Adept representatives of light, middle and heavy rare earths metals, the trivalent lanthanoids La, Eu, and Lu were selected for synergistic solvent extraction by using an ionic liquid, 1-methyl-1,3-butylimidazolium-bis (trifluoromethanesulfonyl)imide. The process was carried out with mixtures of a chelating extractant (HL), 3-methyl-4-(4-methylbenzoyl)-1-phenyl-pyrazol-5-one (HPMMBP) and two novel synthesized compounds. The latter consisted of unsymmetrical NH-urea fragments attached to a p-cresol scaffold (S1 and S2). To elucidate the mechanistic aspects of the extraction process and reveal the nature of the formed metal species in the organic ionic phase, the slope analysis of the extraction data was deduced and compared the results of solution chemistry. From this, the effects caused by CHON based molecular synergistic agents on the extraction-separation efficiency of the aforementioned rare earth elements were investigated. Furthermore, the synergistic enhancement and separation factors between lanthanoid metals were also evaluated. Finally, testing was carried out for the applicability of these new enforced ligands for the competitive removal of rare earth ions from leach solution containing various s- and d-cations.