A Wonderful Bond That Wasn't There: Reformulation of a Compound 'Containing a Ta-Ta Bond without Bridging Ligands' as [(Cy_2N)_2ClTa(mu-H)]_2

摘要

Recently Gambarotta et al. reported the X-ray crystal structures of two compounds prepared by the reaction of TaCl_5 and LiNCy2 (Cy = cyclohexyl). In the [(Cy_2N)_2TaCl-(CyNC_6H_(10))] molecule 1, a pentavalent tantalum atom is surrounded by a chloride ion, two "normal" amido groups and a modified amido group in which one of the hydrogen atoms of a cyclohexyl group is lost to give a Ta C bond. The formula of the other compound was proposed to be [(CyN)_2ClTa]_2 (2). The latter complex was remarkablein at least three ways: (1) each Ta atom had undergone a 2e reduction, (2) it had an unsupported Ta--Ta bond, and (3) the structure was highly unsymmetrical in the sense that what might have been expected to be chemically equivalent bonds were v. ry different in length. For example, according to the published data and those provided as Supporting Information, the Ta--Cl distances were 2.401(3) and 2.453(3) A, the Ta-N distances varied from 2.05(1) to 2.28(1) A, and the C-C distances ranged from 0.86 to1.70 A. The refinement data indicated that it had been necessary to refine five carbon atoms isotropically.