Explain the formation of H2 molecule on the basis of valence bond theory.

Explanation

Outer electronic configuration of iron (Z=26) in ground state is `3d^6 4s^2`. Iron in this complex is in +2 oxidation state. Iron achieves the +2 oxidation state by the loss of two 4s-electrons. The resulting `Fe^(2+)` ion has outer electronic configuration of `3d^6`. It may be pointed out that `F^(-)` jon provides a weak ligand field and is unable to pair up the unpaired electrons of. the 3d-orbitals. Further, it has been experimentally observed that this complex is highly paramagnetic thus suggesting the presence of a number of unpaired electrons. To account for these observed facts, it is proposed that in this complex, the six equivalent hybrid orbitals are obtained by the `sp^3d^2` hybridisation of one 4s and three 4p and two 4 d atomic orbitals. Six pairs of electrons, one from each fluoride ion occupy the six vacant hybrid orbitals so formed. The resulting complex ion has an octahedral geometry and is strongly paramagnetic due to the presence of unpaired electrons. The hybridisation and formation of the complex is illustrated below.

As the d-orbitals involved in the hybridisation belong to the outer i.e, 4th shell, it is termed as outer orbital complex or high spin complex.