Stabilized by “carbene” donor ligands, the Pd complex 1 catalyzes the Heck olefination of aryl halides unexpectedly efficiently and yet has long‐term stability at elevated temperatures. The active Pd0 species can be generated during the Heck reaction or deliberately prepared by reduction of 1 with, for instance, hydrazine or sodium formate. Another similar catalyst can be synthesized in situ from Pd0 complexes and 1,3‐dimethyldihydroimidazoline‐2‐ylidene.

Silicone nanofilaments (see figure) are grown by a simple chemical vapor deposition method on different substrate materials. The filaments are flexible, and have lengths of up to several micrometers and diameters of up to 150 nm. The dense and entangled arrangement of these filaments yields a superhydrophobic coating that is also optically transparent and antireflective.

Stable mono-and dicarbene adducts of late transition metals are readily accessible either by reaction of imidazolium salts with metal complexes bearing basic ligands, or by the cleavage of chloroand acetato-bridged dinuclear metal complexes with the free carbenes (e.g., 1,3-dimethylimidazoline-2-ylidene) . A general novel method for the deprotonation of N-substituted azolium salts in liquid ammonia-the liquid ammonia route-is described. This method yields not only the known free monomeric 1,3-dimethylirnidazoline-2-ylidene in quantitative yield, but also otherwise hardly accessible derivatives. For example, imidazoline-2-ylidenes with linear, branched, cyclic, heteroatom-substituted and even chiral hydrocarbon residues can be obtained. The nucleophilic behaviour of 1,3-dimethylimidazoline-2-ylidene is reported and compared with that of other donor ligands. Novel carbene complexes of Ru", Rh', Pd", 0s" and Ir' are presented. Reaction of the potentially chelating ligand 1,1'-(1,2-ethyIene)-3,3'-dimethyldiKeywords carbenes -Group 8-10 complexesheterocycles * structure elucidation imidazoline-2,2-diylidene with [(COD)-RhCI], yields a dinuclear complex, in which two rhodium centres are linked by the dicarbene bridge. Four single-crystal X-ray diffraction structures of new metalcarbene complexes (Rh, 0s) are reported. N-Heterocyclic carbene complexes of Group 8-10 transition metals are both thermally and chemically very stable. They do not show the typical reactivity of metal-carbon double bonds. For a number of reasons, these complexes must be regarded as donor adducts of the Lewisbasic imidazoline-2-ylidene ligand and the Lewis-acidic organometallic fragment.

To date the only free carbenes of the imidazoline-2-ylidene type to have been described in the literature are those bearing simple hydrocarbon or haloalkyl and -aryl substituents. We report here a novel, versatile and high-yielding method for the synthesis of N-heterocyclic carbenes in a mixture of liquid ammonia and aprotic organic solvents. Deprotonation of the imidazolium precursor salts proceeds under mild conditions within a few minutes at temperatures below -30 "C, and side reactions are thus avoided. The imidazolium salts are much more soluble in organic solvents if liquid ammonia is added. Furthermore, the acidity of the C-2 protons appears to be enhanced by hydrogen bonding. Not only are the known free ylidenes conveniently and quantitatively accessible by this procedure, but also novel functionalized derivatives that are not accessible by known procedures. Imidazoline-2-ylidenes with linear, branched, cyclic, heteroatom-substituted (0, N, P) and chiral hydrocarbon residues are accessible through the novel route. Stable carbene-metal "adducts" are conveniently obtained by treating the free carbenes with chloro-or acetato-bridged dinuclear metal complexes, or by displacement of coordinated ligands such as carbon monoxide, THF or acetonitrile by the free carbenes. The syntheses of novel imidazolium salts, N-heterocyclic carbenes and carbene adducts of Ru", Rh', Wo and sulfur are reported, and the structures of five products analysed by single-crystal X-ray diffraction. N-Heterocyclic carbenes bearing functionalized side chains are important because a number of these complexes show excellent activity in catalytic reactions. They do not show the typical reactivity of metal-carbon "double bonds" and are remarkably stable both thermally and chemically. For a number of reasons, they are best viewed as donor adducts of the highly Lewis basic imidazoline-2-ylidene ligands and the Lewis acidic organometallic fragments. The new synthetic procedure reported here makes Nheterocylic carbenes a generally accessible class of useful ligands in coordination chemistry and catalysis.

COMMUNICATIONSon tantalum or zirconium were used as molecular precursors. In these cases, however, alkyl groups are detached during the grafting process as a consequence of prot~Iysis.'~] In our case, all alkyl groups first remain at the metal since the acidic surface silanols attack the nitridomolybdenum moiety, whereas only in the second step an I-elimination occurs, yielding the corresponding alkylidene species. Further work in this area is in progress.