Question

Explain the structure of octahedral complex, [CO(NH₃)₆]³⁺ on the basis of valence bond theory.

Answer 1

(1) Hexaamminecobalt (III) ion,

[CO(NH₃)₆]³⁺ is a cationic complex, the oxidation state of cobalt is + 3 and the coordination number is 6.

(2) Electronic configuration : ₂₇CO[Ar]¹⁸3d⁷4s²

Electronic configuration : CO³⁺[Ar]¹⁸3d⁶4s°4p°

(3) Since NH₃  is a strong ligand, due to spin pairing effect, 

All the four unpaired electrons in 3d orbital are paired giving two vacant 3d orbitals.

(4) Since the coordination number is Co³⁺ion gets six vacant orbitais by hybridisation of two 3d vacant orbitais, 

One 4s and three 4p orbitais forming six d² sp³ hybrid orbitais giving octahedral geometly. It is an inner complex.

(5) 6 lone pairs of electrons from 6NH₃ ligands are accommodated in the six vacant d²sp³ hybrid orbitals. 

Thus,

Six hybrid orbitals of Co³⁺ overlap with filled orbitals of NH³ forming 6 coordinate bonds giving octahedral geometry to the complex.

Since the complex has all electrons paired, it is diamagnetic.