Three new luminescent cyclometalated Pt(II) complexes, [Pt(L)Cl] (1), [Pt₂(L⁻)₂] (2), and [Pt(L)(PPh₃)]ClO₄ (3·ClO₄) (where HL = 2-phenyl-6-(1H-pyrazol-3-yl)-pyridine), were synthesized and characterized by X-ray crystallography. HL represents a new class of C,N,Npyrazolyl cyclometalating ligands containing a Cphenyl, a Npyridyl, and a Npyrazolyl donor moiety, as well as a 1-pyrazolyl-NH, that can also be available for metal coordination and other chemical interactions. Complex 1 possesses intense intraligand transitions at 275−375 nm and moderately intense metal-to-ligand charge transfer (¹MLCT) (dπ(Pt) → π*(L)) transition at 380−410 nm. The room temperature solid-state emission λmax of 1 occurs at 580 nm and is attributable to the ³MMLCT (dσ*(Pt) → π*(L)) transition. It also displays strong phosphorescence in acetonitrile solutions at room temperature with an emission λmax at 514 nm, which can be tentatively assigned to the ³MLCT (π*(L) → dπ(Pt)) transition. Complex 1 can be deprotonated in organic solvents to yield a cycloplatinated dimer 2, which shows a relatively high room-temperature luminescent quantum yield of 0.59 in DMF (λmax = 509 nm). Substitution of the ancillary chloro-ligand in 1 by triphenylphosphine yields 3, which also possesses a good room-temperature luminescent quantum yield of 0.52 in DMF (λmax = 504 nm) and a better solubility in water. Complex 3 is synthesized to demonstrate the pH dependence of luminescent properties of this C,N,Npyrazolyl cyclometalated Pt(II) system. Such a pH response is ascribable to the protonation/deprotonation of the 1-pyrazolyl-NH on the C,N,Npyrazolyl cyclometalating ligand. The pKa of the 1-pyrazolyl-NH in 3, measured in 1:2 (v/v) aqueous DMF solutions, is approximately 4.0. Copyright © 2007 American Chemical Society.