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

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(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 sp3dhybrid 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.

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