There is a vast literature of EPR of transition metal complexes. See the books by Abragam and Bleaney, by Mabbs and Collison and by Pilbrow for leading references to the older literature. For more recent articles, search SciFinder.

Hello.

Particularly there is a book by Wilfred Hagen, Biomolecular EPR Spectroscopy, which contains very useful insights, examples and hints. Generally speaking (there may be many exceptions) if you have high spin species you will surely need liquid N2 or liquid He temperatures. High spin Co(II) (S = 3/2) needs liquid He. High spin Fe(III) (S = 5/2) is very variable and you can see it sometimes at even 100-140 K. Low spin Co(II) and Fe(III) can be seen at liquid N2 or sometimes higher temperatures. Ru(III) I´m not sure, but I think also can be seen at liquid N2 temperatures, or even higher.

Hope this helps