Why `[Co(NH_3)_6]^(3+)` is an inner orbital complex where is `[Ni(NH_3)_6]^(2+)` is an outer orbital complex ?

Explanation

In `[Co(NH_(3))_(6)]^(3+)`, Co present as `Co^(3+)` and has `3d^(0)` configuration. In the presence of `NH_(3)`, the 3d-electrons pair up leaving two d-orbitals empty to the involved in `d^(2)sp^(3)` hybridization forming inner orbital complex in the case of `[Co(NH_(3))_(6)]^(3+)`.

In `[Ni(NH_(3))_(6)]^(2+)`, Ni is in +2 oxidation state and has `d^(8)` configuration.

Since, (n - 1) d-orbitals are not available but the nd-orbitals are used in bond formation i.e., in hybridization, the complex is called outer orbital complex.