The sterically encumbered complex [CpBIGFe(CO)2]2 (1) (CpBIG = pentakis(4-n-butylphenyl)cyclopentadienyl) forms in solution radicals, which enable the selective cleavage of a E–E single bond (E = P, As) in the cage molecules P4, As4, P4S3 and P4Se3, respectively, already at room temperature.