[Cr(phen)(ox)2]−: a versatile bis-oxalato building block for the design of heteropolymetallic systems. Crystal structures and magnetic properties of AsPh4[Cr(phen)(ox)2]·H2O, [NaCr(phen)(ox)2(H2O)]·2H2O and {[Cr(phen)(ox)2]2[Mn2(bpy)2(H2O)2(ox)]}·6H2O