilicity
The rate of attack on a complex by a given
ligandrelative 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:reactionintermediateisformed
byincludingtheincomingligandinthe
coordinationsphereandhashigher
coordinationnumberthanreactantsorproducts
•Lowercoordinationnumbercomplexes•
Ratesdependontheenteringgroup

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
HClfor many hours, one NH3 is
substituted by Cl-.
[Co(NH3)6]3+ + HCl
[Co(NH3)5Cl]2+ + NH4 +

21. Factors Affecting Lability
complexes
SizeofthecentralmetalionSmallerthesizeofthe
metalion,greaterwillbetheinertnessbecausethe
ligandsareheldtightlybythemetalion.Charge
onthecentralmetalionGreaterthechargeonthe
metalion,greaterwillbetheinertnessofthe
complex.SincetheM-Lbondsarestronger.

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
•Asforascomplexesinsolutionsare
concernedtherearetwokindsofstabilities
•Thermodynamicstability–Measureof
theextenttowhichthecomplexwillbe
formedorwillbetransformedintoanother
species,whenthesystemhasreached
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. LabileandInertcomplexes
•Thecomplexeswhichrapidlyexchange
theirligandswithotherspeciesarecalled
labile.
•Iftheligandexchangereactionrateisslow
thentheyarecalledinertcomplexes.
•Butthereactivenatureshouldnotbe
confusedwiththestability.

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. FACTORSAFFECTING STABILITYOFTHE
COMPLEXES
Propertiesofthemetalion
•Chargeandsize
•Naturalorder(or)Irving–Williamorderofstability•
ClassaandClassbmetals
•Electronegativityofthemetalion

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. Classametalsformmorestablecomplexeswith
ligandsinwhichcoordinationatomsarefrom
secondperiod.(N,O,F)
•Classbmetalsformmorestablecomplexeswith
ligandshavingthirdperiodelementsasligating
atoms.(P,S,Cl)•Classbmetalsarehaving
capacitytoformpibondswiththeligandatoms.
Theexpansionispossibleonlyfromthethird
perioddonoratoms.
•Borderlinemetalsdonotshowanynoticeable
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. Chelateeffect
•Thestabilityofthecomplexofametalionwitha
bidentateligandsuchasenisinvariablysignificantly
greaterthanthecomplexofthesameionwithtwo
monodentateligandsofcomparabledonorability,i.e.,
forexampletwoammoniamolecule.
•Theattainmentofextrastabilitybyformationof
ringstructures,bybiorpolydentateligandswhich
includethemetalistermedaschelateeffect.

64. Whychelatesaremorestable?Supposewe
haveametalioninsolution,andweattachtoit
amonodentateligand,followedbyasecond
monodentateligand.Thesetwoprocessesare
completelyindependentofeachother

65. Whychelatesaremorestable?
•supposewehaveametalionandweattach
toitoneendofachelatingligand
•Attachmentofthesecondendofthechelate
isnownolongeranindependentprocess
onceoneendisattached,theotherend,
ratherthanfloatingaroundfreelyinsolution,
isanchoredbythelinkinggroupin
reasonablycloseproximitytothemetalion.
•Thereforemorelikelytojoinontoitthana
comparablemonodentateligandwouldbe.

72. Stericfactors
•whenbulkygroupsarepresentnearor
ontheligatingatom,thestericforces
comeintoplay.
•Presenceofbulkiergroupsnear
coordinationsitesreducethechancesof
ligandgettingclosertothemetal.
•Evenwhencomplexisformed,toget
relievedfromthesterichindrancethe
bondmaydissociate.Thisreducesthe
stabilityofcomplex

74. Determination of Stability
Constants
Threemethodswereusedtodeterminethe
stoichiometryofthecomplex,themoleratio,
continuousvariationandthesloperatiomethod.
Moleratiomethod:-
Thecontinuousvariationmethod:-
Slope-ratiomethod:-

75. Mole ratio method
The method was described by Yoe and Jones ( ' and it is applied
as fojlows:-
Inaseriesof100cm3separatoryfunnels,5cm3aliquotofthe
metalsolution(XM)werecompletedto11cm3withtheaddition
of6cm3ofpH8bufferandthenextractedwithvarying
amountsofoximesolution(XM)inamylalcohol.Theorganic
layerweretransferedtovolumetricflasks(25cm3)anddiluted
tothemarkwithamylalcohol.

76. Theabsorbancesweremeasuredagainst
solutionofligandasablankinlcmcellsatX
nm.andplottedagainstthemoleratioof
Ligand/Metal.Thecorrespondingpointatthe
molarratioaxistointersectingpointgives
directlytheligandtometalionratio.

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.
M
n+
+ X [MXn]
n+
Kf =
[MXn]
n+
( M
n+
) x (L)
x

80. Theresultingfromcontinuousvariationmethodwere
plottedagainstmolefractionofthemetal.Atriangular
shapedcurvewasobtained.Theratioofthe
stoichiometrywasdeterminedfromthemolefractionat
thepointofintersectionformedbytheextrapolation
ofthelegsofthetriangle

88. Method of Continuous Variations
Themethodofcontinuousvariations,alsocalled
Job’smethod,isusedtodeterminethestoichiometry
ofametal-ligandcomplex.Inthismethodwe
prepareaseriesofsolutionssuchthatthetotalmoles
ofmetalandligand,n
total,ineachsolutionisthe
same.If(n
M)
iand(n
L)
iare,respectively,themolesof
metalandligandinsolutioni,then
n
total=(n
M)
i+(n
L)
i

89. The relative amount of ligand and metal in each
solution is expressed as the mole fraction of ligand,
(X
L)
i, and the mole fraction of metal, (X
M)
i,
X
L)
i=(n
L)
i/n
total
(X
M)
i= 1 –(n
L)
i/n
total=(n
M)
i/n
total
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. Ifwemonitorthecomplexationreactionatawavelength
wherethemetal–ligandcomplexabsorbsonly,agraphof
absorbanceversusthemolefractionofligandwillhave
twolinearbranches—onewhentheligandisthelimiting
reagentandasecondwhenthemetalisthelimiting
reagent.Theintersectionofthesetwobranches
representsastoichiometricmixingofthemetalandthe
ligand.Wecanusethemolefractionofligandatthe
intersectiontodeterminethevalueofyforthemetal–
ligandcomplexML
y.
y=(n
L/n
M)=(X
L/X
M)=(X
L/1–X
M)
Theillustrationbelowshowsacontinuousvariationsplot
forthemetal–ligandcomplexbetweenFe
2+
ando-
phenanthroline.Asshownhere,themetalandligand
formthe1:3complexFe(o-phenanthroline)
3
2+
.

91. THANK
YOU