N-heterocyclic carbenes (NHC) are excellent ligands for late transition metals particularly Au(I).  Their strong sigma donation, ease of preparation and simple functionalization make NHC ligands ideal molecules for stabilizing extended metal-metal assemblies.  Here, we report the preparation, coordination chemistry, and optical properties of several amino-, pyridyl- and picolyl-substituted imidazolium-based NHC ligands with Au(I) and Ag(I).  Simple reaction of the imidazolium salt and Ag₂O readily produces the homoleptic [Ag(NHC)₂]⁺ complex which cleanly reacts with Au(SC₄H₈)Cl in CH₂Cl₂ to form the corresponding Au(I) complex.  Addition of one equivalent of AgBF₄ in a coordinating solvent such as acetonitrile, benzonitrile, or benzylnitrile produces helical one-dimensional Au(I)–Ag(I) coordination polymers with very short metal-metal separations ranging from 2.8 to 3.0 �.  In the solid state these polymers are intensely photoluminescent; however, upon dissolution the polymer dissociates in to its respective monomeric components.  The nature of the excited state is not well-understood, but substitution of the auxiliary ligands changes the Au(I)–Ag(I) separation and hence its luminescent properties. 

Increasing the distance between the nitrogen donor atom and the carbene moiety transitions the chemistry from polymers to discrete dimetallic and trimetallic complexes.  Like their corresponding polymers, these complexes also contain short metal-metal separations and are photoluminescent.  The preparation and physical properties of these complexes as models for the extended polymeric systems is also discussed.