Three heterometallic aggregates, [(Co-II)(2)(Gd-III)(2)((BuPO3)-Bu-t)(2)-((O2CBu)-Bu-t)(2)((HO2CBu)-Bu-t)(2)(NO3)(4)].NEt3 (1), [(Co-II)(2)(Co-III)(2)(Gd-III)(3)(mu(3)-OH)(2)((BuPO3)-Bu-t)(2)((O2CBu)-Bu-t)(9)(deaH)(2)(H2O)(2)] (2), and (Co-III)(2)(Gd-III)(5)(mu(2)-OH)(mu(3)-OH)(2)((BuPO3)-Bu-t)(2)((O2CBu)-Bu-t)(10)((HO2CBu)-Bu-t)(deaH)(2)].MeOH (3), were successfully isolated in reactions of [Co-2(mu-OH2)((O2CBu)-Bu-t)(4)].((HO2CBu)-Bu-t)(4), Gd(NO3)(3).6H(2)O, Bu-t-PO3H2, and diethanolamine (deaH(3)) by varying the stoichiometry of the reactants and/or changing the solvent. The structures of the final products were profoundly affected by these minor changes in stoichiometry or a change in solvent. The metaloxo core of these complexes displays a hemicubane or a defective dicubane-like view. Bond valence sum calculations and bond lengths indicate the presence of Co-II centers in compound 1, mixed valent Co centers (Co-II/Co-III) in compound 2, and only Co-III centers in compound 3 as required for the charge balances and supported by the magnetic measurements. Magnetic studies reveal significant magnetic entropy changes for complexes 13 (-Delta S-m values of 28.14, 25.06, and 29.19 J kg(-1) K-1 for 3 K and 7 T, respectively). This study shows how magnetic refrigeration can be affected by anisotropy, magnetic interactions (ferro- or antiferromagnetic), the metal/ligand ratio, and the content of Gd-III in the molecule.