In [Co (NH3)6]3+,Co is in +3 oxidation state with the configuration 3d6. In the presence of NH3 a strong ligand,the 3d electrons pair up leaving two d-orbitals empty.Hence, the hybridization is d2 sp3 forming an inner orbital octahedral complex.In [Ni (NH3)6]2+,Ni is in +2 oxidation state with the configuration 3d8.In presence of NH3, the 3d electrons do not pair up.The hybridization involved is sp3 d2 forming an outer orbital octahedral complex.