Stability of metal complexes

V.SANTHANAMDEPARTMENT OF CHEMISTRY

SCSVMV

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 constantsM + 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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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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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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Statistical effect

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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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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 = K1.K2.K3. ….. Kn

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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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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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• But when this complex exchanges its CN- ligands with 14C 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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Explanation of lability and inertness according to VBT

• VBT classifies octahedral complexes into two types.

• Inner orbital complexes – d2sp3

• Outer orbital complex – sp3d2

• The two d-orbitals involved in the hybridization are the eg set of orbitals.

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Outer orbital complexes

• The complexes having sp3d2 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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Inner orbital complexes

• These complexes generally have d2sp3 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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• 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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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.

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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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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

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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 radiusComplex ion Charge on the

ionIonic 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 chargeComplex ion Charge on the

ionIonic 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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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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CFSE as a function of no of d-electrons

00.20.40.60.8

11.21.4

0 1 2 3 4 5 6 7 8 9 10 11

no of d-electrons

CFSE

in m

ultip

les

of Δ

.

Crystal Field Stabilization Energy (CFSE) of d0 to d10 M(II) ions:

Ca2+ Mn2+ Zn2+

double-humpedcurve

Ni2+

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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

logK

1(ED

TA) .

Log K1(EDTA) of d0 to d10 M(II) ions:

Ca2+

Mn2+

Zn2+

double-humpedcurve

= CFSE

rising baselinedue to ioniccontraction

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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

logK

1(en

) .

Log K1(en) of d0 to d10 M(II) ions:

double-humpedcurve

Ca2+ Mn2+

Zn2+

rising baselinedue to ioniccontraction

= CFSE

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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

logK

1(tp

en).

Log K1(tpen) of d0 to d10 M(II) ions:

Ca2+

Mn2+

Zn2+

double-humpedcurve

N N NN

N Ntpen

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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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• 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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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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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.

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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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The chelate effect or chelation is one of the most important ligand effects in The 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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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(NH3)6]2+ + 3 NH2CH2CH2NH2 (en)

[Ni(en)3]2+ + 6NH3

log = 9.7

[Ni(H2O)6]+2 + 3 NH2CH2CH2NH2 (en)

[Ni(en)3]2+ + 6H2O

log = 18.3

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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 reaction is negative

Complex Formation: Major Factors

[Ni(H2O)6] + 6NH3 [Ni(NH3)6]2+ + 6H2O

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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 - TS(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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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

GkJmol-1

-42.5

-37.2

-60.7

HkJmol-1

- 53.2

-57.3

-56.5

SJK-1mol-1

- 35.5

- 67.3

+13.8

log

7.44

6.52

10.62

Cd2+ + 4 MeNH2 [Cd(MeNH2)4]2+

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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 2 = 7.7 H = -46 kJ/mol S = -8.4 J/K/mol

Chelate Formation: Entropy Gain

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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.

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HOOH

O

OSH

SH

(R,S)-2,3-dimercaptosuccinic acid

As, Cu, Pb, Hg

HS OH

SHM+

S

SOH

M

Dimercaprol

AsHgAuPb

D-Penicillamine

ZnAsHgAuPb

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CH2NCH2

CH2

C

CCH2 N

CH2

CH2 C

C

O

O

O

O

O O

OO

EDTA

*

* *

***

Important Chelating Ligands

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AnticoagulantCa2+

EDTA: another view

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Macrocylic Ligands

Important Chelating Ligands

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[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

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number of chelate rings Metal

complexNo. 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 16.251santhanam SCSVMV

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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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

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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

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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.

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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

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EXPERIMENTAL DETERMINATION EXPERIMENTAL DETERMINATION

OF OF STABILITY CONSTANTS STABILITY CONSTANTS

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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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It is known that ,CM = [M2+] + [ML2+]

CL = [L] + [ML2+]A = ε [ML2+] . C[ML2+] . l

C[ML2+] = A / ε [ML2+] .lSo

[M2+] = CM - (A / ε [ML2+] .l)[L] = CL - (A / ε [ML2+] .l)

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• 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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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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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+]

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• 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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