178. Discuss the nature of bonding in the following coordination entities on the basis of valence bond theory. 
 [Fe(CN)₆]^4–  

[Fe(CN)₆]^{4 –}

Outer electronic configuration of iron (Z = 26) in ground state is 3d⁶4s². Iron in this complex is in +2 oxidation state. Iron achieves +2 oxidation state by the loss of two 4s electrons. The resulting Fe²⁺ ion has outer electronic configuration of 3d⁶.
Since CN^- is strong field ligand thus cause pairing.


It has been experimentally observed that this complex is diamagnetic as such has no unpaired electron. To account for this two unpaired electrons in the 3d - subshell pair up, thus leaving two 3d-orbitals empty. These two vacant 3d-orbitals along with one 4s-orbital and three 4p-orbitals hybridise to give six equivalent d²sp³ hybridised orbitals. Six pairs of electrons, one from each cyanide ion, occupy the six vacant hybrid orbitals so produced. The resulting complex ion has an octahedral geometry and is diamagnetic due to the absence of unpaired electrons.