In
[Co(NH3)6]3+ ,
Co3+ ion is having 3d6 configuration
In presence of
NH3 ligand, pairing of electrons takes place and it becomes diamagnetic complex ion.
∴‌‌[Co(NH3)6]3* is octahedral with
d2 sp3 hybridisation and it is diamagnetic in nature.
In case of [CoF6]3-, Co is in +3 oxidation state and it is having 3d6 configuration.
In presence of weak field
F - ligand, pairing does not take place.
∴‌‌ln[CoF6]3−,Co3+ is
sp3d2 hybridised with four unpaired electrons, so it is paramagnetic in nature.