In coordination compounds , how to decide whether itss hybridisation will be sp3d2 or d2sp3.. itss based on inner or outer orbital complex but how will we got to know that for every compound ..plzz explain..??

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 thus 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... 

Approved

In sp3d2 all s,p and d orbitals are from the same energy level. But in d2sp3, d orbital is from (n-1)th energy level

The difference between sp3d2 and d2sp3 hybrids
lies in the principal quantum number of the d
orbital.
In sp3d2 hybrids, the s, p, and d orbitals have the same
principal quantum number—High Spin.
In d2sp3 hybrids, the principal quantum number of the d
orbitals is one less than s and p orbitals—Low Spin.

To know for all compounds refer

http://www.google.co.in/url?q=http://www.chemistry.mtu.edu/pages/courses/files/ch1120-srsylves/Chapter%252020.pdf&sa=U&ei=GnwTUajyAsvtrQeRz4CQDw&ved=0CCIQFjAD&usg=AFQjCNFvZz9u09e-03rFiFs4eYoTyAoPWQ