TETRAHEDRAL VERSUS SQUARE-PLANAR ARRANGEMENT OF CYCLOPENTADIENYL LIGANDS IN BIMETALLIC ORGANOSAMARIUM COMPLEXES - X-RAY CRYSTAL-STRUCTURE OF [(C5H4ME)2(THF)SM(MU-CL)]2

The effects of cyclopentadienyl ring size on the geometry of bimetallic organosamarium complexes have been studied by comparing the X-ray crystal structure of [(C5H4Me)2(THF)Sm(mu-Cl)]2, prepared from KC5H4Me and SmCl3 in THF, with C5Me5 analogs. The complex crystallizes from THF at - 30-degrees-C in space group Pbcn with a = 20.312(5), b = 9.626(2), c = 16.225(3) angstrom, V = 172.5(12) angstrom3 and D(calc) = 1.74 g cm-3 for Z = 4. Least-squares refinement of the model based on 1759 reflections [\F(o)\] > 2.0sigma(\F(o)\)] converged to a final R(F) = 5.0%. The complex adopts a geometry which has a molecular two-fold rotation axis perpendicular to the Sm2Cl2 plane and a crystallographic inversion center. Hence, both methyl groups of each (C5H4Me)2Sm unit are located on the side opposite of the THF ligands, which are trans to each other, and the four C5H4Me ring centroids define a square plane. The Sm-Cl distances are 2.759(3) and 2.819(3) angstrom.