OBSERVATIONS ON RARE EARTHS .82. NUCLEAR MAGNETIC RESONANCE AND CALORIMETRIC STUDIES OF COMPLEXES OF TRIPOSITIVE IONS WITH SUBSTITUTED PYRIDINE MOLECULES

Direct calorimetric measurements indicate that the ions Pr3+ and Nd3+ react exothermally with substituted pyridine molecules in anhydrous acetonitrile with enthalpy changes < –3 kcal mol−1 for the formation of 1:1 complexes and still less for the formation of higher order species. At ligand to metal ion mole ratios above six, the 3:1 complexes appear to be the most stable ones. Substitution of nitrate for perchlorate ion or of Nd3+ for Pr3+ ion does not affect the measured enthalpy change. Large isotropic shifts of resonances of the ligand protons are observed upon coordination to these paramagnetic cations. These shifts can be resolved qualitatively into contact and pseudocontact shifts of nearly equal magnitude but opposite sign. The contact shift at the ortho position of the pyridine ring is ca. –1000 cps from the diamagnetic value. In spite of the weakness of the covalent interaction, large contact shifts result, in part as a consequence of the large J values of the cations. © 1969, American Chemical Society. All rights reserved.