Kinetic Stability

Deals with rate and mechanism of chemical reactions.

Thermodynamic stability

It refers to equilibrium constants and deals with bond energies ,stability constants ,redox potentials etc.

•thermodynamic stability is the measure of extent of formation of a complex at equilibrium.

•If the intraction between metal ion and ligand is strong the complex formed is thermodynamicly more stable. It can be expressed in terms of equilibrium Constant ,Ks (stability constant)

#Reciprocal of stability constant is called dissociation Constant.

Stability Constants of Complexes

Consider, M + nL ⇆ MLn

Reaction can proceed as:

M + L ⇋ ML ,K1= [ML]/[M][L]

ML + L ⇋ ML2 , K2= [ML2]/[ML][L]

Similarly MLn is formed as

ML2 + L ⇋ ML3 ,K3= [ML3]/[ML2][L]

. . .
. . .

MLn-1 + L ⇋ MLn ,Kn=[MLn]/[MLn-1][L]

There will be ‘n’ such equilibrium where ‘n’ repersents maximum Coordination no. Of the metal ion for the Ligands ‘L’.

•Equilibrium Const. K1,K2 ,K3,……,Kn are known as Stepwise Stability Constants.

K1> K2> K3>……>Kn

There is another way of expressing Reaction as Overall Stability Const. Or Commutative Stability Constant.

M + nL ⇋ MLn

M + L ⇋ ML ,β1= [ML]/[M][L]

M + 2 L ⇋ ML22= [ML2]/[M][L]2

M + 3 L ⇋ ML33= [ML3]/[M][L]3

M + nL ⇋ MLn n= [MLn]/[M][L]n

Relation between K & β

Let us consider β3 , multiply and devide by [ML][ML2]

relation between K and B

β3= K1K2K3

In general βn = K1.K2.K3……Kn


Logβ= logk1 +logk2 + logk3 +…….+logKn

¶ Complex is regarded as stable complex if its logβ is more than 8.

Greater is the value of stability Constant ,greater will be the stability of the complex ion.

Complex Stability constant(K) Stability order
[Cu(NH3)4]2+ 1×1012 4
[Co(NH3)6]2+ 1.6×1035 1
[Ag(NH3)2]+ 1.67×107 5
[Ag(CN)2] 5.4×1018 2
[Ag(S2O3)2]3+ 1.6×1013 3

The ability of complex to permit quick exchange of its one or more ligands in its coordination Sphere by other ligands is called lability of the Complex

Complexes for which such Substitution reaction are rapid are called labile ; whereas those complexes for which such substitution reactions proceed slowly or do not proceed are called Inert.

A + B → AB + C → AC

If stability constant for A + B → AB is large than AB is thermodynamiclly stable .but if in presence of another C , AB + C → AC is fast reaction then AB is kinetically unstable.


Ni2+ + 4CN → [Ni(CN)4]2- β=1022

This complex is thermodynamically very stable .however the rate of exchange of CN ligands with 14CN is very fast and therefore kinetically complex is labile.

[Ni(CN)4]2- + 14CN → [Ni(CN)314CN]2- + CN(fast,aqueous medium)

•Inert Complexes are those whose Substitution reaction have half life periods more than a minute.

[Co(NH3)5H2O]3+ ⇆[Co(NH3)5(CN)]3+

Half life ~ 7min ,which shows inertness of complex hence [Co(NH3)5H2O]3+ is Stable.

Another example ,[Co(NH3)6]3+ is thermodynamiclly unstable but kinetically it is inert ,remains undecomposed even over a period of several days in aq. Medium i.e. NH3 is not replaced by H2O ligand.

Some Generalisation of stability of coordination complexes

•Metal Complexes without extra stability (such as chelating, LFSE ) are labile.

•Very Small ions are less Labile ,because of strong M-L bond and steric effect.

•All Complexes of s-block (except Be2+ & Mg2+) are labile.

•Complexes of M3+ F-block are very labile.

•Complexes of d10 ions (Zn2+, Cd2+, Hg2+) are normally labile.

•Across d-series M2+ are generally labile with distortion, Cu2+ most labile.

•Across d-series M3+ are distinctly less labile than M2+.

•Transition complexes with electronic Configuration d3 and d6 (Cr3+ , Co3+) ,are non-labile due to large LFSE.

•4d,5d metal complexes are usually nonlabile because of Large LFSE and Strong M-L bonding.

Nucleophilicity :it is defined as Rate of Attack on a complex by a lewis base relative to the rate of attack by a reference lewis base.

•Incoming ligand L has greater effect on rate (Keq) is used to Rank the ligands in order of their Strength in the space.

Factor affecting the Stability of Complexes

a)Nature of Central Metal ion

Smaller the size and larger the charge on metal ion, more stable are the Complexes i.e. Larger the charge/radius ratio of ion.

A smaller ,more highly charged ions allows Closer approach of the ligands and greater force of attraction results into stable complex.

