Predict the shape of hexafluoroferrate(III) on the basis of valence bond theory.

Explanation

Outer electronic configuration of Co (Z = 27) in ground state is `4s^2 3d^7`. Cobalt hi this complex is in +3 oxidation state. This is achieved by the loss of two 4s and one 3d electron. The resulting `Co^(3+)` ion has `3d^6` outer electronic configuration. 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^3 d^2` hybridisation of one 4s, three 4p and two 4d vacant 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 four unpaired electrons. The hybridisation and the formation of the complex is illustrated below.

The formation of this complex involves dorbitals of the outer shell and hence is called outer orbital or high spin or hypoligated complex.