Factors Affecting the Stability of Complexes

Factors Affecting the Stability of Complexes

In general, the stability of compounds means the compounds exist under suitable conditions and may be stored for a long period of time. However when the formation of complexes in solution is studied, two types of stabilities(i.e. thermodynamic stability and kinetic stability) are considered.

In thermodynamics scence, the equilibrium constants of a reaction are the measure of the heat released in the reaction and entropy change during the reaction.
Greater amount of heat evolved in the reaction, the most stable the reaction products are.
Secondly, greater the increase in entropy during the reaction, greater is the stability of products.
The kinetic stability of complexes refers to the speed with which transformation leading to the attainment of equilibrium. In the kinetic sense, it is more proper to call the complexes inert or labile complex rather than stable or unstable complex. The complexes in which the ligands are rapidly replaced by others are called labile, while those in which replaced slowly are called inert complexes.
Followings are some important factors which affects the stability of complexes-
A. The Nature of the Central Metal Atom
1. Charge on the Central Metal Ion
2. Size of Metal Ion
3. Electronegativity or Charge Distribution of Metal Ion
4. Chelate Effect
5. Macrocylic Effect
B. The Nature of the Ligand
1. Basic Strength
2. Size and Charge of Ligands
3. Steric Effect
4. Dipole Moment and Polarizability of the Ligands
5. Back Bonding Capacity of Ligands

A. The Nature of the Central Metal Atom

1. Charge on the Central Metal Ion

Greater the charge (i.e. oxidation number) on the central metal ion, greater will be its attraction for the ligands. Hence, greater will be the stability of the complex. When metal ion forms complexes with the same ligand in more than one oxidation state, the complexes of the higher oxidation states are more stable than those of the lower oxidation states.
However, there are few exceptions with ligands like CO, PMe₃, o-phenanthroline, biphenyl, CN⁻ etc. which forms more stable complexes with metals in lower oxidation states. These ligands have vacant antibonding π molecular orbitals for the accomodation of lone pair of electrons donated by metal atoms(back bonding). CN⁻ ion is not only a poor π acceptor but also a good σ donar and therefore, forms complexes with metal atom in higher oxidation states. Example- Fe³⁺ ion carries higher charge than Fe²⁺ ion. So, Fe³⁺ forms more stable complex than Fe²⁺ ion with ligands like X⁻ or NH₃ or H₂O.

2. Size of metal ion

For a given ligand and metals of same oxidation states, stability of the complexes increases with decrease in size of metal cations.
Within a group, where the charge remains constant but the size increases down the group, the stability of the complexe with a particular ligand decreases in the oreder-
Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺.
For high-spin complexes of the divalent ions of first-row transition metals, the order of stability constant for complex formation is-
Mn²⁺ < Fe²⁺ < Co²⁺ < Ni²⁺ < Cu²⁺ > Zn²⁺
The ionic radius is expected to decrease regularly from Mn²⁺ to Cu²⁺. So the stability increases from Mn²⁺ to Cu²⁺.
Mn²⁺ < Fe²⁺ < Co²⁺ < Ni²⁺ < Cu²⁺ > Zn²⁺
Stability of octahedral complexes of Zn²⁺ is lower than that of Cu²⁺ ion because its CFSE is zero.
This stability trend is known as Irving Williams Series.
This order of stability is based on charge to radius ratio (i.e. ionic potential) concept. Greater the ionic potential of the central metal ion, greater would be the stability of the complexes with a particular ligand.

