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New preparative methods for gold carbene complexes have been developed and older ones modified to prepare compounds with specific inherent properties for targeted applications. One of the important areas of application that has grown rapidly and wherein carbene complexes are increasingly significant falls within the field of medicine. Sophisticated theoretical calculations have accompanied many synthetic studies. These aspects are covered in this critical review (65 references).

A whole library of six-membered N-heterocyclic carbene complexes of Ni(ii) and Pd(ii) were prepared by oxidative substitution. In some of these compounds the heteroatom occurs in a position distant from the carbene donor atom. Combined structural and physical data, especially (13)C NMR results, indicate carbene character in such ligands. DFT quantum mechanical calculation at the RI-BP56/SVP level allowed the extraction of valuable chemical information predicting that rNHC (r = remote) ligands would bond more strongly than their nNHC (n = normal) isomers. This result is also corroborated by calculations on the metal complexes themselves. Orbital overlap (mainly sigma) follows the order N(2)HC(5) < nN(1)HC(6) < rN(1)HC(6) when ligands derived from halo-imidazolium and halo-pyridinium salts are compared. In C-C coupling catalysis using Pd(ii) and Ni(ii) complexes, the simple one-N, six-membered carbene complexes are superior to simple two-N, five-membered examples but clear differentiation between nNHC and rNHC precatalysts in the former family, is not always possible.

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