Factors Affecting the Stability of Complexes

A group or system comprising different atoms or ions linked to each other in a close or complicated way is known as a complex. A coordinate complex consists of a central atom, usually metallic and known as the coordination centre. The atoms surrounding the central atoms are known as ligands or complexing agents. A complex is formed when two or more component molecular entities form a loose association or their corresponding chemical entities associate. In the field of inorganic chemistry, the term complex is replaced by the term coordination entity. A metal complex with ligands is commonly known as complex. The overall charge that a complex compound has is contingent on the atom’s oxidation state as well as charges brought by the ligands surrounding it. They are also known as complex ions or coordinate complexes because they have Lewis acid-base complexes. The stability of a complex is an important factor that decides the stability and reactivity of metal complexes. The stability of metal complexes is governed by two different aspects that are thermodynamic and kinetic stability.

Some examples of complexes 

Example 1

If a complex is formed between a Co+3 ion and three (en)3 ligands, the complex will have a charge that is +3 charge, and [Co(en)3]+3is the formula that represents the complex between Co and en=Ethylenediamine.

Example 2

If complex forms between a Ni2+ ion and four CN- ligands, the complex will have a charge of -2, and the formula of the complex is written as [Ni(CN)4]-2.

Factors affecting the stability of complexes 

Several factors affect the stability of the complexes:

Nature of central metal ion

• Charge on metal cation

When the metal cations and the metals are present in their higher oxidation state then they form complexes which are stable as compared to when they are present in their lower oxidation state.

Examples of this effect are NH3, F- etc.

• Size of central metal cation 

When we decrease the size of a metal cation In any complex then the stability of that complex increases. For example, M2+.

Nature of ligands

When we increase the covalent character of any complex then it results in an increment in the stability of complex compounds. Ligands that are capable of forming π bonds in the complexes are CO and CN-.

The chelate effect 

The bidentate or polydentate ligands form five or six-membered coordinate bonds so that a ring structure is formed, with the metal centre usually forming more stable complexes than the others. For example, cadmium ions, [Cd(H2O)4]2+ with methylamine (CH3NH2).

Macrocyclic effect

Macrocyclic ligands are multi-dentate. Additionally, they permit less conformational freedom by virtue of being covalently constrained to their cyclic form.

There are two important as well as well known examples of macrocyclic ligands: Cyclic crown polyether and heme.

Resonance effect

When we increase the ligand and metal pi bond then the electrons dislocate from their actual position and this increases the stability of the complex.

Steric effect

The steric effect plays an important role in the stability of coordination complexes. It is defined as the effect on a molecule or a reaction due to the size of the atoms or groups involved. Steric hindrance and van der Waals repulsion are two common examples of this effect.

Crystal field stabilisation energy

There is an electrostatic interaction between metals and ligands in a coordination compound.

When any complex Metal coordinates with its surrounding ligand then crystal field stabilization energy occurs and it affects the stability of the complexes. Some examples of complexes which have higher crystal field stabilisation energy and lower spin are Pt4+ and Ir3+.

Determination of stability constants

Some of the methods that are used for the determination of stability constants are given as follows.

Spectroscopic methods

The spectroscopic methods are used to determine the stability constant because it helps to make the complexes thermodynamically stable.

Bjerrum’s method

 Bjerrum’s method is a well known potentiometric method for determining the stability of the constant for the formation of the complexes. It is used primarily on the negative ions and simple molecules which bind to the metal ion.

Irving and Rossotti method

This method is named after the scientists Harry Irving and Robert Williams and this method is used for determination of the stability constant for the complexes formed.

The importance of factors affecting the stability of complexes

The Irving and Rossotti effect, steric effect, Bjerrum’s method, and chelate effect are essential factors that affect the stability positively. They increase the stability of complexes, which helps human beings and plants form the compounds they require.

Conclusion 

The complex compounds are of great importance, as they are present in humans, animals, plants, and minerals. Many complexes are used for manufacturing medicines, and that’s why the stability of complex compounds is very important. A stable complex compound forms a stable component, which helps us in different ways. Red blood cells in the human body are also made up of a complex compound–the heme group, a Fe2+- Porphyrin complex. Coordinate compounds play an important role in inorganic chemistry and the complex is known as a composition in which two different types of molecules and elements are present.

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