Journal of Crystallographic and Spectroscopic Research, Vol. 12, No. 2, 1982 Crystal and molecular structure of 075-CsHs)Ta072-C2H4)CI2(PMe2Ph)2, a sterically crowded molecule which exhibits a distorted 775-coordination mode of the cyclopentadienyl ligand JERRY L. ATWOOD,* MICHAEL B. HONAN, and ROBIN D. ROGERS Department of Chemistry University of Alabama University, Alabama 35486 (Received November 11, 1981) Abstract The X-ray structure determination of (r;LCsHs)Ta(~LC2H4)C12(PMe2Ph)2 has revealed that the compound crystallizes in the orthorhombic space group Fdd2 with a = 21.098(9), b = 29.848(9), c = 7.694(3) A, and Dc = 1.70g cm -3 for Z = 8. Least-squares refinement based on 893 independent observed reflections resulted in a final R value of 0.026. The molecule resides on a crystallographic two-fold axis, with disordered ethylene and cyclopentadienyl groups. The Ta-C1 and Ta-P separations are 2.473(4) and 2.649(3) A, respectively. Since the two carbon atoms of the ethylene molecule are indistinguishable from two of the cyclopentadienyl carbon atoms, the Ta-C bond lengths are 2.37(1) and 2.39(2) A for the ethylene and for two of the cyclopentadienyl carbon atoms. The other three Ta-C(cyclopentadienyl) distances are much longer: 2.54(3), 2.54(3), and 2.62(3) A. In order to achieve 18 e- at the tantalum atom, the cyclopentadienyl ligand must be viewed in an r/Lbonding mode. The variation of the Ta-C lengths is thus considered to be steric in origin. The carbon atoms of the CsH~ group are planar to 0.03 A. Introduction Among the Group IVb and Vb metals, the structural chemistry of organometallic derivatives of tantalum is remarkably well developed. This is 205 0277-8068/82/0400-0205503.00/0 9 1982 Plenum Publishing Corporation