BIFFERENCE BETWEEN
Asume they are the same. They are not the same. With sp3d2, the s, p and d orbitals which are hybridized all come from the same energy level, for instance, it has been taught that when sulfur combines with six fluorine atoms to make SF6 that the 3s, 3p and two 3d orbitals hybridize to make the sp3d2 hybrid orbital set.
But d2sp3 is different. In this case the d-orbitals come from the n-1 energy level. Transition metals may exhibit d2sp3 hybridization where the d orbitals are from the 3d and the s and p orbitals are the 4s and 3d.
The bottom line is this, in sp3d2 hybridization all of the orbitals have the same principal quantum number. In d2sp3, the principle quantum number of the d orbitals is one less than the principal quantum numbers of the s and p orbitals. We see d2sp3 hybridization in the transitions metals and sp3d2 hybridization in the nonmetals.
There is one more issue. Chemists today are finding out that in compounds like SF6 there is no involvement of d-orbitals. In other words, there is no sp3d2 hybridization in SF6. A more likely explanation involves what is called "3-center, 4-electron" bonding in which three orbitals overlap axially (in a straight line) and contain a total of 4 electrons. This means that the 3 unhybridized p-orbitals of sulfur are all that is needed to make the six bonds with fluorine atoms.
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