A quick search gave me this Supplementary Data (http://www.rsc.org/suppdata/cc/c2/c2cc33998f/c2cc33998f.pdf), which lists 3 papers as giving the 4.80eV value:

(a) M. Kumada and K. Tamao, Adv. Organomet. Chem.1968, 6, 19. (b) J. Pommerehne, H. Vestweber, W. Guss, R. F. Mahrt, H. Bassler, M. Porsch and J. Daub, Adv. Mater. 1995, 7, 551. (c) B. W. D. Andrade, S. Datta, S. R. Forrest, P. Djurovich, E. Polikarpov and M. E. Thompson, Org. Electron. 2005, 6, 11.

Mr. Filippi,

You can extract this information from the data and references given as attachment, comparing complexes {[FeIII(C5H5)2]+} and {[FeII(C5H5)2]0}, respectively.

Thank you :-)