Chelate Effect
Ni2+
Teeth of a ligand ( teeth dent)
• Ligands
– classified according to the number of donor
atoms
– Examples
• monodentate = 1 chelating agents
• bidentate = 2
• tetradentate = 4
• hexadentate = 6
• polydentate = 2 or more donor atoms
monodentate, bidentate, tridentate etc. where the concept of teeth (dent)
is introduced, hence the idea of bite angle etc.
oxalate ion ethylenediamine
O O 2-
CH2 CH2
C C H2N NH2
O O * *
* *
Coordination Equilibria & Chelate effect
"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.
Coordination Equilibria & Chelate effect
[Fe(H2O)6]3+ + NCS- [Fe(H2O)5(NCS)]2+ + H2O
Kf = [Fe(H2O)5(NCS)]2+/ [Fe(H2O)6]3+[NCS-]
Equilibrium constant Kf formation constant
M + L ML K1 = [ML]/[M][L]
ML + L ML2 K2 = [ML2]/[ML][L]
ML2 + L ML3 K3 = [ML3]/[ML2][L]
MLn-1 + L MLn Kn = [MLn]/[MLn-1][L]
Coordination Equilibria and Chelate effect
• K1, K2…. Stepwise formation constant.
• To calculate concentration of the final
product, use overall formation constant n:
• n = [MLn]/[M][L]n
• = K1 x K2 x K3 x …. x Kn
Coordination Equilibria & Chelate effect
Example: [Cd(NH3)4]2+
Cd2+ + NH3 [CdNH3]2+ K1 = 102.65
[CdNH3]2+ + NH3 [Cd(NH3)2]2+ K2 = 102.10
[Cd(NH3)2]2++ NH3 [Cd(NH3)3]2+ K3 = 101.44
[Cd(NH3)3]2++ NH3 [Cd(NH3)4]2+ K4 = 100.93
Overall: Cd2+ + 4 NH3 [Cd(NH3)4]2+
β4 = K1 x K2 x K3 x K4 = 10(2.65 + 2.10 + 1.44 + 0.93) = 107.12
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
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 reaction is 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
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+
G H S
Ligands log
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
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
Kinetic stability
Inert and labile complexes
The term inert and labile are relative
“A good rule of thumb is that those complexes that react
completely within 1 min at 25o should be considered labile and
those that take longer should be considered inert.”
Thermodynamically stable complexes can be labile or inert
[Hg(CN)4]2- Kf= 1042 thermodynamically stable
[Hg(CN)4]2- + 4 14CN- = [Hg(14CN)4]2- + CN-
Very fast reaction Labile
Chelating agents:
(1) Used to remove unwanted metal ions in water.
(2) Selective removal of Hg2+ and Pb2+ from body when poisoned.
(3) Prevent blood clots.
(4) Solubilize iron in plant fertilizer.
Important Chelating Ligands
2,3-dimercapto-1-propanesulfonic
acid sodium (DMPS)
Mn+
DMPS is a effective chelator with two groups thiols - for
mercury, lead, tin, arsenic, silver and cadmium.
SH O
HO
OH
O SH Zn
(R,S)-2,3-dimercaptosuccinic acid As
D-Penicillamine
Hg
As, Cu, Pb, Hg Au
Pb
SH S
M+
M As
OH Hg
HS OH S Au
Pb
Dimercaprol
Important Chelating Ligands
O EDTA O
*O C CH2 CH2 C O*
*
N *
CH2 CH2 N
*O C CH2 CH2 C O*
O O
EDTA: another view
Ca2+
Anticoagulant
Important Chelating Ligands
Macrocylic Ligands
