DFT calculations using the B3LYP functional support the suggestion that the [(terpy)(H2O)Mn-IV(mu-O)(2)Mn-III(H2O)-(terpy)](3+) (terpy=2,2':6,2"-terpyridine) complex functions as a synthetic O-2 catalyst. The calculated barrier for O-O bond formation with water is 23 kcal/mol. In this complex, as well as in models of the oxygen evolving complex in PSII, the active species is a Mn-IV-oxyl radical. From comparisons with inactive Mn-V-oxo complexes, it is proposed that radical formation is actually a requirement for O-2 formation activity in Mn-complexes.