3. Electronegativity or Charge Distribution of Metal Ion

The stability of complex ion is also related to the electron charge distribution on the metal ion.
Metal ions may be classified into two types:
a. Class 'a' Metals
These are characterized by fairly electropositive, small size, high oxidation states (+3 or >+3) and no easily distorted outer electrons.
Alkali metals, alkaline earth metals, lighter transition metals in high oxidation states (+3 or >+3) are of class 'a' metal. Class 'a' metals form most stable complexes with ligands favouring electrostatic bonding, generally with first member of group 15, 16 and 17 (i.e. N, O or F donar atoms).
Class 'a' metals are also called hard acids or hard metals.
The order of stabilities of complexes of class 'a' metals with the ligands having the following donar atoms is as folloows-
F⁻ > Cl⁻ > Br⁻ > I⁻
O >> S > Se > Te
N >> P > As > Sb
b. Class 'b' Metals
These are characterized by less electropositive, have fully d-orbitals and form their most stable complexes with ligands which in addition to possess lone pairs of electrons, have empty π-orbitals available to accomodate electron pair form the d-orbitals of the metal.
Class 'b' metals forms stable complexes with ligands having donar atoms from third and subsequent periods (i.e. heavier members of the N, O and F groups).The metal ligand bonding nature is covalent in these complexes.
The stability order is reverse of class 'b' metals.
Class 'b' metals are also called as soft acids or soft metals.
The order of stabilities of complexes with ligands having the following donar atoms is as follows-
F⁻ < Cl⁻ < Br⁻ < I⁻
O << S < Se < Te
N << P < As &t; Sb

Furthermore, class 'b' metals forms complexes with CO, o-phenanthroline, olefines because these have vacant π-orbitals of low laying energy.

4. Chelate Effect

The stability also depends upon the formation of chelate rings. Complexes formed by chelating ligands such as ethylene diamine (en), ethylene diamine tetra acetic acid (EDTA), etc. are more stable than those formed by monodentate ligands such as H₂O or NH₃. Chelation increases the stability of complexes is called chelate effect.
If L is an unidentate ligand and L-L is a bidentate ligand and if the donor atoms of L and L-L are the same element, then L-L will replace L.
The major factors responsible for the special stability of chelate can be attributed to increase in entropy as the reaction leading to the formation of the chelate results in increase in population of product species when compared to reactant species. However, with monodentate ligand, the reaction results in no change in population.
The enhanced stability of complexes containing chelating ligands is of great importance in biological systems and analytical chemistry.
The chelate effect is maximum for 5- and 6-membered rings. In general, rings provide greater stability to the complex.

5. Macrocyclic Effect

If a multidentate ligand is cyclic and there are no unfavourable stearic effects, the complexes formed are more stable than corresponding complexes without cyclic ligands. This is called macrocyclic effect.

B. The Nature of the Ligand

1. Basic Strength of the Ligands

Greater the basic strength of the ligand, more easily it can donate electron pairs to the central ion and hence more easily it can form complexes of greater stability. The ligands that bind H⁺ firmly form stable complexes with metal ions. Thus F⁻ should form more stable complexes than Cl⁻, Br⁻ or I⁻ and NH₃ should be better ligand than H₂O which in turn should be better than HF. This behavoiur is observed for alkali, alkaline earth and other electropositive metals like the first row transition elements.
Example- for a given M⁺² dipositive 3d- series transition metal ion, order of stability of complex with ammonia, water and fluoride ion is-
NH₃ > H₂O > F⁻

2. Size and Charge of Ligand

If a ligand is small, it can approach the metal ion more closely forming a stable bond. Similarly, a highly charged ligand would also form a strong bond with the metal. Thus the high charge and small size of a ligand leads to the formation of stable complexes. For example the stability of the complexes of a given metal ion with halide ions used as ligands is in the order; F⁻ > Cl⁻ > Br⁻ > I⁻. This order is applicable for class a metals.
When class b metals such as Pd, Ag, Pt, Hg etc are used the order is reversed i.e. F⁻ < Cl⁻ < Br⁻ < I⁻.

3. Steric Effect

When a bulky group is either attached to or is present near a donor atom of a ligand, repulsion between the donor atom of the ligand and the bulky group is produced and this mutual repulsion weakens the metal-ligand bonding and hence makes the complex less stable.

4. Dipole Moment and Polarizability of the Ligands

For neutral ligands, higher the magnitude of dipole moment, higher the stability of the complex.
Example- the order of stability of complexes of ligands having nitrogen as donar atom is-
ammonia > ethylamine > diethylamine > triethylamine.
Due to the greater electrostatic interactions between the metal ion and the ligands, polarity and ploarizability of ligand results in higher K for the complexes.

5. Back Bonding Capacity of Ligands

Ligands like CO, CN⁻, NO, PR₃, alkenes and alkynes have special ability to form π-bonding with central metal -ion useually forms more stable complexes than others.

What is meant by stability of a coordination compound in solution? State the factors which govern the stability of complexes.

Stepwise and Overall Stability Constant

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