Heterocyclic oxygen donor ligands

While UOi+ forms a very large number of complexes with oxygen-donor ligands of all types, particular effort has been devoted to carboxylic acids, from the simplest (formic, acetic, oxalic acids) to polyfunctional, aromatic and heterocyclic acids. One motive for investigating these compounds is the possible role of simple carboxylic acids as reductants [3, 5] of excited UVI, generating U,v which can then reduce Puiv to PuIn which is more readily separable than Pulv from UVI in the treatment of nuclear waste. Another significant role has been proposed for carboxyl-functionalized polymers which show potential in the solid-phase extraction of UV1 from dilute solution [13]. 

Numerous adducts, mostly of type MX4L2 (X = Cl, Br, I), are known with nitrogen, phosphorus, arsenic, oxygen, and sulfur donors. They are synthesized by reaction of solid tetrahalides with the ligand or by reduction of pentahalides with the ligand, mainly bidentate or heterocyclic N-donors or phosphines. NbCLt(THF)2 (29) is of particular value since the lability of the THF molecules makes it a useful synthon in Nb chemistry. 

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