## 1 Introduction  
The poisoning action of toxic metal ions represents one of the most important problems for health. In this context, metal chelation therapy was proposed more than 50 years ago for the treatment of pathologies produced in the body by an overload of a metal ion, and it is the most efficient therapeutic approach [1, 2]. The choice of the most adequate chelating agent is related to the specificity and stability of the metal-chelator complexes. Metal chelation therapy involves the use of a chelating agent, which consists of an organic molecule that possesses a chelator located at one terminus of the agent and an active functionality located at the other end of the molecule to connect with the vector molecule [3]. Among chelators, the most used are macrocyclic ligands, which are to be preferred with respect to their acyclic counterparts from a thermodynamic and kinetic point of view. Their drawback is related to their slow binding kinetics, which necessitates high temperatures. Coumarin and its derivatives can form complexes with different metal ions, and therefore, in principle, can be used in specific chelation therapies. This class of ligands can form complexes with several toxic metal ions, and has attracted the interest of researchers, also in consideration of their antibiotic, anticoagulant, anticancer and anti-inflammatory properties [4]. Complexes of this class of ligands with several transitions, lanthanide and actinide metal ions have been studied previously to gain insight into their coordination chemistry and their biological activity [5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]. Some reviews summarize the advances in various medicinal applications of metal complexes of coumarins [16, 17, 18, 15]. It was reported that the biological activity of some coumarin derivatives significantly improves by binding to metal ions; this behavior could be attributed to an increase in the lipophilicity of the systems. However, the relationship between the structure and properties of complexes is still unknown. Nevertheless, to achieve insight into the factors controlling the metal complex biological activity, it is first necessary to know the binding properties of coumarins to the metal ions. In 2001, attempts to determine the structure of a copper(II) complex of coumarin-3-carboxylic acid (HCCA), analyzing spectroscopic data, suggested a binuclear structure [4]. Synthesis was carried out using copper(II) chloride as the salt yielding a very insoluble product. Overall complexes of Ni(II), Co(II), Zn(II) and Mn(II) with HCCA were studied at experimental and theoretical levels. The complexes were characterized by elemental analyses, FT-IR, \({}^{1}\)H-NMR, \({}^{13}\)C-NMR and UV-Vis spectroscopy, and by magnetic susceptibility measurements [19]. Recently, we have reported on the complexation of HCCA with Al(III) and Fe(III), in aqueous solutions at 37 \({}^{\circ}\)C and in 0.16 M NaCl, to evaluate the selectivity of this ligand towards bioavailable metal cations [20]. HCCA exhibits two different coordination sites (Figure 1): it can play as monodentate or bidentate ligand via carboxylic moiety involving one or both oxygen atoms, respectively, as well as bidentate ligand through the lactone and the carboxylic oxygen to realize a high stabilized 6-membered metallocycle. Simultaneous bidentate and monodentate coordination mode by the free carbonyl oxygen was shown in the presence of lanthanides [12]. In view of its potential application as a chelating agent, herein we present a study on the complexation ability of coumarin-3-carboxylic acid towards neodymium(III) and dioxouranium(VI) (uranyl, UO2\({}^{2+}\)) under the physiological conditions of temperature and ionic strength (i.e., 37 \({}^{\circ}\)C and 0.16 mol/L NaClO4), evaluating the stability constants of the complexes and the corresponding structures by potentiometric measurements and UV-Vis spectrophotometry.