~ Pergamon 0277-5387(95)00468-8 Poh'hedron Vol. 15, No. 12, pp. 2071 2078, 1996 Copyright c 1996 Elsevier Science Lid Printed in Great Britain. All rights reserved 0277 5387/96 $15.00+0.00 SYNTHESIS AND CRYSTAL STRUCTURES OF HYDRIDOTRIS(3,5-DIMETHYLPYRAZOLYL)BORATE TANTALUM(V) (:N-2,6-Pr2C6H3)(X)CI (X = C1, Bu"O) JAMES M. BONCELLA,* MARY L. CAJIGAL, A. SCOTT GAMBLE and KHALIL A. ABBOUD Department of Chemistry and the Center for Catalysis, University of Florida, Gainesville, FL 32611-7200, U.S.A. (Received 7 July 1995; accepted 15 September 1995) Abstract The reaction between Ta(~NAr)(CI)3(THF)2 and one equivalent of potassium hydridotris(3,5-dimethylpyrazolyl)borate (Tp') gave the compound Tp"Ta(--NAr)(C1)2 (1) as an orange crystalline solid. The salt, [H2B:(p-3,5-Me2HC3N2)][(ArNn)TaC14(3,5- Me2HC3N2H)] (2) was also formed as a by-product in this reaction. These compounds were characterized by NMR spectroscopy as well as by single-crystal X-ray diffraction studies. Reaction of 1 with excess Na/Hg in wet THF gave Tp'Ta(=NAr)CI(OBu n) (3) in good yield. The butoxide ligand in 3 is formed by the ring opening of a THF molecule of the solvent. Single-crystal X-ray diffraction studies of I and 3 show the presence of a TEN triple bond and the expected trans influences of the CI , OR- and imido ligands on the Ta--N bond lengths of the coordinated pyrazole ligands. We have been interested in the use of the tridentate chelate ligand hydridotris(3,5-dimethylpyrazolyl) borate (Tp') as an ancillary ligand in high oxidation state Group 6 metal complexes that also contain alkylidene and imido ligands. 1 Our interest in these compounds arises because they are precursors to olefin metathesis catalysts when they contain the alkylidene functionality. The Tp' ligand system pro- vides a significant degree of steric bulk and occupies three coordination sites, which results in kinetically inert, six-coordinate complexes. We have extended the use of the Tp' ligand to tantalum imido com- plexes and now report the synthesis, charac- terization and crystal structures of several of these complexes. During the course of this work, the preparation of several related oxo and imido com- plexes has been reported. 2 * Author to whom correspondence should be addressed. RESULTS AND DISCUSSION The reaction of KTp' with one equivalent of Ta(=NAr)(CI)3(THF)2 (Ar = 2,6-Pr~C6H3) 3 gave the orange crystalline complex, Tp'Ta(=NAr)(C1)2 1 as well as a yellow crystalline compound that was subsequently characterized as the salt, [H2Bz(#-3,5- Me2HC3N:)] [(ArN=)TaCI4(3,5-Me2HC3N2H)] [2 ; eq. (1)]. Compound 1 is soluble in aromatic hydro- carbons and can be separated from 2 by extraction of the crude reaction product with toluene. It is air stable for weeks to months in the solid state, but it slowly decomposes in solution when exposed to the atmosphere. The ~H NMR spectrum of 1 displays peaks at 5.32 and 5.78 ppm in a 2:1 ratio, as well as four methyl peaks in the region from 1.8 to 2.9 ppm in a 6 : 3 : 6 : 3 ratio, consistent with a six-coordinate structure which has two sets of equivalent pyrazole rings, as shown in eq. (1). The observation of two sets of peaks that correspond to two different PP 2071