Novel neutral mixed-ligand lr(N boolean AND C boolean AND N)(N boolean AND C)X complexes (N boolean AND C boolean AND N = 1, 3-bis (3-methylpyrazolyl)benzene (bpzb), 1,5-dimethyl-2,4-bis(3-methylpyrazolyl)benzene (dmbpzb), and 1,5-difluoro-2,4-bis(3-methylpyrazolyl)benzene (dfbpzb); N boolean AND C = 2-phenyl pyridine (ppy); and X = Cl or CN) have been synthesized and characterized. An X-ray single-crystal structure of the complex lr(dmbpzb)(ppy)Cl shows that the nitrogen atom in the ppy ligand occupied the trans position to the carbon atom in the tridentate N boolean AND C boolean AND N ligand of dmbpzb with the lr-C bond length of 1.94(1) angstrom, whereas the coordinating carbon atom occupied the trans position of chlorine. Electrochemical data show that the complexes exhibit an oxidation lr(III/IV) process in the potential range of +0.5 similar to 0.9 V and two irreversible reductions at approximately -2.6 and -3.0 V against Fc(0)/Fc(+), respectively. All of the lr(III) complexes do not emit phosphorescence at room temperature, although strong phosphorescence is exhibited at 77 K with the 0-0 transition centered at around 450 nm and lifetimes of 3-14 mu s. DFT calculations indicate that the HOMOs are mainly localized on iridium 5d pi and chlorine p pi*, whereas the LUMOs are mainly from the ppy ligand pi* orbitals. The phosphorescence originates from a (LC)-L-3 state mixed with the (MLCT)-M-3 and (XLCT)-X-3 ones. Temperature-dependent lifetime measurements of lr(dfbpzb)(ppy)Cl reveal the existence of a thermal deactivation process with a low activation energy (1720 cm(-1)) and very high frequency factor (2.3 x 10(13) s(-1)). An unrestricted density functional theory indicates that the dd state, in which both the lr-N (pyrazolyl) bond lengths increase considerably, exists almost at the same energy as that for the phosphorescent state. A thorough analysis based on the potential energy surfaces for the T-1 and S-0 states allows us to determine the reaction pathway responsible for this thermal deactivation. The calculated activation energies of 1600 similar to 1800 cm(-1) are in excellent agreement with the observed values.