CRYSTAL-STRUCTURE AND MOLECULAR-GEOMETRY OF TA(=CHCME3)2(MESITYL)(PME3)2, A BIS(ALKYLIDENE) COMPLEX OF TANTALUM WITH REMARKABLY OBTUSE TA=C(ALPHA)-C(BETA) ANGLES OF 154.0 (6) AND 168.9 (6)-DEGREES

Mesitylbis(neopentylidene)bis(trimethylphosphine)tantalum(V), Ta(═CHCMe3)2(mesityl)(PMe3)2, previously prepared by Schrock and co-workers, has been subjected to a single-crystal X-ray structural analysis. The complex crystallizes in the centrosymmetric monoclinic space group C2/c (C2h6; No. 15) with a = 17.966 (3) Å, b = 18.217 (4) Å, c = 18.531 (3) Å, β = 97.99 (2)°, V = 6006 (2) Å3, and Z = 8. Diffraction data were collected with a Syntex P2X four-circle automated diffractometer by using a coupled θ(crystal)-2θ(counter) scan. The structure was solved by means of Patterson, Fourier, and least-squares refinement techniques. All nonhydrogen atoms were located and refined, the final discrepancy indices being RF=5.3% and RwF=3.7% for all 3946 reflections with 4.5 > 2θ > 45° (Mo Kα radiation). The central tantalum atom is a distorted trigonal-bipyramidal coordination environment, with the phosphine ligands in axial sites (Ta-P(1) = 2.565 (2) Å, Ta-P(2) = 2.569 (2) Å, < P(1)-Ta-P(2) = 166.34 (7)°). The three equatorial sites are occupied by an η1-mesityl ligand (Ta-C(11) = 2.303 (6) A and two η1-neopentylidene ligands (Ta-C(1) = 1.932 (7) Å, Ta-C(6) = 1.955 (7) A). The Ta═C(α)—C(β) angles in the two neopentylidene ligands are extraordinarily obtuse for angles at formally sp2-hybridized carbon atoms and have values of 154.0 (6) and 168.9 (6)°, respectively. © 1979, American Chemical Society. All rights reserved.