Solution structures of europium(III) complexes of ethylenediaminetetraacetic acid

Coordination of ethylenediaminetetraacetic acid (EDTA) with europium(III) has been studied at different concentrations in solution using F-7(0) --> D-5(0) excitation spectroscopy and excited-state lifetime measurements. EDTA forms with Eu(III) ion three different species in equimolar solutions at room temperature. At low pH values EuEDTAH is formed and at higher pH values than 1.5 two EuEDTA(-) complexes, which differ from each other with one water molecule in the first coordination sphere of the Eu(III) ion, total coordination number and coordination geometry are also formed. When the concentration of EDTA is higher than the concentration of Eu(III), an EuEDTA(EDTAH)(4-) species where the second EDTA is weakly coordinated to EuEDTA(-), is formed. If the concentration of Eu(III) ion is higher than EDTA, the extra Eu(III) ions associate with EuEDTA(-) and link to one of the carboxylate groups of EDTA thus causing a shortening of the excited-state lifetime of the EuEDTA(-) complex.