The hybridisation of coordination compounds is based on valence bond theory.. if the metal in the coordinate sphere is linked to strong ligands (for e.g. CN-) then the electrons of (n-1)d orbital pair up against hund''''s rule and the vacant spaces therefore formed is used up to combine with the ligands.. this is d3sp3 coz the inner d orbital participates..On the other hand if the ligand is weak (for e.g. F-) then the electrons of (n-1)d orbital of the metal does not pair up..this is sp3d2 coz outer d orbital will participate in bond formation with the ligands...