text: Imidazol-2-ylidenes, triazol-5-ylidenes (and less so, diaminocarbenes) have been shown to coordinate to a plethora of elements, from alkali metals, main group elements, transition metals and even lanthanides and actinides. A periodic table of elements gives some idea of the complexes which have been prepared, and in many cases these have been identified by single crystal X-ray crystallography.
  Stable carbenes are believed to behave in a similar fashion to organophosphines in their coordination properties to metals. These ligands are said to be good σ-donors through the carbenic lone pair, but poor π-acceptors due to internal ligand back-donation from the nitrogen atoms adjacent to the carbene centre, and so are able to coordinate to even relatively electron deficient metals. Enders  and Hermann have shown that these carbenes are suitable replacements for phosphine ligands in several catalytic cycles. Whilst they have found that these ligands do not activate the metal catalyst as much as phosphine ligands they often result in more robust catalysts. Several catalytic systems have been looked into by Hermann and Enders, using catalysts containing imidazole and triazole carbene ligands, with moderate success. Grubbs  has reported replacing a phosphine ligand (PCy) with an imidazol-2-ylidene in the olefin metathesis catalyst RuCl(PCy)CHPh, and noted increased ring closing metathesis as well as exhibiting "a remarkable air and water stability". Molecules containing two and three carbene moieties have been prepared as potential bidentate and tridentate carbene ligands.