[anna.kolinski]

Hi everyone,

I have a (meridonial) octahedral structure with 3 fluorine ligands, and 3 other ligands that are different. One of the different ligands is a P ligand. How do I know which Fluorine ligand will be coupled to first by the P ligand?


I think the P ligand will first couple to the Fl ligands that are at a 90 degree angle to it, and then it will couple to the Fl ligand that is at a 180 degree angle to it, but I'm not sure if that's right.

Any help/ direction much appreciated

Thank you

[Babcock_Hall]

What do you mean couple first?  I don't think of coupling in that way.  Do you mean which coupling constant will be largest?

[anna.kolinski]

For example if P couples to the 2 CO ligands which are at a 90 degree angle to it, it will form a triplet.

If it then couples to the CO ligand which is at a 180 degree angle to it, it will split the triplet into a triplet of doublets.

I hope this makes sense! Thank you for your comment

[Babcock_Hall]

http://folk.ntnu.no/audunfor/7.%20semester/Spektro%20VK/NMR/Nyttige%20artikler/notes-5-hmr-3-coupling.pdf

Scroll down to the section Simple Multiplets.  The pattern one sees is dependent on the relative magnitudes of the two coupling constants.

[anna.kolinski]

Oh I see... In that case surely each CO ligand will have the same coupling constant? Then since the other 3 ligands are unique, each CO environment is unique as they are in a meridonial arrangement, and so 3 separate doublets will be seen?

Thank you very much for your reply

[anna.kolinski]

After further research could it be that maybe trans coupling constant is greater than cis coupling constant and therefore a triplet will be seen for P coupling to the 2 trans CO ligands, and then this triplet will be split into a triplet of doublets by the cis CO ligand?

Sorry for the spam and confusion!

[Babcock_Hall]

I don't know which coupling constant will be larger.  As far as I am aware, people use triplet of doublets as a synonym for doublet of triplets, regardless of which coupling constant is larger.  The size just changes the appearance.  If you look at the two examples from my link, the one on the left has a larger doublet coupling constant, and the one on the right has a larger triplet coupling constant.