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Stability of Coordination Compounds- Stability of Coordination Compounds and Factors Affecting the Stability, Practice Problems and FAQs

Stability of Coordination Compounds- Stability of Coordination Compounds and Factors Affecting the Stability, Practice Problems and FAQs

Attaining stability is the ultimate desire of all material and living species. Understanding determining stability and ways of attaining stability is an important task.

Humans attain stability in getting their desire fulfilled. Materials get stability on attaining the lowest energy.

Tell me which sodium or magnesium is more stable, and how to find out? The ionisation enthalpy and electronic configuration of the metals can help you to decide. The higher the stability more the energy is needed to remove an electron from it.

Ionisation enthalpy of sodium = 494.6 KJ/mole

Ionisation enthalpy of magnesium is 737.6 KJ/mole

Magnesium having higher ionization enthalpy is more stable than sodium.

image

What about coordination complexes? If you have [Cu(NH3)4]2+ and [Cu(CN)4]2-, will you be able to correctly determine which complex is more stable? I believe the answer is NOOO. As you can see, the ligand differences are evident, but we are unable to determine what basis should be used to find out the correct stability order. 

That is what you are going to find here.

Table of Contents

  • Coordination compounds
  • Stability of coordination compounds
  • Aspects influencing the stability of a complex ion
  • Practice Problems
  • Frequently Asked Questions:

Coordination compounds

Coordination complex is a compound containing a score of a central metal atom or ion coordinately attached to atoms/ ions/ groups called ligands. The core is the coordinate complex, which can be positive, neutral or negatively charged.

The complex is formed by a reaction between the central metal atom/ ion and the ligands in an equilibrium reaction in a liquid medium

M+nL ⇌ MLn

Stability of coordination compounds

The degree of association between the metal ion and the ligands involved in the state of equilibrium is defined as the stability of a complex in the solution. Stronger the stability of the bond between the metal and ligands stronger the stability of the coordination complex and the lesser the dissociation back into metal and the free ligands in solution. The equilibrium lies towards to left for a stronger coordination complex/compound.

Hence, the magnitude of the (stability or formation) equilibrium constant for the association helps to measure the stability of the coordination complex.

Consider a complex formation

M+nL ⇌ MLn

image

The equilibrium constant for the reaction is thus known as the stability constant and is denoted by Ks.

The Lewis acid-base reaction can be used to describe the interaction of a metal ion and a ligand. If the interaction is strong, the complex formed will be thermodynamically more stable. The complex is more thermodynamically stable as the numerical value of Ks increases.

The greater the stability constant, the greater the proportion of MLn in solution. Because free metal ions are rare in solution, the metal ion (M) is usually surrounded by solvent molecules, which will complete with the ligand molecules, L, and be represented as

M(H2O)n+nL ⇌ MLn+n H2O

The above overall reaction is divided into steps, with formation constants K1 , K2 , K3 and………. Kn  for each step, as shown below:

image

Multiply all the equationsimage

When the system has reached equilibrium, the overall stability constant, logKs, is related to thermodynamic stability. The majority of the measurements are from aqueous solutions, implying that the complex is formed by the ligand displacing water molecules from the metal ion's aqua complex.

Example: consider the stability constants of two copper complexes:image

The stability constants for cyanide complex is very large in comparison to corresponding ammine complex. This implies that the cyanide complex is more stable than the ammine complex for the same metal copper ion and the cyanide(an CN-) ion is a stronger ligand than NH3 the molecule.

Aspects influencing the stability of a complex ion

The complex ion's stability is determined by the following factors:

(1) The central ion's nature

The stability of a complex is determined by the nature of the central metal ion, which is as follows:

(i) Central metal ion charge density: In general, the higher the charge density on the central ion, the more stable its complexes.

Greater an ion's charge and the smaller the size, i.e., (the greater an ion's charge/radius ratio), the greater the stability of its complex. Fe3+ ions have a higher charge than Fe2+ ions but are rough of the same size.

As a result, the charge density of Fe3+ ions is greater than that of Fe2+ ions and so, ferric ion complexes are more stable than ferrous ion complexes.

(ii) Metal ion size: As the size of the metal ion decreases, so does the stability of the complex. When we consider bivalent metal ions, the stability of their complexes (regardless of the ligands) increases as the ionic radius of the central metal ion increases, and thus the order of stability is

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

This is known as Irving Williams' order of stability.

(iii) Chelate effect: The formation of chelate rings affects stability as well. L-L will replace L if L is a unidentate ligand and L-L is a bidentate ligand and if the donor atoms of L and L-L are the same element.

The chelate effect refers to the stabilisation caused by chelation. In biological systems and analytical chemistry, the increased stability of complexes containing chelating ligands is critical.

The chelate effect is strongest in 5- and 6-membered rings. Rings, in general, provide greater stability to the complex.

