STRUCTURE OF DIALUMINIOHEXALUTETIOPENTAKIS(HEXANIOBATE) - COMPARISON WITH EUROPIUM AND ERBIUM ANALOGS

The structure of a6.5H19.5[{Lu3O(OH)(3)(H2O)(3)}(2)Al-2(Nb6O19)(5)] . 38H(2)O was determined by X-ray diffraction analysis and compared with isomorphous Eu- and Er-complexes to investigate the effect of the ion ic radii of the lanthanide cations (Eu3+:1.066; Er3+:1.004; Lu3+:0.977 Angstrom, for eight-fold coordination) on the molecular structure. A central [{Lu3O(OH)(3)(H2O)(3)}(2)](8+) core in the [{Lu3O(OH)(3)(H2O)( 3)}(2)Al-2(Nb6O19)(5)](26-) anion was attached by both three equatoria l [Nb6O19](8-) and two axial [A1(Nb6O19)](5-) groups with approximatel y D-3 point symmetry. The three Lu atoms in a [Lu3O(OH)(3)(H2O)(3)](4) group, a half [{Lu3O(OH)(3)(H2O)(3)}(2)](8+) core, achieved eight-fo ld coordination with O atoms; one mu(3)-O, two OH-, one terminal H2O a nd four O atoms belonging to the equatorial [Nb6O19](8-) groups. The m ean Lu...Lu distance within each of the two Lu-3 traids is 3.676(6) An gstrom, while that between the two different Lu-3 traids is 4.69(5) An gstrom. The former distance is short compared with that of Eu-complex (3.76(2) Angstrom) but somewhat long compared with that of the Er-comp lex (3.66(1) Angstrom). The latter distance is comparable to the corre sponding distance (4.69(6) Angstrom) of the Eu-complex, and slightly l ong compared with that of the Er-complex (4.62(4) Angstrom). The small size of the Lu3+ cation was reflected by a contraction of the [{Lu3O( OH)(3)(H2O)(3)}(2)](8+) core. This effect represents a shift in the th ree equatorial [Nb6O19](8-) groups toward the center of the complex by about 0.13 and 0.05 Angstrom, compared with Eu- and Er-complexes, res pectively.