Coordination Compounds: Six and Four Coordination Complex, Limitation of Crystal Field Theory (For CBSE, ICSE, IAS, NET, NRA 2022)

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Six Coordination Complex

  • Consider a simple examples such as and .
  • In both the complexes, the oxidation state of cobalt is , but is paramagnetic and is diamagnetic.
  • The formation of complex may be considered as a series of hypothetical steps.
  • First the appropriate metal ion is taken e. g. .
  • Cobalt atom has the outer electronic configuration . Thus, ion will have the configuration and the electrons will be arranged as:
Six Coordination Complex
  • ion forms both paramagnetic (outer orbital) and diamagnetic (inner orbital) complexes depending upon the nature of ligands.
  • If ion combines with six fluoride ligands in , empty atomic orbitals are required on the metal ion to receive the coordinated lone pair of electrons.
  • The orbitals used are one , three and two . These are hybridized to give a set of six equivalent hybrid orbitals.
  • A ligand orbital containing a lone pair of electron forms a coordinate bond by overlapping with an empty hybrid orbital of the metal ion.
  • In this a bond is formed with each ligand. The d-orbitals used are the and .

It is shown below:

Six Coordination Complex
  • The outer orbitals are used for bonding, so this is called an outer orbital complex.
  • This complex will be high-spin paramagnetic because it has four unpaired electron.
  • An alternative octahedral arrangement in is possible when the electrons on metal ion are rearranged as shown below:
Six Coordination Complex
  • The inner d-orbitals are used , so this is called an inner orbital complex.
  • There is no unpaired electron, the complex will be low-spin diamagnetic.

Four Coordination Complex

1.

(i)

Four Coordination Complex

(ii)

Four Coordination Complex

(iii)

Four Coordination Complex

The resulting complex will be tetrahedral with two unpaired electrons. So, it will be paramagnetic.

2.

(i)

Four Coordination Complex

(ii)

Four Coordination Complex

(iii)

Four Coordination Complex

The resulting complex will be tetrahedral. It has no unpaired electrons and will be diamagnetic.

Limitation of Crystal Field Theory

  • This theory cannot easily explain the colour of complexes.
  • This theory is difficult to extend quantitatively.

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