The reaction of Hoveyda's catalyst [Ru{η2-(CHAr)}(PCy3)Cl2] (1; Ar = o-O-i-Pr-C6H4) with the diphosphine PhCH2N(CH2PCy2)2 (2) gives the new air-stable green ruthenium carbene complex [Ru{η1-(CHAr)}{η2-(Cy2PCH2)2N(CH2Ph)}Cl2] (3A), in which 2 models a dendritic branch of poly(diphosphine) dendrimers DAB-dendr-[N(CH2PCy2)2]n (G1, n = 4; G2, n = 8; G3, n = 16; G4, n = 32). The complex 3A reversibly dimerizes in concentrated solution, a trend favored at low temperature. The structure of 3A was also confirmed by DFT calculations, which also establish the dimeric structure of 3B and the fact that the dimerization energy of 3A is small. Facile halide abstraction is shown by MALDI-TOF mass spectroscopy, and reaction with AgPF6 gives the air-stable green dicationic dimer 5, whose structure has been confirmed by DFT calculations and whose reactions with ligands (I- and DMSO) gives monomeric alkylidene complexes. The diiodo analogue of 3A, 7A, is also synthesized by addition of NaI to either 3A or 5 and dimerizes more readily than the dichloro analogue 3A. On the basis of this chemistry, metallodendrimers DAB-dendr-[PCy2CH2NCH2PCy2Ru(CHAr)(PPh3)(Cl)2]n (8−11) derived from the four first generations of DAB polyamines containing, respectively, 4, 8, 16, and 32 ruthenium branches have been synthesized and characterized by elemental and standard spectroscopic analysis. Dimerization of the ruthenium alkylidene species of these dendrimers is found to increase upon dilution, which is taken into account by intradendritic dimerization and larger extension of the branches, providing more freedom for dimerization in dilute solution. These dendritic ruthenium−carbene complexes are shown to initiate the ROMP of norbornene at room temperature to form star-shaped metallodendritic polymers. Interestingly, the metallodendrimer G1 initiates the ROMP of norbornene much faster than the model ruthenium complex 3, the overall rate order being G1 > G2 > G3 > model. The dramatic positive dendritic effect is rationalized in terms of the labilization of a ruthenium−phosphorus bond at each Ru within the dendrimers. Such a speculative dissociative metathesis mechanism (3A, 16e → 14e) would be in accord with the limited ROMP activity, the lack of RCM activity, the instability in air, and the DFT calculations showing that the interaction of 3A with ethylene is repulsive. The second dendritic effect, negative among the generations, is taken into account by the increasing bulk as the generation number increases, slowing down the approach of Ru by norbornene. Cleavage of the polynorbornene branches of these metallodendritic polymer stars using ethyl vinyl ether followed by SEC analysis shows that the observed masses are close to the theoretical ones, indicating that dendritic-star polymers have formed in the ROMP process.< Leer menos