Oxalate V(V) forms a surprising complex: cis-[VO2(C2O4)2]3- whose ammonium salt (NH4)3[VO2(C2O4)2].2H2O is bright yellow and forms large, maybe even single crystals, but solution tends to be unsaturated till the end. Preparation of this is quite hard to describe: Mixing same molar NH4VO3, H2C2O4 and (NH4)2C2O4 together in solution is OK, but then it darkens soon even if you are very precise. Use a few milliliter H2O2 to oxidize it back it turns red, and then in a few days it darkens and repeat H2O2 addition... finally it does not becomes dark but yellow, then go to vaporize at RT. V(IV) forms a complex: [VO(H2O)(C2O4)2]2- whose ammonium salt (NH4)2[VO(H2O)(C2O4)2].H2O can hardly be crystallized without ethanol. However, a DMAP(4-dimethylaminopyridinium) salt can be made. In this salt, oxo and aquo are trans. It is sparingly soluble. Follow the steps to prevent toxic DMAP vapor. Steps: 1. Roast 1x ammonium metavanadate till it becomes black, then red 2. Add 1.5x oxalic acid an...