Question

Explain the geometry of [CoF₆]^3- on the basis of valence bond theory.

Answer 1

(1) Hexafluorocobaltate(III) ion,

[CoF]^3- is an anionic complex, the oxidation state of cobalt is +3 and the coordination number is 6. 

(2) Electronic configuration : ₂₇Co [Ar]¹⁸ 3d⁷ 4s² 4p°4d° 

Electronic configuration : Co³⁺[Ar]¹⁸3d⁶ 4s° 4p° 4d°

(3) Since F is a weak ligand, there is no spin pairing effect and Co³⁺ possesses 4 unpaired electrons.

(4) Since the coordination number is 6, 

The Co³⁺ ion gets six vacant orbitals by hybridisation of one 45 orbital, three 4p orbitals and two 4d orbitals forming six sp³d² hybrid orbitals giving octahedral geometry.

(5) 6 lone pairs of electrons from 6F^- ligands are accommodated in the six vacant sp³d² hybrid orbitals. 

Thus,

Six hybrid orbitals of Co³⁺ overlap with filled orbitals of F^- forming 6 coordinate bonds giving octahedral geometry to the complex. It is an outer complex.

As the complex has 4 unpaired electrons it is paramagnetic.

Magnetic movement μ is,

μ = \(\sqrt{n(n+2)}\)

= \(\sqrt{4(4+2)}\) 

= 4.9 B.M.