A series of square planar platinum(II) complexes with the mer-coordinating tridentate ligand, pip & sub2;NCN & sup-; (pip & sub2;NCNH = 1,3?bis(piperdylmethyl) benzene) has been synthesized. The lowest emissive excited states of these complexes can be tuned by varying the properties of the monodentate ligand. Colorless halide complexes and the pyridine adduct exhibit weak red emissions originating from a lowest triplet ligand field (3LF) excited state. In contrast, yellow-green emissions from Pt(pip & sub2;NCN)(4-phenylpyridine))+; originate from a lowest 3TT?TT* state. Similarly, (Pt(pip & sub2;NCN))2(mƯL)2+; dimers exhibit emission from a lowest 3LF excited state (L=1,2-bis(4-pyridyl)ethane), a lowest 3TT-TT* excited state (L=trans-1,2-bis(4-pyridyl)ethylene or 4,4'-bipyridine) or a lowest triplet metal-to-ligand charge-transfer excited state (L=pyrazine), depending on the bridging ligand. A series of platinum(II) terpyridyl complexes with aryl ligands has been synthesized. The complexes undergo two one-electron reduction processes near -0.9 and -1.4 V vs. Ag/AgCl. In the cyclic voltammograms of complexes unsubstituted at the 2 and 6 positions of the aryl ligand, the first reduction exhibits larger (>59 mV) differences between the anodic and cathodic peak potentials than observed for complexes with methyl groups at the 2 and 6 positions. The accumulated data are consistent with the formation of dimers that reduce at different potentials than the monomers, causing the first reduction wave to appear broadened. Protection of the axial sites of the metal center appears to interfere with dimerization. 77 K glassy solution emission measurements confirm that steric effects can be utilized to control intermolecular metalʺʺʺmetal and ligandʺʺʺligand stacking interactions in solutions of these complexes. The first examples of platinum complexes that undergo reversible and cooperative thermal two-electron transfer have been prepared. The tpy ligand of Pt(tpy)(pip & sub2;NCN))+; is tridentate and the pip & sub2;NCN & sup-; ligand is monodentate, bonded through the central aryl group. Cyclic voltammograms of this complex exhibit two reversible oneƯelectron tpy-centered reductions and a nearly reversible two-electron platinum-centered oxidation at ""0.40 V vs. Ag/AgCl. Electrochemical studies of a series of model complexes establish that both potentially tridentate ligands are required for reversible two-electron transfer. A detailed analysis leads to the suggestion that the ligands preorganize around the platinum(II) center, resulting in a 5- or 6-coordinate electrochemically active species.