Reactions of monovalent ions with a versatile building block [Cr(phen)(C2O4)2]-. Molecular and crystal structures of NaCr(phen)(C2O4)2·3H2O, KCr(phen)(C2O4)2·DMF·0.5H2O, 4-NH3py[Cr(phen)(C2O4)2]·2H2O, N-(n-butyl)4[Cr(phen)(C2O4)2], and PPh4[Cr(phen)(C2O4)2]