The ortho-metalated Pt(II) complexes Pt(ppy)(CO)Cl (1), Pt(ptpy)(CO)Cl (2), and Pt(ppy)(Hppy)Cl (3) (where ppy and ptpy are respectively the ortho-C-deprotonated forms of 2-phenylpyridine and 2-p-tolylpyridine and Hppy is 2-phenylpyridine) have been prepared. Crystal structure determination performed on a yellow needle of 2 shows that this compound crystallizes in the orthorhombic system, space group Pna2(1) (No. 33), with a = 7.2025(14) Angstrom, b = 19.082(5) Angstrom, 8.7393(22) Angstrom, V = 1201.9(5) Angstrom(3), Z = 4 molecules/cell, and R = 0.046 (R, 0.056) for 1747 unique reflections. The CO Ligand is coordinated trans to the nitrogen atom of the ortho-metalated ligand and exerts a strong trans effect resulting in a relatively long Pt-N bond [2.114(19) Angstrom]. Characterization of 1 and 2 by IR and H-1 NMR spectra reveals that these complexes have analogous geometries. Complex 3 crystallizes in the monoclinic system, space, group P2(1)/c (No. 14), with a = 12.023(5) Angstrom, b = 9.547(4) Angstrom, c = 16.870(7) Angstrom, beta = 103.559(5)degrees, V = 1882.5(13) Angstrom(3), Z = 4 molecules/cell, and R = 0.044 (R(w) = 0.061) for 2361 unique reflections. This structure shows both the bidentate ppy ligand and the monodentate Hppy with the nitrogens of these ligands trans to each other. The UV/vis electronic absorption spectra of 1-3 have intense bands in the near-UV region (similar to 375 nm) which have been assigned as metal to ligand charge transfer (MLCT) transitions, and higher energy bands were assigned as ligand-centered transitions. Each complex exhibits relatively long-lived structured emissions in the solid state at ambient temperature and at 77 K and in 77 K glassy toluene solutions. These emissions are proposed to originate from triplet MLCT states. Notably, in solution both the lifetime and spectrum of 2 proved to be a function of the concentration, a phenomenon interpreted in terms of the propensity of square planar d(8) complexes to oligomerize. In contrast, the more sterically hindered complex d3 displayed no such tendency toward oligomerization.