The ability of a sulfonyl amidophosphate (SAPh) compounds having the structural fragment RSO2NHP(O)R to act as a β-diketone homologue for the complexation of Ln(III) ions has been explored. We report the synthesis and magnetic properties of five mononuclear complexes of formula LnL3Phen (L stands for deprotonated bis(methyl(phenyl)amino)phosphoryl)benzenesulfonamide, C6H5SO2NP(O)[N(CH3)(C6H5)]2, and Phen for phenanthroline) with Ln = TbIII, DyIII, HoIII, ErIII, and YbIII, along with the X-ray structure of the Dysprosium and Ytterbium compounds. In-field SMM behaviors were observed for the Dy and Er derivatives.