MULTIPLE BONDS BETWEEN MAIN-GROUP ELEMENTS AND TRANSITION-METALS .126. HETEROCYCLIC CARBENES AS PHOSPHANE-ANALOG LIGANDS IN METAL-COMPLEXES

Carbenes L derived from imidazole and triazole, as generated from the corresponding azolium precursor salts, smoothly react with simple metal carbonyls such as M(CO)6 (M = Cr, Mo, W), Fe(CO)5, and Ni(CO)4 to give the substitution products M(CO)5L, Fe(CO)4L, Ni(CO)3L, and Ni(CO)2L2, respectively, owing to the pronounced C-nucleophilicity of the free carbenes L. Dicarbenes L L form chelate complexes of type M(CO)4(LL) upon treatment with M(CO)6 (M = Cr, Mo, W). The new bis(carbene)nickel(O) complex Ni(CO)2L2 and the first member of a metal complex M(CO)4(LL) exhibiting a chelating dicarbene of the triazole series have been characterized by single-crystal X-ray diffraction. It is evident from the present study that heterocyclic mono- and (chelating) dicarbenes of the aza-type are congeners of phosphanes and diphosphanes, resp., with regard to their metal-coordination chemistry. Both in terms of ligand properties and metal complex synthesis, they closely resemble electron-rich phosphanes, e.g. trimethylphosphane.