REACTIONS OF M(C5H4R)CL4 COMPLEXES (R=H, CH3) WITH PENTADIENYL ANIONS - PATHWAY VARIATIONS FOR NIOBIUM VS TANTALUM

The reactions of Nb(C5H5)Cl-4 or Nb(CH3C5H4)Cl-4 compounds with 4 equiv of various pentadienyl anions lead initially to Nb(RC(5)H(4))(eta(5)-Pdl)(eta(3)-Pdl) complexes, for Pdl = C5H7 or 2,4-C7H11 (C7H11 = dimethylpentadienyl). For the 2,4-C7H11 complexes, subsequent intramolecular coupling between the two Pdl ligands occurs on standing at room temperature, leading to Nb(RC(5)H(4))(eta(8)-2,4,7,9-tetramethyl-1,3,7,9-decatetraene) complexes. These contain one eta(4)-cis-diene and one eta(4)-trans-diene coordinated to the metal center. This has been confirmed crystallographically for the R = H complex, which crystallizes in the orthorhombic space group Pcc2 with a = 14.061(3) Angstrom, b = 14.273(3) Angstrom, c = 8.045(1) Angstrom, and Z = 4. For related tantalum reactions, a different course is followed, leading to isolation of Ta(C5H5)(2-)(eta(3)-Pdl) complexes (Pdl = 2,3-C7H11; 2,4-C7H11; 1,5-(Me(3)Si)(2)C5H5), apparently via disproportionation. A structural study on the 2,3-C7H11 complex revealed that its mode of coordination involved localized Ta-C and Ta-olefinic interactions, through the 1, 4, and 5 positions of the open dienyl ligand. The compound crystallizes in the monoclinic space group P2(1)/e with a = 7.887(2) Angstrom, b = 14.433(3) Angstrom, c = 12.645(3) Angstrom, and beta = 97.93(2)degrees for Z = 4. However, for the 1,5-(Me(3)Si)(2)C5H5 complex, the more usual allylic mode of coordination is observed. This compound crystallizes in the monoclinic space group P2(1)/n, with a = 7.422(1) Angstrom, b = 23.934(5)Angstrom, c = 13.059(2) Angstrom, and beta = 97.52(1)degrees for Z = 4.