what scheme of hybridisation is proposed for Co in [Co(NH3)6]3+ ??
In [Co(NH3)6]3+. complex, inner d-orbitals are used in hybridisation; so hybridization is d2sp3.
The hybridization scheme is shown in the following diagram.
Now as ammonia is a strong field ligand so pairing of electrons will take place to give six vacant d2sp3 hybrid orbitals into which six ammonia molecules can donate electron pair.