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