The complex ${\left[\mathrm{Ni}{\left({\mathrm{NH}}_3\right)}_6\right]}^{2+}$ involves 

• This type involves mixing of one ‘s’ orbital and three ‘p’ orbital’s of equal energy to give a new hybrid orbital known as sp³.
• Hybridization is defined as the concept of mixing two atomic orbitals with the same energy levels to give a degenerated new type of orbitals. This intermixing is based on quantum mechanics

${\mathrm{Ni}}^{2+}\left(3{\mathrm{d}}^8\right)$ ion in the given complex involves  ${\mathrm{sp}}^3{\mathrm{d}}^2$ hybridization