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