Our continued research on the use of heavy metal cluster complexes as a new class of X-ray contrast agents in medical diagnostic imaging is described. A series of 2:3 cluster-ligand complexes, [((W3SO3)-S-IV)(2)L-3](4-) (L = linear polyaminopolycarboxylate ligands), were isolated from the reaction of aqua ion [(W3SO3)-S-IV(H2O)(9)](4-) (prepared in large quantities through an improved literature process) with respective ligands in refluxing DMF. The salts of [((W3SO3)-S-IV)(2)L-3](4-) complex anions were fully characterized using routine techniques such as elemental analysis, MS. HPLC, UV-vis, IR, and NMR. The solid structures of two complex anions, [((W3SO3)-S-IV)(2)(PDTA)(3)](4-) and [((WSO3)-S-IV)(2)(HO-PDTA)(3)](4-), were determined by X-ray crystallography. They are the first examples wherein two (W3SO3)-S-IV clusters are complexed and linked by three ligands that contain two terminal iminodiacetate (bis-IDA) groups. Complexation of the unstable aqua ion [(W3SO3)-S-IV(H2O)(9)](4-) With ligands has imparted desired biological compatibility to the tungsten metal cluster. These complexes are stable and highly soluble in H2O. The potential utility of such tungsten cluster complexes as X-ray contrast agents was evaluated in both in vitro and in vivo animal studies. in addition, the syntheses of several new linear polyaminopolycarboxylate ligands used in this study are reported.