[Cr(dmbipy)(ox)2]-: a new bis-oxalato building block for metal assembling.Crystal structures and magnetic properties of XPh4[Cr(dmbipy)(ox)2].5H2O(X = P and As), {Ba(H2O)2[Cr(dmbipy)(ox)2]2}n.17/2nH2O and {Ag(H2O)[Cr(dmbipy)(ox)2]}n.3nH2O