The radiochemical yields of [Cp50Fe(CO2]• in neutron-irradiated [CpFe(CO)2]2 and [CpFe(CO)2]I are 6.9 and 14.8%, respectively. Other prominent products are [CpFe(CO)2]2-59Fe, 59Fe(CO)5, and 59FeCp2, in both target compounds.Thermal treatment at 75 °C increases the yields of 59Fe(CO)5, and of the radioactive target compound in both cases, and does not affect the yields of the other two products. The radioactive radical [Cp59Fe(CO)2]• is stable to 75° in the solid [CpFe(CO)2]2, and on dissolution is rapidly scavenged by iodine transfer from carrier [CpFe(CO)2]I.It is suggested that complex molecular products are not reformed following the nuclear activation, but are rather preserved during and despite the nuclear reaction. An effective "hot zone" need not contain more than two or three molecules.Production of [Cp59Fe(CO)2]• and [Cp59Fe(CO)2]2 is observed in neutron irradiated solutions of Fe(CO)5 and benzene with either FeCp2 or C5H6.