CRYSTAL AND MOLECULAR-STRUCTURE OF PENTA(4-METHYLBENZYL)TANTALUM, [TA(CH2C6H4-4ME)5]

Reaction of the dichloride [TaCl2(CH2-C6H4-4Me)3] (1) (CH2-C6H4-4Me = 4-methylbenzyl) with the dialkylmagnesium compound [Mg(CH2-C6H4-4-Me)2] leads to the homoleptic 4-methylbenzyl compound [Ta(CH2-C6H4-4Me)5] (2). The solid state structure of 2 shows a distorted square-pyramidal geometry for the central TaC5 core. Four of the 4-methylbenzyl ligands are sigma-bound to the tantalum with Ta-CH2-C angles of 108.4(5)-125.7(5)-degrees, while the remaining ligand has an acute Ta-CH2-C angle of 90.4(5)-degrees. In solution all alkyl groups are equivalent by H-1 and C-13 NMR even at -60-degrees-C, indicating facile exchange of non-equivalent alkyl groups on the NMR time-scale.