The electron paramagnetic resonance linewidth of aquo gadolinsuggested 8-coordinate (6-9), 9-coordinate (10,11), and ium(III) ion changes with the counter-ion identity and concentra-10-coordinate structures (12), as well as the possibility of an tion in aqueous solutions. The EPR linewidth of 2 m M gadoliniequilibrium between 8-and 9-coordinate species (14,15). um(III) chloride increases from 49.2 to 89.0 mT when carbonate Recently, Merbach and co-workers have concluded that aquo ion is added and decreases to 17.3 mT when nitrite ion is added. gadolinium(III) ion is 8 coordinate, [Gd(H 2 O) 8 ] 3/ , based These observations suggest association reactions between aquo on neutron diffraction measurements at high concentrations gadolinium(III) ion and anions that change the electron spin reof gadolinium nitrate (6).
laxation rates of the aquo ion. The concentration dependence of
In the present work, we find that the linewidth of the the gadolinium(III) EPR linewidth is consistent with binding conelectron paramagnetic resonance spectra of the aquo gadolinstants for nitrite and nitrate ion with the aquo gadolinium(III) ium(III) ion is a strong function of the type and concentraion of 37 { 6 and 2.3 { 0.3 L mol 01 respectively. The decreases in the EPR linewidth factors with these association reactions are tion of the anion present in the solution, which suggests that difficult to understand unless the anion reactions increase the symthe solution phase species may be complicated by the anion metry of the metal center. Although first-coordination reactions equilibria. We report here EPR and proton relaxation dispermay not be ruled out, the decrease in EPR linewidth is more sion studies that suggest a concentration-dependent equilibconsistent with an outer-sphere association reaction that also rerium between the aquo gadolinium(III) ion and anions that duces the coordination number of the metal center from 9 to 8. may change the coordination number from 9 at low concen-