A series of bis-N-heterocyclic carbenes of rhodium and iridium have been obtained and characterized. The formation of the M-C bond has been studied according to experimental and theoretical data, showing that two different mechanisms are operative for the first (single proton deprotonation of the bisimidazolium salt, or oxidative addition followed by deprotonation of the metal hydride) and second (oxidative addition of the second bisimidazolium C-H bond, yielding a NHC-IrIII-H species) metalation processes. In the case of complexes with long linkers Between the imidazolium rings, reductive elimination of HCl affords bisimidazolylidene complexes of IrI. According to the theoretical studies we concluded that thermodynamic parameters would determine the formation of the NHC-IrIII-H species, while IrI-NHC species would be kinetically favored in the case of complexes with long linkers between the azole rings. The crystal structures of a series of Ir-bis(NHC) complexes are described. © 2006 American Chemical Society.