A series of new dimeric complexes with composition [Ln2(hfa)6(4-cpyNO)3] (2:3 adduct, Ln = SmIII–DyIII, TmIII) were synthesized and fully characterized. X-ray diffraction data of [Tb2(hfa)6(4-cpyNO)3]·CHCl3 confirmed the dimeric structure; Tb ions are nine-coordinated by six O atoms from three hfa− ligands and three O atoms from three bridging 4-cpyNO molecules. The photophysical properties (absolute quantum yields and luminescence lifetimes) of the EuIII and TbIII 2:3 adducts are presented and compared with those of the 2:2 adducts.