2) The ligand's nature

(i) Nucleophilicity and Basic strength: More the nucleophilicity of a ligand, the easier it is to donate its lone pairs of electrons and thus the greater the stability of the complexes it forms. Thus, CN- , F- ions, as well the NH3 molecules, which are strong bases, are also good ligands and form a large number of stable complexes.

Order in terms of basic nature : CN- > NH3 > F-

(ii) Ligand size and charge: The higher the charge and smaller the size of anionic ligands, the more stable the complex formed. As a result, F- ion produces more stable complexes than Cl- ion.

Practice Problems

Q1. Consider the stability constants of two Nickel complexes:

image

Predict which complex is more stable with explanation?

Solution: Higher stability constants for ethylene diamine complexes indicates higher stability in comparison to corresponding ammine complexes. They can be explained, due to the ethylenediamine being a bidentate ligand that can do chelation and hence form a more stable complex than NH3 a molecule.

Q2. Consider the stability constants of two cobalt complexes:

image

Predict which complex is more stable?

A. [Co(CN)6]3- is more stable than [Co(CN)6]4-.
B. [Co(CN)6]4- is more stable than [Co(CN)6]3-.
C. Both [Co(CN)6]3- and [Co(CN)6]4- are equally stable
D. Can not be predicted.

Solution: The larger the charge/radius ratio of an ion, the more stable its complex becomes. The Co3+ ion has a larger charge than the Co2+ ion, but they are about the same size. Hence based on charge, we can say that the greater the charge, the more firmly a central atom can bind the ligands, hence more is more stability. 

Hence, [Co(CN)6]3- is more stable than [Co(CN)6]4-.

Hence, option (A) is the correct option.

Q3. Arrange the following complexes in the correct order of stability?

A. [Ni(NH3)6]2+<[Zn(NH3)6]2+<[Mn(NH3)6]2+<[Cu(NH3)6]2+
B. [Zn(NH3)6]2+<[Ni(NH3)6]2+<[Mn(NH3)6]2+<[Cu(NH3)6]2+
C. [Mn(NH3)6]2+<[Cu(NH3)6]2+<[Zn(NH3)6]2+<[Ni(NH3)6]2+
D. [Mn(NH3)6]2+<[Ni(NH3)6]2+<[Cu(NH3)6]2+<[Zn(NH3)6]2+

Solution: When we consider bivalent metal ions, the stability of their complexes (regardless of the ligands) increases as the ionic radius of the central metal ion increases, and thus the order of stability is as Irving Williams' order of stability(Mn2+<Fe2+<Co2+<Ni2+<Cu2+<Zn2+). 

Hence, the correct order is an option (D).

Q4. Arrange the following complexes in the correct order of stability?

A. [Co(F)6]4-<[Co(Cl)6]4-<[Zn(NH3)6]2+<[Mn(CN)6]4-
B. [Mn(CN)6]4-<[Co(Cl)6]4-<[Co(F)6]4-<[Zn(NH3)6]2+
C. [Mn(CN)6]4-<[Co(F)6]4-<[Co(Cl)6]4-<[Zn(NH3)6]2+
D. [Co(Cl)6]4-<[Co(F)6]4-<[Zn(NH3)6]2+<[Mn(CN)6]4-

Solution: When we consider bivalent metal ions, the stability of their complexes (regardless of the ligands) increases as the ionic radius of the central metal ion increases, and thus the order of stability is as Irving Williams' order of stability(Mn2+<Fe2+<Co2+<Ni2+<Cu2+<Zn2+). 

Also, Ligand size and charge play an important role in the stability of complexes. The higher the charge and smaller the size of anionic ligands, the more stable the complex formed. As a result, F- ion produces more stable complexes than Cl- ion.

Hence, [Co(F)6]4- should be more stable than [Co(Cl)6]4-

Hence, the correct order is an option (B).

Frequently Asked Questions:

Question 1. What is the Macrocyclic effect?
Answer: When a multidentate ligand is cyclic and no unfavourable stearic effects exist, the complexes formed are more stable than corresponding complexes without cyclic ligands. This is known as the macrocyclic effect.

Question 2. Is temperature a component in the stability constant?
Answer: The stepwise stability constants of complexes formed at various temperatures were determined. These values decrease as temperature increases, indicating that the complexation process is more favourable at lower temperatures.

Question 3. Why are chelating ligands better at forming stable coordination compounds?
Answer: Chelating complexes are more stable than unchelated complexes because the ligand is attached to the metal ion at multiple points. As a result, there is a strong attraction between the metal and the ligand.

Question 4. How do you tell the difference between inert and labile complexes?
Answer: Inert complexes undergo slow substitution, whereas labile complexes undergo rapid substitution. This is primarily because inert complexes are more thermodynamically stable complexes.

Related Topics

Oxidation number of elements in coordination compounds

Organometallic Compounds

EAN Rule

Ligands

Bonding in coordination compounds

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