Formation and reactivity of the [Cp ' Ta-2(H)(2)](+) cation (Cp ' = eta(5)-(BuC5H4)-Bu-t)

The complex Cp'Ta-2(H)(OCOCF3)(2) (2) is synthesized by the acidolysis of C p'2TaH3 (1) (Cp' = eta(5)-(BUC5H4)-B-t) in the presence of an excess of tri fluoroacetic acid. 1 undergoes a loss of H- upon the reaction of electrophi le (H+ or CPh3+). The resulting cation [Cp'Ta-2(H)(2)](+) ([3](+)) can be i solated as a solvento adduct [Cp'Ta-2(H)(2)(SMe2)]BF4 (4 . BF4) by caning o ut the reaction in dimethyl sulfide. Oxidation of 1 by means of ferrocenium hexafluorophosphate gives 4 . PF6. Coordination of various neutral two-ele ctron-donor L ligands affords the corresponding cationic species [Cp'Ta-2(H )(2)(L)](+) as BF4 or PF6 salts with L = tetrahydrothiophene (5), CN-Bu-t ( 6), PMe2Ph (8), and CO (9). Microanalytical, mass spectrometric, and infrar ed and NMR (H-1, F-19, P-31) spectroscopic data for the complexes are repor ted, along with the X-ray crystal structure of [Cp'Ta-2(H)(2)(SMe2)]BF4 (4 . BF4). The structural and spectroscopic properties of the dihydride comple xes (4-8, 12, and 13) are consistent with a central position of the ligand at the tantalum center. The oxidation of 1 in the presence of acetonitrile results in the formation of the two isomeric azaalkenylidene complexes [Cp' Ta-2(H)(NCHMe)]PF6 (9/9'. PF6), but only one isomer, 10 . PF6, is obtained with pivalonitrile. By heating in acetonitrile, complex 4 . BF4 loses a mol ecule of dimethyl sulfide and is transformed to the mixture 9/9'. PF6. The cationic heterobimetallic mu-hydrido complex [Cp'Ta-2(H)(2)(mu-H)Nb(CO)Cp-2 ]PF6 (11 . PF6) is synthesized by reaction of [3](+) with Cp2Nb(CO)H, The n eutral dihydride complexes Cp'Ta-2(H)(2)(X), with X = SEt (12), CN (13), ar e formed by treatment of [3](+) with the corresponding anionic species. The central position of the ligand X is proposed from 1H NMR spectroscopic dat a. When treated with potassium hydroxide, [3](+) leads to the formation of the known oxo complex Cp'Ta-2(=O)H.