reaction of (NH4)2Ce(NO3)6 with sodium cyclopentadienide (C5H5Na) in THF at a molar ratio of 1:6 to give a brown crystal of (h5-C5H5)3Ce譕C4H8; (tricyclopentadienyl)erbium tetrahydrofuranate complex was synthesized by the reaction of ErCl3譶THF with potassium cyclooctadienide (C8H11K) at a molar ratio of 1:1 at -78癈, and then C5H5Na in THF (at molar ratio of ErCl3譶THF to C5H5Na 1:2) was added to the above solution at room temperature to give the pink crystal of (h5-C5H5)3Er譕C4H8. The crystal structure determination of both complexes showed that both crystals belong to monoclinic space group P21/n. The Ce complex structure is different from those of (h5-C5H5)3Ln譕C4H8 (Ln = La, Pr, Nd, Gd, Dy, Y, Lu). They are not isostructural, but the Er complex is isostructural with them. The distance Ce-O, average Ce-C(h5) and Ce-Cent (centroid of cyclopentadienyls) are not consistent with the lanthanide contraction regularity, but those of Er-O, average Er-C(h5) and Er-Cent are consistent with it. It means that there are two discontinuity points at Ce and Dy but no "gadolinium break phenomenon" is observed in the isostructural series of (h5-C5H5)3Ln譕C4H8 because the bond lengths of Ln-O and Ln-C(h5) and the Ln-centroid distance of the elements (Y, Er, Lu) are not greater than those of Gd.

Key words: CRYSTAL STRUCTURE DETERMINATION, CYCLOPENTADIENE P, TETRAHYDROFURAN, CERIUM COMPLEX, ERBIUM COMPLEX, ERBIUM CHLORIDE