Fe3+ + 6CN ⇆[Fe(CN)6]3- logβ=31 (More Stable)

Fe2+ + 6CN ⇆ [Fe(CN)6]4- logβ=8.3 (Less Stable)

•As the size of metal ion decreases ,the stability of Complex increases.

All ions have Same Charge but size decreases from Mn2+ to Cu2+
Ion Mn2+ Fe2+ Co2+ Ni2+ Cu2+ Zn2+
Ioni Radius(pm) 91 83 82 78 69 74
therefore order of stabitity of complex is Mn2+< Fe2+< Co2+ < Ni 2+ < Cu 2+ < Zn 2+

•thus Smaller the ion more is Stability.

Example :

Complex BeOH+ MgOH+ CaOH+ BaOH+
Stability Constant 107 120 30 4

•Most Stable Complexes are formed with metal ion having high Charge Density.

Electronegativity :

A Strong electron attracting central ion i.e one with high electronagetivity ,would form stable Complexes bcoz bonding between M and L is due to electron donation from L→M , hence strong electron attracting central ion will give stable Complex.

•Certain metal ion Can Also exert a more Significant Polarizing effect on the ligand Causing an increase in bond Stability.

•Metal ion Such as Li+, Mg2+ ,Al3+ etc which attract electron weakly ,from Stable Complexes with those ligands having Slightly electronagetive atoms such as O,N,F.

•On other Hand ,Heavy Metal Such as Ag , Pd, Pt, Au,Hg, Pb etc. Form stable complexes with those ligands containing P,S,As,Br,I which can accept electron from metal to form π-bonds.

Example : Ag form Insoluble halide Salt AgX and Stable halide complex ion AgX2 in which order of stability is I> Br> Cl> F

Complex AgF AgCl AgBr AgI
logβ 0.3 3.2 4.5 8.0

Similarly Hg2+ ,Pb2+, Cd2+ ,Bi3+ metal ion and other electronagetive metal ions form water insoluble sulphide Salts.These Salts are very stable and their formation indicates that these metals prefer sulphur Containing ligands due to Considerable Covalent character in M-Sulphur bonds.

b) Temperature & Pressure

•Volatile ligands may be lost at higher temp. This is examplified by the loss of water by hydrates and ammonia.

Example : [Co(NH3)6]Cl3 (∆175-180℃) → [Co(NH3)5Cl]Cl2 + NH3

•Transformation of certain Coordination Compounds from one to another

AgHg[AgI4] (red)(45℃)⇆ Ag2[HgI4] (yellow)

c)Nature of Ligand

i)Size & charge of ligand

:higher the charge and smaller the size ,more stable is the Complex formed.

Thus F form more Stable Complexes than Cl , Br ,I

FeF2+ logβ= 6.0 , FeCl2+ logβ = 1.3

similarly O2- form more stable Complexes then large S2- ion ,provided metal ion is not a heavy metal like Pt,Pd ,Ag ,Au etc.

ii)Basic Character

:formation of metal ligand bond involves the donation of electron pair from the ligands to the empty orbitals of metal ion.

•Greater the Ease of donation of electron pair by the ligand, greater will be the stability of the complexes formed by it.

In other words ,Greater the Basic strength of the ligand greater will be its tendency to form a metal complex.

Thus CN ,F, NH3 ,Which are strong bases are good ligands to form stable complexes.

iii)Steric Effects :

Because of Stearic Strain ,ligands containing large bulky groups forms less stable Complexes than do analogous Smaller ligands.

•this is due to the fact that the bulky groups present near a donor atom cause mutual repulsion among the ligands making M-L bonds to be weak and reducing Stability of the Complex.

Example :

•H2NCH2CH2NH2 forms more stable complex than its Substituted derivative (CH3)2NCH2CH2N(CH3)2

•Ni2+ ion form more stable complex with 8-hydroxyquinoline then its Substituted derivative 2methyl-8-hydroxyquinoline.

iv) Back Bonding effect

Ligands which have back bonding ability ,form quite stable Complexes ,CN ,CO,NO2 ,C2H4 etc forms stronger complexes with the transition metals.

v) Role of Solvent:

Selected Solvent must be a weaker ligand than the ligand whose complex is to be synthesised .therefore syntheses of halo Complexes such as [NiCl4]2- ,[CoCl4]2- are carried out in non-aqueous Solvent like alcohol.

d)Chelate Effect

Complexes formed by Chelating ligands are more Stable than those formed by monodentate ligands.

This Enhanced Stability of Complexes Containing Chelating ligands is Called Chelate Effect.

M stands for: Mn2+ Fe2+ Co2+ Ni2+ Cu2+
LogK of [M(NH3)4(H2O)2]2+ _ 3.7 5.3 7.8 12.6
logk of [M(en)2(H2O)2]2+ 4.9 7.7 10.9 14.5 20.2
Logk of [M(trien)(H2O)2]2+ 5.8 8.8 12.8 14.7 20.6

¶ [Cd(H2O)4]2+ + 4CH3NH2 ⇆Cd(CH3NH2)4]2+ K=3×106

[Cd(H2O)4]2+ + 2(en) ⇆Cd(en)2]2+ +4H2O K=3.2×1010

#Chelate Effect Can be easily understood on the basis of favorable (+ve) entropy change during chelation.

