What is a good (small) model system for ligand metal charge transfer practises?

Question

I am looking for an easy to calculate model system in which I could see a ligand metal charge transfer. Basically I am looking for a model complex, that might have the metal/ligand in different oxidation states: $\ce{ M^{III}(Lig)}$ versus $\ce{M^{II}(Lig^{\bullet +})}$ or $\ce{M^{IV}(Lig^{\bullet -})}$. I am calling it a charge transfer since I am not sure how else to call it.
I am also quite aware, that the description of these systems is necessarily a multireference case, so ordinary DFT methods will fail. (At least broken symmetry is necessary.)

In principle I was thinking about an iron heme complex that might exhibit that feature.

However, I have the feeling, that this kind of system is already pretty demanding to calculate the electronic structure. Since I would like to use it as an exercise and for a tutorial achieving alternative occupations I would prefer a much simpler case. It should eventually treated with broken symmetry DFT and CAS/MR methods.

Does anyone know of such a system or would suggest one. It does not have to be a real one. Purely hypothetical is just as fine.

Answer 1

AFAIK, permanganate ion is colored because of intense line, corresponding to oxygene-manganese charge transfer. This is noted in wikipedia article on charge-transer complex, but, unfortunately, without quote, and I'm unable to come with one right now

Edit: Aha. Here there is a MO diagram on $\ce{MnO4-}$ ion and extensive discussion of (mis)understanding of its spectra, including statement I made above.

Answer 2

you can try $\ce{Cu^{2+}--Cys}$ vs $\ce{Cu^+--Cys}$, much smaller and it is a model of $\ce{Cu-S}$ clusters in eg $\ce{Cu}$-binding proteins

I'm interested in these systems too, maybe we can share information?

Goedele