Stability of Complex Ions &

Chelate Effect

DR KARIMAH KASSIM

•As ligand is added to the solution of metal ion, ML first forms more predominantly than any other complex in the series.

•As addition of ligands is continued, the ML2 concentration rises rapidly while the concentration of ML drops.

•Then ML3 becomes dominant, ML and ML2 becomes unimportant and so on until the highest complex MLN is formed to the nearly complete exclusion of all others at very high ligand concentrations.

CHELATE EFFECT

• As a general rule, a complex having bidentate or polydentate ligands are more stable than an equivalent complex having monodentate ligands only.

• Bidentate and polydentate ligands are also called chelating agent because of their ability to hold the metal atom like a claw (from the Greek chele, meaning ‘claw’).

• Chelation confers stability to a metal complex.

Looking at the value of log b4:Reaction (c) has the biggest value, therefore, the chelated complex [Cd(en)2]2+ has the highest thermodynamic stability.Looking at the value of Ho:All the three reactions do not show very different values, therefore the extra stability the chelated product has over the non-chelated products is not enthalpy driven.Looking at the value of So:The formation of the chelated complex, [Cd(en)2]2+, produces the most positive entropy change, showing that its stability over the non-chelated products is entropically driven.

Macrocyclic Effect

• A macrocycle is a ring of at least nine atoms of which at least three are donor atoms. An example of a macrocycle is porphyrin, a naturally occuring substance, as given below:

NH

N

N

HN

PorphyrinA 16 membered ring macrocycle with 4 nitrogen donor atoms.

Macrocyclic Effect

• The macrocyclic effect is a name for the greater stability of complexes with macrocyclic ligands over complexes with analogous acyclic ligands.

• The macrocyclic effect describes a further degree

of stabilisation than the chelate effect

NH

NH HN

HNNH

NH HN

HN

La Lb

The formation of zinc(II) complexes by the two ligands above have the reaction following the thermodynamics below:

Zn2+(aq) + L(aq) ZnL2+ (aq)

ZnLa2+ ZnLb

2+

log K 11.25 15.34

- Ho (kJ/mol) 44.4 61.9

So (J deg-1 mol-1) 66.5 85.8

• The bigger value of the log K for the formation of the macrocyclic complex (ZnLb

2+) shows that the macrocyclic complex is more stable than the acyclic complex.

• The more negative enthalpy value for the production of ZnLb

2+ indicated that it is is more thermodynamically stable than ZnLa

2+. More energy is given off during the formation, so the product has lower energy i.e. more stable.

• The entropy of the formation of ZnLb2+ is also

more positive (more favoured) than that of ZnLa

2+.