[Cr(bpym)(C2O4)2]− in designing heterometallic complexes. Crystal structures and magnetic properties of PPh4[Cr(bpym)(C2O4)2]·H2O and [Ag(bpym)][Cr(C2O4)2](H2O)2]·2H2O (bpym=2,2′-bipyrimidine)