Stability of Coordination Compounds in Solution

Contents

1 Stability Of Coordination compounds in Solution
2 Factors affecting the stability of a complex ion
- 2.1 (1) The nature of the central ion
- 2.2 (2) The nature of the ligand

The stability of a complex in solution means the degree of association between the metal ion and the ligands involved in the state of equilibrium.

The magnitude of the (stability or formation) equilibrium constant for the association express quantitatively the stability.

For example: The formation of [Cu(NH₃)₄]²⁺complex may be expressed as :
Cu²⁺ (aq) +4NH₃ (aq) ⇔ [Cu(NH₃)₄]²⁺ (aq)

The equilibrium constant for the reaction, is therefore, called the stability constant and is represented by K_s as

The interaction between metal ion and ligand may be regarded as Lewis acid-base reaction. If the interaction is strong, the complex formed would be thermodynamically more stable and the value of stability constant (K_s) will be large.

The larger the numerical value of K_s, the more thermodynamically stable is the complex.

Let us consider a general reaction:

M+4L ⇔ ML₄

The stability constant may be written as:

Larger the stability constant, the higher the proportion of ML₄ that exists in solution. Since free metal ions rarely exist in the solution so that metal ion (M) will usually be surrounded by solvent molecules which will complete with the ligand molecules, L and be successively represented as :

[M(H₂O)_n] + 4L ⇔ [ML₄] +nH₂O

The above overall reaction proceeds in steps with formation constant K₁, K₂, K₃ and K₄, for each step as represented below:

where K₁, K₂, K₃ and K₄ are called stepwise stability constants

Alternatively, we may write the overall stability constant as

M+ 4L ⇔ ML₄

β₄ = [ML₄] /[M] [L]⁴

The stepwise and overall stability constants are therefore, related as:

β₄ = K₁ x K₂ x K₃ x K₄

β_n = K₁ x K₂ x K₃ x K_{4……………..}K_n

For example: The steps involved in the formation of the complex ion, tetraamminecopper(III) ion, we have the following:

β₄ =[Cu(NH₃)₄]²⁺/[Cu²⁺][NH₃]⁴

Then, logβ may be used as a measure of stability of the complex.

For example:For [Cu(NH₃)₄]²⁺, the four constants are :

log β₄= logK₁ + logK₂ + logK₃ + logK₄

= 4.0 + 3.2 +2.7 + 2.0

log β₄= 11.9

For example: for [Cd(NH₃)₄]²⁺

log β₄ = 2.6 +2.1 + 1.4 + 0.9 = 7.0

Thus, the log β₄ value of [Cu(NH₃)₄]²⁺ and [Cd(NH₃)₄]²⁺ complexes indicate that [Cd(NH₃)₄]²⁺ is more stable than [Cd(NH₃)₄]²⁺ complex.

In the second group of qualitative analysis of basic radicals, Cu²⁺ does not form precipitate as CuS when H₂S is passed through the solution containing [Cd(NH₃)₄]²⁺ complex, while Cd²⁺ ions form precipitate as CdS because of instability of [Cd(NH₃)₄]²⁺ complex ion.

The reciprocal of stability constant gives the instability constant or dissociation constant of coordination compound, Thus,

Dissociation constant of complex = 1 / Stability constant

The overall stability constant, β_n is related to thermodynamic stability when the system has reached equilibrium. Most of the measurements are made from aqueous solutions, which means that the complex is formed by the ligand displacing water molecules from the aqua complex of the metal ion.

The values indicate that CN¯ ion ia a stronger ligand than NH₃ molecule because the stability constants for cyanide complexes are very large in comparison to corresponding ammine complexes.

Now, let us compare the stability constant of two nickel complexes:

Ni²⁺ + 6 NH₃ → [Ni(NH₃)₆]²⁺ K= 6.1 × 10⁸

Ni²⁺ + 3 en → [Ni(en)₃]²⁺ K= 4.6 × 10¹⁸

The K values indicate that the bidentate ligand NH₂CH₂CH₂NH₂forms a considerably more stable complex than ammonia.

Factors affecting the stability of a complex ion

The stability of the complex ion depends upon the following factors:

(1) The nature of the central ion

The stability of a complex depend upon the nature of the central metal ion as:

(i) Charge on the central metal ion:In general, the greater the charge density on the central ion, the greater is the stability of its complexes.

The greater the charge and the smaller the size of an ion, i.e., (the larger the charge/radius ratio of an ion), the greater is the stability of its complex. Fe³⁺ ion carries higher charge than Fe²⁺ ion but their size is about the same.

Hence charge density is higher on Fe³⁺ than on Fe²⁺ ion. The complexes of Fe³⁺ ion are, therefore more stable.

Fe³⁺+ 6CN¯ → [Fe(CN)₆]^3–

K= 1.2 x10³¹

Fe²⁺ + 6CN¯ → [Fe(CN)₆]^4–

K = 1.8 x 10⁶

(ii) Size of metal ion: As the size of the metal ion decreases, the stability of the complex increases. If we consider the bivalent metal ions, than the stability of their complexes (irrespective of the ligands) increases with increase in the ionic radius of the central metal ion as:

Ion	Mn²⁺	Fe²⁺	Co²⁺	Ni²⁺	Cu²⁺	Zn²⁺
Ionic Radius	91	83	82	78	69	64

Therefore, the order of stability is

Mn²⁺ < Fe²⁺ < Co²⁺ < Ni²⁺ < Cu²⁺ < Zn²⁺

This order is called Irving William’s order of stability.

(iii) Electronegativity or charge distribution of metal ion: The stability of complex ion is also related to the electron charge distribution on the metalion.

Metal ions may be classified into two types:

(a) Class ‘a’ acceptors: These are fairly electropositive metals and include the metals of groups 1 and 2, inner transition metals and the early members of the transition series (groups 3 to 6) which have relatively a few electrons beyond an inert gas core. These form most stable coordination entities with ligands containing N, O or F donor atoms.

(b) Class ‘b’ acceptors: These are much less electropositive and include heavy metals such as Rh, Pd, Ag, Ir, Pt, Au, Hg, Pb, etc. having relatively full d-orbitals. These form most stable complexes with ligands whose donor atoms are the heavier members of the N, O and F groups.

(iv) Chelate effect: The stability also depends upon the formation of chelate rings. If L is an unidentate ligand and L-L, a bidentate ligand and if the donor atoms of L and L-L are the same element, then L-L will replace L.

This stabilisation due to the chelation is called chelate effect. 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.

(v) 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.

(2) The nature of the ligand

(i) Basic strength: The more basic a ligand, the greater is the ease with which it can donate its lone pairs of electrons and, therefore, the greater is the stability of the complexes formed by it. Thus, CN¯ and F¯ ions and NH₃ molecules, which are strong bases, are also good ligands and form many stable complexes.

(ii) Size and charge of ligands: For anionic ligands, the higher the charge and the smaller the size, the more stable is the complex formed. Thus, F¯ ion gives more stable complexes than does Cl¯ ion.