for [Co(NH3)6]^(+3) in many books hybridisation is given as d2sp3. this is correct.

but for compound [Fe(NH3)6]^(+2) the hybridisation is given as sp3d2. WHY?

Both Co3+ and Fe 2+ are d^6 confguration. and nh3 is strong field ligand. so it should also form inner spin complex in fe2+ compound.

PLEASE HELP. I WILL BE VERY GRATEFUL OF YOU.....

for hybridisation it does not depend fully on the ligand tendency. it also depends the behaviour of the compound i.e paramag. and diamag. actually considering properties like this the series is formed for ligands. as fe[NH6](+2) is paramagnetic in nature thus sp3d2 is most appropriate for it. the series is experimental. eg. check for [MnBr4](2-) mag. moment=5.59 (5 unpaired electrons) as dsp2 or sp3 ans- sp3 and for [MnBr6](4-) u= 5.59 it is sp3d2

NH3 is a weak filled Ligand, but look at the charges present on Fe and Co. If Co is in +2 charge then obviously it shows sp3d2 hybridization but since the charge on central metal atom increases so tendency of ligand towards central metal atom increases thus it splits the d-orbital more and acts as strong filled ligand, energy differences between t2g and eg increases so pairing of electron take place in t2g first so eg orbitals remains vaccant thus its hybridisation is d2sp3.