Question

Explain the formation of [Co(NH₃)₆]³⁺ ion on the basis of valence bond theory.

Based on valence bond theory, predict the magnetic behaviour of the [Co(NH₃)₆]³⁺ complex.

Answer 1

1. The oxidation state of Cobalt is +3.
2. The valence shell electronic configuration of Co³⁺ is represented in the box diagram, as shown below:
   
3. The number of ammine ligands is 6, and the number of vacant metal ion orbitals required for bonding with ligands must be six.
4. Complexes are low-spin, so pairing of electrons will take place prior to hybridisation.
5. Electronic configuration after pairing would be 
   
6. Six orbitals available for hybridisation are two 3d, one 4s, three 4p orbitals.
   
   The orbitals for hybridization are decided from the number of ammine ligands, which is six. Here, (n − 1)d orbitals participate in hybridization since it is the low-spin complex.
7. Electronic configuration after complex formation.
   
8. According to Valence Bond Theory (VBT), the Co³⁺ ion undergoes d²sp³ hybridization in an octahedral field.
9. As all electrons are paired, the complex is diamagnetic.
10. If there are unpaired electrons, the complex is paramagnetic.