In addition to the metal-centered dd transition that is widely accepted as a dominant radiationless decay channel, other factors may also play important roles in governing the loss of phosphorescence efficiency for heavy-transition-metal complexes. To conduct our investigation, we synthesized two dicarbonylruthenium complexes with formulas [Ru(CO)(2)(BQ)(2)] (1) and [Ru(CO)(2)(DBQ)(2)] (2), for which the cyclometalated ligands BQ and DBQ denote benzo[h] quinoline and dibenzo[f,h] quinoxaline, respectively. Replacing one CO ligand with a P donor ligand such as PPh2Me and PPhMe2 caused one cyclometalated ligand to undergo a 180 degrees rotation around the central metal atom, giving highly luminous metal complexes [Ru(CO)L(BQ)(2)] and [Ru(CO)L(DBQ)(2)], where L = PPh2Me and PPhMe2 (3-6), with emission peaks lambda(max) in the range of 571-656 nm measured in the fluid state at room temperature. It is notable that the S-0-T-1 energy gap for both 1 and 2 is much higher than that of 3-6, but the corresponding phosphorescent spectral intensity is much weaker. Using these cyclometalated Ru metal complexes as a prototype, our experimental results and theoretical analysis draw attention to the fact that, for complexes 1 and 2, the weaker spin-orbit coupling present within these molecules reduces the T-1-S-0 interaction, from which the thermally activated radiationless deactivation may take place. This, in combination with the much smaller (MLCT)-M-3 contribution than that observed in 3-6, rationalizes the lack of room-temperature emission for complexes 1 and 2.