When monodentate ligands such as NH3 ,H2O are replaced by chelating rings then overall entropy of Solution increases due to more free ions. i.e. ∆S is +ve and ∆G has more -ve value.

◆There are Several Cases where ∆H° for formation of Chelate is +ve but still the Chelate is stable .it is so because ∆S° of such Chelates is so large and +ve that ∆G° = ∆H° -T∆S° still remain Negative.

Example :

[Ni(en)2(H2O)2]2+ + N(CH2CH2NH2)3 ⇆ [Ni(N(CH2CH2NH2)3)(H2O)2] + 2en

∆H° = +13kj , T∆S°=23.7kJ ,So that ∆G° = -10.7KJ and complex is stable

Stability in terms of Nearness

When one ligand get attached to metal ion then other end Cannot go far away and probability of it getting attached to metal ion is large.which makes it in better position to attack the metal ion than a free unidentate ligand moving in the solution at random.

Number of Chelate Rings

Larger the number of Chelate ring in a complex ,the greater is the stability of Complex.

Complex No. Of Chelate Rings logβ
[Cu(NH3)4]2+ 0 12.6
[Cu(en)2]2+ 2 20.0
[Cu(trien)]2+ 3 20.5

Trien= triethylenetetraamine: H2N(CH2)2NH(CH2)2(NH)(CH2)2NH2

structure of trien ,Chelate of triethylenetetraamine

Is most stable of given above due to larger no. Of Chelate rings.

Size Of Chelate Rings

Metal Chelates having 5 membered rings are most stable. Chelates having 6 membered rings are slightly less stable in case of Single bonds.

Example: en ,oxalate ion ,glycine ,dMG ,form quite stable 5-membered Chelate Rings.

•Chelates having 6-membered ring with double bond or unsaturation forms more stable Chelate than Corresponding 5-membered Chelate .example acetylacetonate (acac) Complexes containing 6-membered rings and double bonds in ring form stable complexes with trivalent metals such as [Ti(acac)3] ,[Cr(acac)3] ,[Co(acac)3].

pH Spectrophotometry

(determination of binary formation constant by pHSpectrophotometry)

Many methods of great diversity are now being used for determination of stepwise stability constant however most widely used ,most accurate reliable method has been based on the potentiometric measurement of hydrogen Ion concentration. This method is based onthe fact that pH of Solution is directly influenced by complex formation which get accompanied by the displacement of a proton from acidic ligand.

The magnitude of the observed pH change is then employed for determining the stability Constant of metal Complex by Bjerrum’s method ,Calvien & wilson method etc.

pH spectrophotometry
pH spectrophotometry

It was shown by Calvin & Wilson that pH measurement done during titration with alkali solution ligands in the presence and absence of metal ion could be used for formation.function (nh ,n , pl ) & stability const. Can be calculated.

Glass Electrode method

-it is used to measure the pH of Solution under study.

Example-1 :

NH4+ ⇆ H+ + NH3 K= [H+][NH3]/[NH4+]

[NH3]= K[NH4+]/[H+]

K[NH4+] = Const (K’)

In acidic solution [NH4+] is large wrto. NH3 So,NH4+ conc. Does not change as [H+] increases.

Thus , [NH3] = K’ /[H+]


Ag+ + NH3 ⇆ [AgNH3]+ K1= 3.2×103

[AgNH3]+ + NH3 ⇆[Ag(NH3)2]2+ K2=8.5×102

If Initial conc [NH3]° is known than [NH3] at any time can be determined by Gas electrode method.

n(avg. no. Of NH3 bounded to Ag+) = [NH3]° – [NH3] /[Ag+]

Avg molecule of NH3 Coordinated to Ag+ ion can be determined by plotting the conc. Of NH3 vs n.
pH spectrophotometry

Spectroscopic Method


Fe3+ + CNS ⇆ [Fe(CNS)]2+ (bright red colour)

Let the initial conc ,[Fe3+]° = [CNS]°

Then conc. Of [Fe(CNS)]2+ at any time is determined by spectroscopic method by intensity of bright red colour.

[Fe]3+ at that time = [Fe3+]° – [Fe(CNS)]2+

[CNS] at that time = [CNS]° -[Fe(CNS)]2+

K= [Fe(CNS)]2+/ [Fe3+][CNS]

Asmus method :

this method is used for the determination of stability of weak complexes.

Consider the reaction :

M + nHL ⇆ MLn + nh

K=[MLn][H]n / [M][HL]n

Suppose the absorbance and molar extinction coefficient of the complex [MLn] are A and E respectively.

[MLn] = A/E

[M] = C- A/E ,C:refers to the concentration of the metal M .

pH spectrophotometry Asmus method

This Repersents the straight line equation ,if at constant pH value ,we plot the graph between log(A/E)/( C-A/E) & log[HL] , a straight line will be obtained having the slope ‘n’ and intercept logk-nlog[H] from the value of intercept , K can be Calculated.