The synthesis, electrochemistry, and photophysics of a series of square planar Pt(II) complexes are reported. The complexes have the general structure C<^>NPt(O<^>O),where C<^>N is a monoanionic cyclometalating ligand (e.g., 2-phenylpyridyl, 2-(2'-thienyl)pyridyl, 2-(4,6-difluorophenyl)pyridyl, etc.) and O<^>O is a beta-diketonato ligand. Reaction of K2PtCl4 with a HC<^>N ligand precursor forms the chloride-bridged dimer, C<^>NPt(mu-Cl)(2)PtC<^>N, which is cleaved with beta-diketones such as acetyl acetone (acacH) and dipivaloyl methane (dpmH) to give the corresponding monomeric C<^>NPt(O<^>O) complex. The thpyPt(dpm) (thpy = 2-(2'-thienyl)pyridyl) complex has been characterized using X-ray crystallography. The bond lengths and angles for this complex are similar to those of related cyclometalated Pt complexes. There are two independent molecular dimers in the asymmetric unit, with intermolecular spacings of 3.45 and 3.56 Angstrom, consistent with moderate pi-pi interactions and no evident Pt-Pt interactions. Most of the C<^>NPt-(O<^>O) complexes display a single reversible reduction wave between -1.9 and -2.6 V (vs CP2Fe/CP2Fe+), assigned to largely C<^>N ligand based reduction, and an irreversible oxidation, assigned to predominantly Pt based oxidation. DFT calculations were carried out on both the ground (singlet) and excited (triplet) states of these complexes. The HOMO levels are a mixture of Pt and ligand orbitals, while the LUMO is predominantly C IN ligand based. The emission characteristics of these complexes are governed by the nature of the organometallic cyclometalating ligand allowing the emission to be tuned throughout the visible spectrum. Twenty-three different C<^>N ligands have been examined, which gave emission lambda(max) values ranging from 456 to 600 nm. Well-resolved vibronic fine structure is observed in all of the emission spectra (room temperature and 77 K). Strong spin-orbit coupling of the platinum atom allows for the formally forbidden mixing of the (MLCT)-M-1 with the (MCLT)-M-3 and (3)pi-pi* states. This mixing leads to high emission quantum efficiencies (0.02-0.25) and lifetimes on the order of microseconds for the platinum complexes.