Nephelauxetic effect and Nephelauxetic series

Definition of Nephelauxetic effect.

Electrons in the partly filled d-orbitals of a metal ion repel one another and give rise 

to a number of energy levels depending upon the arrangement of these electrons in the d-orbitals . 

The energy of each of these levels can be expressed in terms of some inter electronic repulsion parameters calledRacah repulsion parameters  B and C .

The energy gap between two such energy levels which have the same spin multiplicity 

can be expressed in terms of Dq and B and the energy gap between two energy levels having different spin multiplicities is expressed in terms of Dq, B and C . 

It is observed experimentally that the magnitude of these inter electronic  repulsion  parameters always decreases on the complexation of the metal ion .

This is possible only if inter electronic repulsion between the d electrons of the metal ion decreases on complexation.

The magnitude of inter electronic repulsion is inversely proportional to the 

distance  ‘r’ between the regions of maximum charge density of the d orbitals which are occupied by the electrons. 

This  repulsion would decrease only if the distance ‘r’ increases or if the lobes of d orbitals extended in space are shown below,

The extension of the lobes of the  d orbitals, that is, the expansion 

of d electron charge cloud during complexation by the transition metal ion, is called Nephelauxetic  effect.

Why Nephelauxetic  effect of cyanide ion is greater than water molecule ?

Expansion of  d-electron  clouds takes place during complexation of trasition metal ion which is termed Nephelauxetic  effect. 

This happen  because there is to some extent mixing of metal ligand orbitals through which metal d-electron can enter the ligand  orbitals.

The more the covalent nature of the ligand the more is the Nephelauxetic   effect.             

The order of ligands in Nephelauxetic effect are shown below,

So the nature of bonding in cyano complex of a given metal is more covalent than that in aquo-complex. 

Hence CN^- ion has a greater nephelauxetic  effect than  H₂O molecule.

Summary 

• What is nephelauxetic  effect ?
• Why nephelauxetic  effect of CN^- ion is greater than H₂O molecule?

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