The $\mathrm{d}-$ orbital involved in ${\mathrm{sp}}^3\mathrm{d}$  hybridisation is 

The linear combination of a 4s−orbital with three 4p−orbitals (4p_x, 4p_y, and 4p_z) and one 3d−orbital $(3{\mathrm{d}}_{{\mathrm{Z}}^2})$ results in five sp³d-orbitals.