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The α-diimine ligand ArNC(Me)C(Me)NAr (Ar = 2,6-C6H3-iPr2) reacts with Pd(OAc)2 in the presence of HBF4·Et2O, or [Pd(MeCN)4](BF4)2 in the absence of acid, to give ethylene polymerization catalysts. The reactions of a related β-diimine ligand with [Pd(MeCN)4](BF4)2 and (1,2-dimethoxyethane)NiBr2, and the polymerization activity of catalysts derived from it, are discussed.

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The synthesis, characterization and X-ray crystallographic structure determination are described for a stable thiazol-2-ylidene 2. This thiazol-2-ylidene is the first example of a stable, crystalline carbene in which the singlet carbene center bears a sulfur substituent. The carbene 2 is the closest stable analog of the important thiamin (vitamin B,) carbene. Although the thiazol-2-ylidene 2 is sufficiently stable to isolate at room temperature in the absence of moisture and oxygen, it will dimerize to form a normal carbene-carbene dimer (2}*. With the isolation and structure determinations of both 2 and ( 2 4 , this system is the first in which a stable carbene and its corresponding dimer have been isolated and characterized. Additionally reported is the "reduced" 2H-thiazoline 2 . H2 in which the former carbene center in 2 has been reduced to a methylene (CH,) group. Thiazol-2-ylidenes with smaller groups on nitrogen (e.g. mesityl or methyl) are too unstable to allow easy isolation. The dimer from 3,4,5-trimethylthiazol-2-ylidene possesses a very unusual geometry in which one of the former carbene centers is planar while the second is pyramidal.

The air-stable crystalline carbene 1,3-dimesityl-4,5-dichloroimidazol-2-ylidene (2) is produced from the chlorination of 1,3-dimesityimidazol-2-ylidene (1) by 2 equiv of carbon tetrachloride. The physical and spectroscopic properties of the carbene are reported. The features which contribute to the exceptional stability of the carbene are discussed. Further reaction of 2 with carbon tetrachloride leads to reduction of CCl 4 to dichlorocarbene. The formation of the mixed carbene "dimers" (olefins) from in situ generated dichlorocarbene and various imidazol(in)-2-ylidenes is also reported. The tellurones derived from 1 and 2 are synthesized and compared.

The syntheses, characterizations, and X-ray crystallographic structure determinations are described for adducts of stable nucleophilic carbenes with pnictinidenes. The adducts between 1,3-dimesitylimidazol-2-ylidene and phenylphosphinidene, phenylarsinidene, (trifluoromethyl)phosphinidene, and (pentafluorophenyl)arsinidene are reported. These carbene-pnictinidene adducts are formed by the direct reaction of a stable nucleophilic carbene with the corresponding pnictinidene cyclic oligomers. The synthesis and structure of the adduct between 1,3-dimesitylimidazolin-2-ylidene and phenylphosphinidene from the reaction of 1,3-dimesitylimidazolin-2-ylidene with phenylphosphorus dichloride are also reported. These carbene-pnictinidene adducts possess strongly polarized pnictinidene-carbene bonds. The C-Pn-C angles are all typically small at 97-102 degrees, and there is only a 4% shortening of the nominal Pn=C double bond compared to the Pn-C single bond to the second substituent on the pnictogen. The (31)P NMR shifts of the phosphorus adducts suggest strongly shielded phosphorus centers in accord with the polarized structures.