Homonuclear and heteronuclear metal complexes with a cyclic tetracopper(II) unit. Syntheses, crystal structures, and magnetic properties of [CuII4(dmap)3(OH)(O2CCH3)2(HO2CCH3)(H2O)][PF6]2, [CuII4(dmap)2(O2CCH3)4][PF6]2, [CuII4(dmap)2(O2CCH3)2(OH)2][HgII(O2CCH3)Cl2]2[HgIICl2], and [CuII4(dmap)2(O2CCH3)3(OH)2(H2O)][PF6] (dmap = 1,3-bis(dimethylamino)-2-propanolato)