ZIRCONIUM AND HAFNIUM DIENE AND DIENYL HALF-SANDWICH COMPLEXES - SYNTHESIS, POLYMERIZATION CATALYSIS AND DEACTIVATION PATHWAYS - THE MOLECULAR-STRUCTURES OF )-C3H5)(2,3-ME2C4H4)(ETA-C5H3(SIME3)(2)-1,3)](M=ZR OR HF) AND (3)-CH2CMECMECH2B-(C6F5)(3))(ETA-C5H3(SIME2)-1,3)]

The reduction of [MCl3Cp ''] [M = Zr or Hf; Cp '' = eta-C5H3(SiMe3)(2) -1,3] with sodium amalgam in the presence of dienes gave the compounds [MCl(diene)Cp ''] which are alkylated with MeMgBr or RMgCl (R = allyl ) to give [MX(diene)Cp ''] (X = CH3 or eta(3)-C3H5; diene = 2,3-dimeth ylbuta-1,3-diene or isoprene). The reduction of [ZrCl(CHCMeCH)Cp ''] w ith an excess of Na-Hg leads to the binuclear eta(4)(5e)-butadienyl co mplex [Zr(mu-eta(1):eta(4)-C4H3Me2-2,3)Cp ''](2), also formed from {Zr Me(C4H4Me2)Cp ''] by methane elimination. The butadiene complex [Zr(et a(3)-CH2CMeCHCH2)(eta(2)-C4H6)Cp ''] by methane elimination. The butad iene complex [Zr(eta(3)-CH2CMeCHCH2)(eta(4)-C4H6)Cp ''] is obtained di rectly from [ZrCl3Cp ''] and MeCHCHCH2MgCl. The complexes [M(allyl)(di ene)Cp ''] react with B(C6F5)(3) to give the zwitterionic complexes [C p '' eta(3)-C3H5)-{eta(3)-(C4H4RRB-)-R-1-B-2(C6F5)(3)}] which contain a 14-electron [CpM(allyl)(2)](+) core stabilised by agostic bonding of the B-CH2 methylene hydrogens. These zwitterions catalyse the polymer isation of ethene to high molecular weight polyethene. Catalysts with similar activities are obtained by the activation of [M(allyl)(diene)C p ''] with [CPh3]-[B(C6F5)(4)]. The thermal stability of the zwitterio nic active species depends strongly on the steric requirements of the dienyl ligands and decreases sharply in the order R-1 = R-2 = Me > R-1 = Me, R-2 = H much greater than R-1 = H, R-2 = H; i.e. the dimethylbu tadiene derivatives are stable at room temperature, while in the latte r case decomposition is significant even at -60 degrees C. The complex es CH2CHCHR1){eta(3)-(CH2CRMeCHCH2B)-Me-2(C6F5)(3)}Cp ''] (R-1 = H, R- 2 = Me, 9a; R-1 = Me, R-2 = H, 10) decompose via an unusual C-H activa tion pathway, with alkene elimination and concomitant migration of a C 6F5 substitutuent from boron to zirconium, to give the catalytically i nactive borylidene complexes [Zr(C6F5){eta(4)-(CH2CRCHCHB)-C-1(C6F5)(2 )}Cp '']. The crystal structures of [M(C3H5)(Me2C4H4)Cp ''] (M = Zr or Hf) and [Hf(eta(3)-CH2CMeCMeCH2B(C6F5)(3)}Cp ''] are reported.