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The electron density distribution in a transition-metal dimer containing a semibridging carbonyl is determined through experimental X-ray diffraction and quantum chemical computations. The changes occurring during the evolution from terminal to bridging coordinations are described by a "structure-correlation-like" approach and by a theoretical investigation along the conversion path. The smooth continuum of conformations observed in the solid state is explained in terms of the mutual interplay of direct M-M and M-CO and indirect M- - -M and M- - -C interactions, which can be characterized by interatomic delocalization indexes, within the framework of Quantum Theory of Atoms in Molecules.

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Pentacoordinate hydrogen atoms were identified by single-crystal neutron diffraction analysis of [N(CH3)4]3[H2Rh13(CO)24]. The hydrogen atoms are located in square pyramidal cavities of the Rh13 cluster, in positions almost coplanar with the Rh4 faces on the surface of the cluster. They are slightly displaced inward, toward the central rhodium atom of the cluster, with average H-Rh(central) and H-Rh(surface) distances of 1.84(2) and 1.97(2) angstroms, respectively. This result shows that hydrogen, which normally forms only one bond, can be attached to five other atoms simultaneously in a large metal cluster.

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The redox condensation of [Fe2(C0)'l2-with [Fe,N(CO),,] -yielded the cluster [Fe,N(CO), ,I3 -. Singlecrystal X-ray analysis showed it to possess an octahedral metal cage, with an interstitial nitride ligand. Under the D, idealized symmetry, all iron vertices are equivalent, being bound to one edge-bridging and two terminal carbonyls. The ion [Fe6N(CO)l,]3-can be oxidized to [Fe,N(~O),,]-or protonated to the hydridic dianion [Fe6H(N)(C0),,]2-. The molecular structure of the latter was determined, and is strikingly similar to that of the parent trianion. Small deformations of the ligand shell or elongations of the Fe-Fe distances are not sufficient to determine the location of the hydride. Electrochemical experiments were consistent with the chemical findings, showing that [Fe,N(CO), ,] -undergoes three irreversible one-electron oxidation steps, ultimately generating [Fe,N(CO),,] -. A lifetime of about 15 s was evaluated for the transient radical [Fe,N(CO), ,I2 -. Thermal activation induces substitution of one carbonyl ligand of [Fe,N(CO),,] -by PPh(C,H,FeC,H,),, yielding [Fe,N(CO), {PPh(C,H,FeC,H,),}] -, the molecular structure of which was also determined. The cluster adopts a butterfly arrangement of iron atoms, having an exposed u,-N atom and the phosphine ligand at a wingtip position. Cyclic voltammetry showed that communication between the two ferrocenyl units of the ligand PPh(C5H4FeCSH5), is rather low in the free state, and is notably improved by co-ordination to the tetrairon cluster. The lSN-labelled [Fe,N(CO), ,I3 -and [Fe,H(N)(CO), ,I2 -complexes were synthesized, and the NMR chemical shifts and IR bands of the interstitial p6-N ligands measured.