IRON-RHODIUM AND IRON-IRIDIUM MIXED-METAL NITRIDO-CARBONYL CLUSTERS -SYNTHESIS, CHARACTERIZATION, REDOX PROPERTIES, AND SOLID-STATE STRUCTURE OF THE OCTAHEDRAL CLUSTERS [FE5RHN(CO)(15)](2-), [FE5IRN(CO)(15)](2-), AND [FE4RH2N(CO)(15)](-) - INFRARED AND NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPIC STUDIES ON THE INTERSTITIAL NITRIDE

The cluster [Fe5RhN(CO)(15)](2-) was synthesized in 40% yield from [Fe 4N(CO)(12)](-) and [Rh(CO)(4)](-) in refluxing tetrahydrofuran, wherea s the analogous anion [Fe5IrN(CO)(15)](2-) was prepared in CH3CN at ro om temperature from [Fe6N(CO)(15)](3-) and [IR(C8H14)(2)Cl](2); the yi elds are higher than 60%. The monoanion [Fe4Rh2N(CO)(15)](-) was obtai ned in 70% yield from [Fe5RhN(CO)(15)](2-) and hydrated RhCl3. The sol id-state structures of the three anions were determined on their [PPh4 ](+) salts: the six metal atoms are arranged in octahedral cages and a re 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 center s. The two rhodium atoms in [Fe4Rh2N(CO)(15)](-) are directly bound. T he N-15 NMR spectra of the three compounds have been recorded; the sig nals of the nitride ligands were found at delta = 514 ppm for the dian ions 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 arrangeme nt, subsequently modified by the perturbations given by different atom ic masses and M-M stretching force constants. The motions of the nitro gen are related to the idealized symmetry of the cage; the M-N force c onstant values depend on the type of metal and on the charge of the an ion. The dianions [Fe5MN(CO)(15)](2-) can be electrochemically oxidize d at -20 degrees C to their short-lived monoanions, which can be chara cterized by EPR spectroscopy. In contrast, the cluster [Fe4Rh2N(CO)(15 )](-) undergoes a single-step 2-electron reduction to the partially st able trianion [Fe4Rh2N(CO)(15)](3-), which was also characterized by E PR spectroscopy. The Fe-Rh nitride clusters are active catalysts for t he hydroformylation of l-pentene, but display low selectivity (35-65%) in n-hexanal and are demolished under catalytic conditions.