The series of novel rhenium(I) tricarbonyl mixed-ligand complexes Re(X)(CO)(3)(N boolean AND N) (N boolean AND N = pyridine-2-aldoxime; X = -Cl, 1; X = -C/N, 2; and X = -C C, 3) has been investigated theoretically to explore the ligand X effect on their electronic structures and spectroscopic properties. The contribution of the X ligand to the highest occupied molecular orbital (HOMO) and HOMO-1 decreases in the order of 3 > 1 > 2, in line with the pi-donating abilities of the X: -C C > -Cl > -CN. The reorganization energy (lambda) calculations show that 1 and 3 will result in the higher efficiency of organic light-emitting diodes than 2. The lowest-lying absorptions of 1 and 3 can be assigned to the {{d(xz), d(yz)(Re) + pi(CO) + pi(X)] -> [pi* (N boolean AND N)]) transition with mixing metal-to-ligand, ligand-to-ligand, and X ligand-to-ligand charge transfer (MLCT/LLCT/ XLCT) character, whereas this absorption at 354 nm (H-1 -> L) of 2 is assigned to {d(xz), d(yz)(Re) + pi(CO) + pi(N boolean AND N)]->[pi* (N boolean AND N)]} transition with MLCT/LLCT/ILCT (intrafigand charge transfer). Furthermore, the absorptions are red-shifted in the order 2, 1, and 3, with the increase of pi-donating abilities of X ligands. The solvent effects cause red shifts of the absorption and emission spectra with decreasing solvent polarity.