A TRIBENZYLIDENEMETHANE-TANTALUM COMPOUND - SOME EXPERIENCES WITH INVERSION TWINNING

The six-electron-donating ligand tribenzylidenemethandiide has been us ed to form a tantalum (group 5) mimic, handiide)di-meibenzylidenemetha ndiidethyltantalum, of a group 4 bent metallocene. The material crysta llizes with two molecules in the asymmetric unit with quite different packaging arrangements, although the overall structures of the two are similar. The Cp and methyl ligands are disordered about a three-fold axis. Crystal data: [Ta{(C7H6)(3)C}(C5H5)(CH3)(2)], trigonal P31c, wit h a = 12.681 (3), c = 16.124 (5) Angstrom, V = 2245.5 (7) Angstrom(3), T = 293 K, Z = 4, M(r) = 558.47, D-x = 1.65 g cm(-3), F(000) = 1104, Mo K alpha, lambda = 0.71073 Angstrom, mu = 4.91 mm(-1), R = 0.020 for 1319 reflections with F-o > 4 sigma(F-o); S = 2.18. Because of crysta l decay, three separate crystals were needed for a full data set. Thes e polar (but achiral) crystals showed apparently differing amounts of inversion twinning, leading to problems in accurately merging the thre e data sets and refining the structure. These problems are discussed b riefly.