Interaction of trivalent lanthanide cations with nitrate anions: a quantum chemical investigation of monodentate/bidentate binding modes

We report ab initio quantum mechanical calculations on Ln(III)(NO3-)(n)(n=1-3) and Ln(NO3)(3)(H2O)(m)(m=4-6) complexes to compare the monodentate vs. bidentate binding mode of nitrate anions to trivalent lanthanide cations (La-III, Eu-III and Lu-III) in the gas phase. In all Ln(III)(NO3-)(n) complexes studied, bidentate coordination is preferred (by about 30 kcal mol(-1) for n=1 and 20 kcal mol(-1) for n=3). In the Ln(NO3)(3)(H2O)m aggregates, however, where the first coordination shell is saturated, the two types of binding modes become of similar energy, leading to different coordination numbers (CNs) and distributions of first and second shell water molecules. For instance, for La(NO3)(3)(H2O)(6), CN ranges from 9 (3 monodentate nitrates + 6 water) to 10 (3 bidentate nitrates + 4 water) or 11 (3 bidentate nitrates + 5 water). Thus, at some point, adding water to the second or to the first shell becomes isoenergetic. As the cation becomes smaller, the preference for monodentate nitrate binding increases, due to avoided repulsions in the first coordination sphere. Thus, water coordination to the lanthanide cations may induce a change of anion coordination mode and of coordination number. The fact that in solid state structures bidentate binding is dominant thus does not result from a marked intrinsic preference.