[Cr(dpa)(ox)2]−: a new bis-oxalato building block for the design of heteropolymetallic systems. Crystal structures and magnetic properties of PPh4[Cr(dpa)(ox)2], AsPh4[Cr(dpa)(ox)2], Hdpa[Cr(dpa)(ox)2]·4H2O, Rad[Cr(dpa)(ox)2]·H2O and Sr[Cr(dpa)(ox)2]2·8H2O (dpa = 2,2′-dipyridylamine)