The coordinatively unsaturated complex [Fe 2 (CO) 5 (μ-PBu t 2 )(μ-PCy 2 )] (Fe Fe) (1) adds spontaneously dppm [dppm = bis(diphenylphosphino)methane] and yields the saturated compound [Fe 2 (CO) 5 (μ-PBu t 2 )(μ-PCy 2 )(dppm-P)] (2). Compounds1 and 2 exist in a reversible thermal equilibrium; heating 2 in toluene produces 1 together with the free ligand dppm, whereas by cooling to room temperature 2 is recovered. A conversion of 2 into the known complex [Fe 2 (CO) 3 (μ-PBu t 2 )(μ-PCy 2 )(μ-dppm)] (3) is only possible by photochemical means. Compound 3 is readily transformed back into 2 by carbon monoxide under normal conditions. The molecular structure of 2 determined by X-ray analysis is described.