An evaluation of the CO releasing ability of iron(II) and molybdenum(II) complexes has facilitated the discovery of the most rapid CO releaser, namely [Mo(CO)3(η5-C5H5)(η1-{O}–C{O}–O–CMeCH-COMeCBr)]BF4 (CORM-F10), reported to date. The rate of CO release is related to the overall solution phase stability of the transition metal carbonyl complex. The cytotoxity and vasodilatory properties of CORM-F10 have been determined.