Question

When does dz2 orbital not take part in hybridization?

Answer 1

Generally there is no proof, but if hybridization is such that we need 2 d orbitals then dx2-y2 and dz2 is considered.

While when there is condition of 3 d orbitals such that sp3d3 hybridization then we consider dxy, dyz, dzx.

These are purely a convention and assumption.

Hope this helps.