A series of Ru(II) complexes, tcc- RuX2(POC4Pyr-P, O)(2) ( X) Cl ( 3), Br ( 4), I ( 5)), containing the hemilabile phosphine pyrenyl ether ligand 4- {2-(diphenylphosphino) phenoxy} butylpyrene (POC4Pyr ( 1)) are reported. The synthesis and spectroscopic properties of both the ligand, POC4pyr ( 1), and ligand oxide, P(=O) OC4pyr ( 2), and the solid- state structure of 1 are reported. Complexes 3 - 5 react rapidly with CO to give complexes ttt-RuX2(CO)(2)(POC4pyr- P)(2) ( X = Cl ( 6), Br ( 7), I ( 8)). No pyrene excimer emission is detected from 3 - 5; however, different intensities of excimer emission are observed for 6 -8. The intensity of excimer emission decreases through the series, with 6 showing the most intense response. The emission is solely due to intramolecular pyrene excimers at low concentrations (<= 10(-4) M). Comparison of the UV - vis and steady-state fluorescence spectra shows overlap between the low energy d - d absorption of 7 and 8 with excimer emission ( 480 nm), suggesting nonradiative energy transfer may be occurring. Once excess CO is removed, complexes 6 - 8 isomerize to cis-dicarbonyl complexes cct-RuX2(CO)(2)( POC4Pyr-P)(2) ( X = Cl ( 9), Br ( 10), I ( 11)). The intensity of excimer emission from 9 - 11 increases with respect to the excimer emission observed for 6 - 8, with 9 showing a significant increase in excimer intensity.