Cyclometalated osmium complexes with the formulas [OS(ppy)(2)(CO)(2)] (1a,b), [Os(dfppy)(2)(CO)(2)] (2a,b), and [Os(btfppy)(2)(CO)(2)] (3a,b) have been synthesized, for which the chelating chromophores ppyH, dfppyH, and btfppyH denote 2-phenylpyridine, 2-(2,4-difluorophenyl)pyridine, and 2-(2,4-bis(trifluoromethyl)phenyl)pyridine, respectively. The isomers 1a-3a, possessing an intrinsic C-2 rotational axis as determined by single-crystal X-ray diffraction analysis, underwent slow isomerization in solution at elevated temperature, giving the respective thermodynamic products 1b-3b, which showed a distinctive coordination, arrangement produced by a 180 degrees rotation of one cyclometalated ligand around the Os(II) metal center. In contrast to the case for 1a,b and 2a, which are inert to substitution, complexes 2b and 3b (or 3a) readily react with PPh2Me to afford the products [Os(dfppy)(2)(CO)(PPh2Me)] (4) and JOs(btfppy)(2))(PPh2Me)] (6), in which the incoming PPh2Me replaced the CO located trans to the carbon atom of one cyclometalated ligand. UV-vis and emission spectra were measured, revealing the lowest excited state for all complexes as a nominally ligand-centered (3)pi pi* state mixed with certain MLCT character. Introduction of the electron-withdrawing substituents on the cyclometalated chelates or replacement of one CO ligand with phosphine at the metal center increased the MLCT contribution in the first excited state, giving a broad and featureless emission with greatly enhanced quantum yields.