HI

How can I explain that F- has a bigger field than I- (Δ bigger) using the ligand field theory ( so using the MO diagram and not the electrostatic theory)

Here is a MO diagram:

https://en.wikipedia.org/wiki/Ligand_field_theory#/media/File:LFTi(III).png2 case:

M(I)

_{6} ^{n-}MF

_{6} ^{n-}Now looking at the MO diagram when I pass from I

^{-} to F

^{-} the electronegativity increases...so the SALC for the 6 "s" F orbitals (the ones on the right in the diagram) are lower in energy than the Iodine one

WIth fluorine so there is a bigger difference between the 6 F salc s and the metal transition orbitals...so a bigger energy gap means a weaker interaction /overlap and so the stabilization energy of bonding MO is less and also the destabilization energy for the e*

_{g} is less .(they are lower in energy)

SO for fluorine I should have a small Δ because e*

_{g} is lower in energy (in comparision with iodine) and the t2g non bonding are also at the same energy...

BUt I know that in the spectrochemical series I have an opposite trend