In acetonitrile solvent, Fe-5(CO)(15)(mu(5)-C), 1, reacts with Ni(COD)(2) at room temperature to afford the iron-nickel complex Fe5Ni(NCMe)(CO)(15)(mu(6)-C), 3. The acetonitrile ligand in 3 can be replaced by CO and NH3 to yield Fe5Ni(CO)(16)(mu(6)-C), 4, and Fe5Ni(NH3)(CO)(15)(mu(6)-C), 6, respectively. When refluxed in acetonitrile solvent, compound 3 loses a vertex to form the square pyramidal Fe4Ni complex Fe4Ni(NCMe)(2)(CO)(12)(mu(5)-C), 7. Compound 7 readily converts to Fe4Ni(NCMe)(CO)(13)(mu(5)-C), 8, by losing one of its acetonitrile ligands. Addition of acetonitrile to 8 gives compound 7. When heated to 110 degrees C under an atmosphere of CO, both compounds 7 and 8 furnish the octahedral Fe4Ni2 complex Fe4Ni2(CO)(15)(mu(6)-C), 9. All six compounds were structurally characterized by single-crystal X-ray diffraction analyses.