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Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: xshadow on January 25, 2021, 05:35:47 AM

Title: explain why F- has a bigger field than I- (ligand field theory)
Post by: xshadow on January 25, 2021, 05:35:47 AM
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://lh3.googleusercontent.com/proxy/SXbQ8rtl02ZvlCM-vJtSy8129TeY-csFpXQDGD4rHQMqbeKJ49cmWtUTu-bd6VojOL3wmKB1m-tSF4zzFDc4xHdpugPgJf8)
https://en.wikipedia.org/wiki/Ligand_field_theory#/media/File:LFTi(III).png



2 case:
M(I)6  n-
MF6  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  ???
Title: Re: explain why F- has a bigger field than I- (ligand field theory)
Post by: Orcio_87 on March 14, 2021, 02:40:45 PM
F- and I- are σ and π donors. π because two p filled orbitals (2p in F- and 5p in I-) share electrons with empty d metal orbitals (orbitals dxy, dxz and dyz). But they are still σ donors because one of they p orbital will be directed towards dx2-y2 or dz2 orbitals.

F- is small electronegative ion so σ bonding will be stronger (I think that size of 2p orbitals is also important).

In turn I- gives good π bonding and that is why gap between dxy, dxz, dyz and dx2-y2, dz2 is smaller.