New CO-releasing molecules, CO-RMs, based on indenyl iron carbonyls have been identified. Half-lives for carbon monoxide (CO) release, (1)H NMR, (13)C NMR, IR, mass spectra, elemental analysis and biological data have been determined for the compounds. Limited correlations have been made between half-lives and ΔG(‡) for CO release and spectroscopic parameters, ν(CO) and δ((13)CO). X-ray structures have been determined for [Fe(η(5)-C(9)H(7))(CO)(2)L][BF(4)] where L is CO, NCMe, PPh(3), P(OPh)(3), NC(5)H(5) or 1-methylimidazole. Improved preparations of [Fe(η(5)-C(9)H(7))(CO)(2)](2) and [Fe(η(5)-C(9)H(7))(CO)(3)][BF(4)] are reported.