[Ilaria]

I am studying the complex cis,fac-[RuCl₂(DMSO)₄], which has 3 DMSO molecules coordinated by the S atom and one by the O atom. Why is there π backdonation when DMSO is coordinated via S, while there isn't when the coordination is via O? In other words, why is DMSO-κS a π-acceptor ligand and why isn't DMSO-κO a π-acceptor ligand?

[kriggy]

I suggest comparing electron configurations of oxygen and sulphur

[critzz]

I think it can be explained in different ways, but here is one explanation involving electronegativity.
Since oxygen is more electronegative than sulfur, it likes to have a negative charge on oxygen rather than on the sulfur atom.
By π-back donation (or accepting electrons from the metal), the S-coordinated DMSO can delocalize electron density on the oxygen atom. (see picture)

[Ilaria]

Thank you for the explanation!
Now I have another question regarding the same complex: I've found in literature that the cis isomer is the facial one, but why couldn't it be the meridional?

[critzz]

From what I've read, the meridional product does exist; however, the facial product is the thermodynamic stable one.
The -S-DMSO is more steric demanding than the O-DMSO, so probably it has to do with sterics.

Here is the synthesis from what i've found.



DOI: 10.1021/cr0307291

[Ilaria]

Thank you, particularly for the article. I didn't know about it, even if the author is my teacher!