PREPARATION, PROPERTIES, AND BONDING ANALYSIS OF TANTALUM(II) ETA-6-ARENE COMPLEXES

The tantalum(III) eta-6-arene complexes (eta-6-C6R6)Ta(DIPP)2Cl (DIPP = 2,6-diisopropyl phenoxide) undergo facile one-electron reductions to afford the paramagnetic Ta(II) arenes (eta-6-C6R6)Ta(DIPP)2. Thus, (eta-6-C6ET6)Ta(DIPP)2Cl (1), itself prepared by the cyclotrimerization of EtC = CEt, reacts with excess NaHg to afford the maroon Ta(II) complex (eta-6-C6Et6)Ta(DIPP)2 (3) in high yield. Alternatively, 3 is prepared from the three-electron reduction of Ta(DIPP)2Cl3 (OEt2) in the presence of EtC = CEt. Similarly, (eta-6-C6Me6)Ta(DIPP)2 (4) is prepared upon reducing (eta-6-C6Me6)Ta(DIPP)2Cl (2) with excess NaHg. (eta-6-C6Et6)Ta(DIPP)2 (3) undergoes one-electron oxidative addition reactions with CH2Cl2 to regenerate 1, with allyl bromide to afford (eta-6-C6Et6)Ta(DIPP)2Br (5), and with CH3CH2I to provide (eta-6-C6Et6)Ta(DIPP)2I (7). (eta-6-C6Et6)Ta(DIPP)2 (3) crystallizes in the monoclinic space group P2(1)/c, with a = 12.396 (4) angstrom, b = 17.344 (6) angstrom, c = 18.622 (4) angstrom, and beta = 99.11 (2)-degrees, with V = 3953.3 angstrom-3 and Z = 4 for a calculated density of 1.31 g cm-3. The eta-6-arene ligand in 3 is characterized by a folded or boatlike distortion, with two carbons making a close approach to the metal. The arene also displays a C2, ''twist-boat'' deformation of the other four arene carbons. Molecular orbital calculations at the extended Huckel and ab initio level on (eta-6-C6H6)Ta(OH)2 are used to investigate the bonding and geometric distortions for these molecules. It is found that the boatlike deformation of the benzene ligand is primarily due to maximization of the overlap between an s hybridized x2 - y2 orbital at the metal and pi* on benzene. The peculiar C2 deformation of the remaining four carbon atoms in the benzene ligand is tied to rotation of the ML2 unit about the metal-benzene axis. At the ab initio level, a fully optimized structure was found to lie 19.8 kcal/mol lower in energy than one where the benzene ligand was constrained to be planar.