This paper presents synthesis and photophysical investigation of a very rare type of the Re^I diimine complexes, [Re(diimine)(CO)₃(OPR₃)]⁺, R = Ph, Cy; diimine – phenanthroline and neocuproine, containing monodentate (unsupported) phosphine oxide ligands. The obtained compounds have been structurally characterized in solid phase by using XRD crystallography, which revealed unusual distortions in the pseudo octahedral rhenium environment, which may be ascribed to intramolecular interligand (phosphine oxide – diimine) interaction rather than to crystal packing effect. Optimization of the ground state structure of these molecules with the DFT method also confirmed intramolecular origin of the observed structural peculiarities. The complexes display phosphorescence in solution and in solid state with the quantum yield up to 11 % and 14 %, respectively, which originates from ³MLCT excited state as demonstrated by DFT calculations. Comparative analysis of the ligand (L) effect onto emission energy in the [Re(neocuproine)(CO)₃(L)] complexes (L = PPh₃, NCMe, OPPh₃, Cl^–) showed that the position of emission wavelength in the corresponding complexes qualitatively correlates with cumulative donor ability of these ligands that is in complete agreement with the ³MLCT character of emission.