CHARACTERIZATION, REDOX PROPERTIES AND STRUCTURES OF THE IRON NITRIDOCARBONYL CLUSTERS [FE4N(CO)(11)(PPH(C5H4FEC5H5)(2))](-), [FE6N(CO)(15)](3-) AND [FE6H(N)(CO)(15)](2-)

The redox condensation of [Fe-2(CO)(8)](2-) with [Fe4N(CO)(12)](-) yie lded the cluster [Fe6N(CO)(15)](3-). Single-crystal X-ray analysis sho wed it to possess an octahedral metal cage, with an interstitial nitri de ligand. Under the D-3 idealized symmetry, all iron vertices are equ ivalent, being bound to one edge-bridging and two terminal carbonyls. The ion [Fe6N(CO)(15)](3-) can be oxidized to [Fe5N(CO)(14)](-) or pro tonated to the hydridic dianion [Fe6H(N)(CO)(15)](2-). The molecular s tructure of the latter was determined, and is strikingly similar to th at of the parent trianion. Small deformations of the ligand shell or e longations of the Fe-Fe distances are not sufficient to determine the location of the hydride. Electrochemical experiments were consistent w ith the chemical findings, showing that [Fe6N(CO)(15)](3-) undergoes t hree irreversible one-electron oxidation steps, ultimately generating [Fe5N(CO)(14)](-). A lifetime of about 15 s was evaluated for the tran sient radical [Fe6N(CO)(15)](2-). Thermal activation induces substitut ion of one carbonyl ligand of [Fe4N(CO)(12)](-) by also determined. Th e cluster adopts a butterfly arrangement of iron atoms, having an expo sed u(4)-N atom and the phosphine ligand at a wingtip position. Cyclic voltammetry showed that communication between the two ferrocenyl unit s of the ligand PPh(C5H4FeC5H5)(2) is rather low in the free state, an d is notably improved by co-ordination to the tetrairon cluster. The N -15-labelled [Fe6N(CO)(15)](3-) and [Fe6H(N)(CO)(15)](2-) complexes we re synthesized, and the NMR chemical shifts and IR bands of the inters titial mu(6)-N ligands measured.