Magnetically Isolated CuIIGdIII Pairs in the Series [Cu(acacen)Gd(pta)3], [Cu(acacen)Gd(hfa)3], [Cu(salen)Gd(pta)3], and [Cu(salen)Gd(hfa)3], [acacen = N,N‘-Ethylenebis(acetylacetoniminate(−)), salen = N,N‘-Ethylenebis(salicylideniminate(−)), hfa = 1,1,1,5,5,5-Hexafluoropentane-2,4-dionate(−), pta =