Question

Which $d$-orbital is involved in $s p^3 d$ hybridization and why?

Answer 1

The d-orbital involved in $s p^3 d$ hybridization is $d_{z^2}$. The three planar triangular hybrid orbitals may be assumed to be formed from one $s$ - and two $p$-orbitals.

i.e., $s p^2$ hybridization and the remaining one $p$-orbital may combine with $d_{z^2}$ to form two axial hybrid orbitals that results in the formation of $s p^3 d$ hybridized orbitals.