A Tribenzylidenemethane-Tantalum Compound: Some Experiences with 'Inversion Twinning'

The six-electron-donating ligand tribenzylidenemethandiide has been used to form a tantalum (group 5) mimic, ( 775 -cyclopentadienyl )( 77 -tribenzy lidenemethandiide )dimethyltantalum, of a group 4 bent metallocene. The material crystallizes with two molecules in the asymmetric unit with quite different packing arrangements, although the overall structures of the two are similar. The Cp and methyl ligands are disordered about a threefold axis. Crystal data: [Ta{ (C7H6 hC}(C5 H5)(CH3hJ, trigonal P3lc, with a= 12.681(3), c = 16.124(5) A, V = 2245.5 (7) A\ T = 293 K, Z = 4, Mr = 558.47, Dx = 1.65 g cm-3, F(OOO) = 1104, Mo Ka, >. = 0.71073 A,μ= 4.91 mm1, R = 0.020 for 1319 reflections with F0 > 4a(F0 ); S = 2.18. Because of crystal decay, three separate crystals were needed for a full data set. These polar (but achiral) crystals showed apparently differing amounts of inversion twinning, leading to problems in accurately merging the three data sets and refining the structure. These problems are discussed briefly.