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The N-heterocyclic carbene (NHC) complexes [(RC 5 H 4 )Ni-(X)(NHC)] (2-5) have been prepared by treating nickelocene [or 1,1Ј-bis(alkenyl)nickelocene] with a suitable carbene precursor. The alkenylcyclopentadienido complexes 4 and 5 undergo chemoselective cross-metathesis with methyl acrylate or methyl vinyl ketone in the presence of the seocnd-generation Grubbs catalyst to yield complexes 6-8, which bear an α,β-unsaturated carbonyl substituent on the cyclopentadienido ligand. The X-ray crystal structure of 2 [monoclinic,

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A new hexanuclear cyclopentadienylnickel carbide cluster (NiCp)(6)(μ(6)-C) (1) was obtained through the thermolysis of the alkene complex [NiCp(CH(3))(η(2)-CH(2)═CHC(4)H(9))] (4). The X-ray molecular structure of 1 (monoclinic; P2(1)/c; Ni-C(carbide) = 1.767(4)-2.109(4) Å) reveals a highly deformed octahedral arrangement of nickel atoms with two octahedron edges opened (Ni-Ni bonding distances = 2.410(1)-2.623(1) Å, Ni···Ni nonbonding distances = 3.107(2) and 3.108(2) Å). Cluster 1 is the first example of a homoleptic, cyclopentadienylnickel carbide cluster. Moreover, (13)C-labeling studies proved that the carbido ligand in cluster 1 originated from the Ni-bound methyl group. This transformation requires a triple C-H bond activation in the methyl group, which has not been observed so far for late transition metal compounds.

Following previous reports on the synthesis and structure of different nickelacyclic complexes analogues of nickelocene, we now deal with the new metallacyclic compounds (eta5-R-cyclopentadienyl){eta4-[1-(eta5-R-cyclopentadienyl)]-2,3,4,5-tetraphenyl-1-nickela-2-cyclopentenyl}nickel (R = H, CH3). The redox ability of the whole series of nickelacyclic derivatives has been also investigated by electrochemical and spectroelectrochemical techniques, and the nature of the frontier orbitals responsible for the rich electron transfer activity of this class of compounds has been supported by theoretical considerations. On the basis of the redox properties of a few neutral members of the series, their chemical oxidation afforded the corresponding monocations and the crystal structures of the pertinent redox couples were determined by X-ray single-crystal analysis.