When the complex formed involves the inner (n - 1) d-orbitals for hybridization (d2sp3), the complex is called inner orbital complex. [Co(NH3)6]3+ involves inner (n - 1) d-orbitals. Therefore, it is called inner orbitals complex. When the complex formed involves the use of outer n d-orbitals for hybridization (sp3 d2), the complex is called outer orbital complex. [Ni(NH3)6]2+ involves (n - 1) d-orbitals. Therefore, it is outer complex.
[N(NH3)6]2+ Ni = 28 = 4s2 3d8
Strong ligand but if pairing occurs only one d orbital will be available so it cant ndergo d2sp3
