The cluster [Fe5RhN(CO)(15)](2-) was synthesized in 40% yield from [Fe4N(CO)(12)](-) and [Rh(CO)(4)](-) in refluxing tetrahydrofuran, whereas the analogous anion [Fe5IrN(CO)(15)](2-) was prepared in CH3CN at room temperature from [Fe6N(CO)(15)](3-) and [IR(C8H14)(2)Cl](2); the yields are higher than 60%. The monoanion [Fe4Rh2N(CO)(15)](-) was obtained in 70% yield from [Fe5RhN(CO)(15)](2-) and hydrated RhCl3. The solid-state structures of the three anions were determined on their [PPh4](+) salts: the six metal atoms are arranged in octahedral cages and are coordinated to 3 edge-bridging and 12 terminal carbonyl ligands and to a mu(6)-N ligand. The Rh and Ir atoms have less terminal COs than Fe, in order to equalize the excess electrons at the d(9) metal centers. The two rhodium atoms in [Fe4Rh2N(CO)(15)](-) are directly bound. The N-15 NMR spectra of the three compounds have been recorded; the signals of the nitride ligands were found at delta = 514 ppm for the dianions and 470 ppm for [Fe4Rh2N(CO)(15)](-); any group 9 atom shifts the resonance of nitrogen to higher fields. The coupling constants J(N-15-Rh-103) are 8-9 Hz. The vibrational patterns of the metal cores have been interpreted on the basis of an idealized M-6 octahedral arrangement, subsequently modified by the perturbations given by different atomic masses and M-M stretching force constants. The motions of the nitrogen are related to the idealized symmetry of the cage; the M-N force constant values depend on the type of metal and on the charge of the anion. The dianions [Fe5MN(CO)(15)](2-) can be electrochemically oxidized at -20 degrees C to their short-lived monoanions, which can be characterized by EPR spectroscopy. In contrast, the cluster [Fe4Rh2N(CO)(15)](-) undergoes a single-step 2-electron reduction to the partially stable trianion [Fe4Rh2N(CO)(15)](3-), which was also characterized by EPR spectroscopy. The Fe-Rh nitride clusters are active catalysts for the hydroformylation of l-pentene, but display low selectivity (35-65%) in n-hexanal and are demolished under catalytic conditions.