Three new heterometallic phosphonate complexes with a novel 3d–4f metal core {Co2Gd4} were isolated. The reactions between CoCl2, GdCl3, ligand sodium salts, and 2,2′-bipyridine in methanol solution lead to new unexpected 3d–4f heterometallic complexes, [Na2CoIII2Gd4(L)6(bpy)2(OR)2]·nSolv (L = SAA3− (1), BSAA3− (2), and NAA3− (3); bpy = 2,2′-bipyridine; OR = OH and OMe; Solv = H2O and MeOH), composed of a rare case of two magnetically independent oxygen bridged Gd2O2 dinuclear units, which are separated by O–P–O bridges. According to the BVS calculations and measured magnetic properties, only CoIII ions are present in the obtained complexes. The magnetic properties of the compounds 1–3 were studied and the entropy changes for the designation of the magnetocaloric effect were calculated. The largest value for the magnetic entropy change −ΔSM of 20 J kg−1 K−1 for complex 1 has been found for Tav = 2.25 K and Hf = 5 T.
