ansa-Cyclopentadienyl-Arene Tantalum Complexes: Structure and Reactivity of Neutral, Cationic, and Dicationic Derivatives

The cationic tantalum complex {[η⁶-Ar-CMe₂-η⁵-C₅H₄]­TaPr}­[B­(C₆F₅)₄] (1; Ar = 3,5-Me₂C₆H₃) serves as a starting material for a series of neutral, monocationic, and dicationic derivatives. The cationic hydride {[η⁶-Ar-CMe₂-η⁵-C₅H₄]­TaH}­[B­(C₆F₅)₄] (2) that results from hydrogenolysis of 1 inserts the di- and trisubstituted olefins cyclopentene and 2-methyl-2-pentene; it reacts with styrene to give the 2,1-insertion product, for which the Ta–CH­(Me)­Ph group is bound in a σ³-allylic fashion. A neutral complex is obtained from 1 by reaction with Br^–, and a dicationic derivative is available by hydride abstraction from 2 using the Lewis acidic trityl cation. All compounds described here retain the unusual ansa-(η⁵-cyclopentadienyl,η⁶-arene) coordination mode of the ligand that stabilizes the formally Ta­(III) center. X-ray structures and DFT calculations show that the metal–arene interaction contains a significant π back-donation component (arene←Ta­(III)) that differs only slightly in the series, despite the variation of charge at the metal center.