I have studied coordination chemistry as part of my school curriculum and we covered Crystal Field Theory as our final theory, and the book says that Ligand Field Theory and Molecular Orbital Theory are used to explain the limitations of CFT but are beyond our scope of learning.

One of the limitations of CFT according to the book is that if the ligands are point charges then it should follow that an anionic ligand should exert more splitting effect but that is not the case as we see in the spectro-chemical series.

A good example would be $\ce{S^2-}$ < $\ce{F-}$ < $\mathrm{EDTA^{4-}}$ < $\ce{CO}$

Also we can see that size is not the determining factor as then $\mathrm{EDTA^{4-}}$ wouldn't be so high as compared to $\ce{F-}$

Is there some intuitive explanation or logic for this which I can follow with my current knowledge till CFT without going too deep into MOT or LFT?

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    $\begingroup$ One of the major flaws in your assumption is that you are using absolute negative charge as the cause for higher splitting. The truth is, it is not the absolute charge, but the overall charge density of the approaching ligand the determines the splitting. Ligands with higher charge density will often cause more splitting than those with low charge density. That explains the positioning of sulphide and fluoride. Chelating ligands also in general, cause more splitting. CO is a special case where synergic bonding strengthens the metal to ligand bond, and hence causes even more splitting $\endgroup$ – Yusuf Hasan Jul 30 '20 at 18:02
  • $\begingroup$ Thanks for the explanation if you can write that as an answer it would be better @YusufHasan $\endgroup$ – FoundABetterName Jul 30 '20 at 18:33
  • $\begingroup$ It would be good if you can add some data showing the variation in different cases you mentioned. Also is this the only factor if not what other factors come to play?@YusufHasan $\endgroup$ – FoundABetterName Jul 30 '20 at 18:39
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    $\begingroup$ You already know the answer to your question, as evidenced by your first paragraph. I don't think there's much of a shortcut to it. The charge density thing only accounts for a portion of the spectrochemical series. The rest can only be explained by MO theory. $\endgroup$ – orthocresol Aug 2 '20 at 9:30
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    $\begingroup$ It's a start. The polyatomic case is a bit more complicated. But if you keep at it I'm sure one day you'll reach it. Suffice it to say that things like CO, CN-, PPh3 etc have strong metal-ligand covalent bonding which is best described using MOT. $\endgroup$ – orthocresol Aug 2 '20 at 13:07

Crystal Field Theory assumes that all of the interactions occuring are purely electrostatic and only concern d-orbitals. This is clearly not true when dealing with some ligands which create a covalent bond.

Most limitations of CFT all stem from the inability to explain non-electrostatic interactions. For this reason, it works well when the electrostatic aspect is the dominant factor but it completely fails for some cases such as neutral CO ligands. While you could easily argue electrostatic forces still exist in this situation, clearly they are not dominant and do not explain any observed behavior while still having an effect.


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