New bis-pyrazole complexes [Pt(C^C*)(RpzH)2]X, containing a cyclometalated N-heterocyclic carbene ligand (HC^C* = 1-(4-(ethoxycarbonyl)phenyl)-3-methyl-1H-imidazol-2-ylidene) were prepared as chloride (X = Cl-, RpzH: 3,5-Me2pzH 1a, 4-MepzH 2a, pzH 3a), perchlorate (X = ClO4-,1b-3b), or hexafluorophosphate (X = PF6-, RpzH: 3,5-Me2pzH 1c) salts. The X-ray structure of 1a showed that the Cl- anion is trapped by the cation through two N-H
Cl bonds. In solution of methanol, acetone and THF at r.t. 1a-3a coexist in equilibrium with the corresponding [PtCl(C^C*)(RpzH)] (B) and RpzH species. In CH2Cl2, this equilibrium takes place just for 2a and 3a, but it is completely shifted to the left at 243 K and 223 K for 2a and 3a respectively. The low lying absorption and the emission bands were assigned to intraligand (ILCT) charge transfer on the NHC group. Quantum yield measurements in PMMA films revealed that 1b, 2b and 1c are amongst the most efficient blue-light emitters, with values up to 100%. Proton abstraction from the coordinated 3,5-Me2pzH in 1b by NEt3 and replacement by Ag+ afforded a neutral [Pt2Ag2] cluster containing Pt→Ag dative bonds.