Abstract The phosphinidene bridged clusters FeCo2(CO)9(PMe) and FeCoWCp(CO)8(PM e) add the diphosphinomethane ligands dmpm and dppm with concomitant opening of two metal-metal bonds. The crystal structure determination of the addition product (Me2P-CH2-PMe2)FeCoWCp(CO)8(PMe) has shown that the Fe -W and Fe -Co bonds have been opened and the chelating diphosphine spans the Co -W bond. This is in contrast to the reaction with monodentate phosphines where the Co -W and Co -Fe bonds are broken. Upon heating the addition products eliminate two moles of CO with formation of the diphos phinom ethane substituted starting clusters, which in turn add CO under pressure to re-form the addition products.