NEET Chemistry Notes Coordination Compounds – Crystal Field Theory (CFT)

NEET Chemistry Notes Coordination Compounds – Crystal Field Theory (CFT)

Crystal Field Theory (CFT)

Crystal Field Theory (CFT)

The splitting of five d-orbitals into lower and higher energy levels due to approach of ligands, is known as crystal field theory. The five d-orbitals in a gaseous metal atom/ion have same energy.

Crystal Field Splitting in Octahedral Coordination Entities

Energy separation is denoted by (the subscript o is for octahedral). This is also known as Crystal Field Splitting Energy (CFSE).



Where, P represents the energy required for electron-pairing in a single orbital (Pairing energy).

Crystal Field Splitting in Tetrahedral Coordination Entities
In tetrahedral coordination entities,

Consequently, the orbital splitting energies are not sufficiently large for forcing pairing and therefore, low spin configurations are rarely observed. Due to less crystal field stabilisation energy, it is not possible to pah electrons and so all the tetrahedral complexes are high spin.

Spectrochemical Series

An arrangement of ligands in order of increasing crystal field strength is spectrochemical series.

Colour in Coordination Compounds

In complex compounds d-orbitals split in sets t_2g and e_g. These have different energies. The difference in energies lies in visible region and electron jump from ground state  t_2g  level to higher state e_g. level.
This is known as d-d transition and is responsible for colour of coordination compounds, d-d transition takes place in d¹ to d⁹ ions, so the ions having d¹ to d⁹ configuration are coloured.
On the other hand, the ions with d° and d¹⁰ configuration do not show d-d transition.

Stability Constants and Stability of Complexes

Stability of a complex can be expressed in terms of stability constant, k. If the complex is  and is the overall formation constant, then

Factors Affecting Stability of Complexes

• The strength of a complex ion depends upon the following factors:
• Higher charge of the central metal ion
  i.e. greater ionic potential  greater is the stability.
• 
• Greater base strength of the ligand, greater will be the stability.
•  Ring formation (chelation) in structure of the complexes is the chief factor, which increases the stability of the complexes in solution,
• If a multidentate ligand happens to be cyclic without any steric effects, a further increase in stability occurs. This is called macrocyclic effect.Importance and Applications of Coordination Compounds
• Hardness of water is estimated by simple titration with Na₂ EDTA. The Ca²⁺ and Mg²⁺ ions form stable complexes with EDTA.
• Some important extraction processes of metals, like those of silver and gold make use of complex formation.
•  Similarly, purification of metals can be achieved through formation and subsequent decomposition of their coordination compounds, e.g. impure nickel is converted to [Ni(CO)₄], which is decomposed to yield pure nickel (Mond’s process).
• Coordination compounds are used as catalysts for many industrial processes. Examples include rhodium complex,[(Ph₃P)₃RhCl], Wilkinson catalyst, is used’for the hydrogenation of alkenes.
• Metals present in toxic proportions in animals and plants are removed by chelate therapy, e.g. Cu and Fe are removed by D-penicillamine and desferrioxime-B.
•  The platinum complex cis-[Pt (NH₃)₂ Cl₂] (cis-platin is used in the treatment of cancer.

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