Valence Bond Theory

Important features of Valence Bond Theory (VBT) are :

(1) The central metal ion provides hybrid orbitals as required by the ligands i.e. coordination number to form complex.

(2) each ligand should have at least one lone pair of electron.

(3) The lone pair of electrons of the ligand overlap with empty hybrid orbitals of metal ion.

(4) If d-orbitals are involved in hybridisation that may be either inner viz (n-1) orbitals or the outer viz (n) d orbitals. The complexes formed by these two ways are known as inner orbital and outer orbital complexes respectively. Generally, the inner orbital complxes are low spin and outer orbital complexes are high spin.

(5) If the complex contains one or more unpaired electrons, the complex is paramagnetic and if it does not contain unpaired electron, it is diamagnetic.

What is the magnetic moment of the co-ordination compound \([FeF_6]^{4-}\)?

\(\sqrt {24}\) B.M.

The correct answer is option 1. \(\sqrt {24}\) B.M.

In \([FeF_6]^{4-}\)

Let the coordination number of \(Fe\) be 'x'. Thus,

\(x + 6(-1) = -4\)

\(x - 6 = -4\)

\(x = -4 + 6 = 2\)

₂₆Fe : [Ar]4s²3d⁶

Fe²⁺ : [Ar]3d⁶      (4 unpaired electrons)

As F^- is a weaker ligand, so it will not involve 3d-orbitals.

4d orbitals are used of Fe²⁺ and the hybridisation of complex is sp³d²

Magnetic moment, \(\mu =\sqrt {n(n+2)}\) 

\(\mu = \sqrt {4(4+2)}\) = \(\sqrt {24}\) B.M.