Reactions of Co-ordinated Ligands.  Part 27.  Formation of h³-Allyl and s-Vinyl Complexes by Nucleophilic Attack on Cationic Molybdenum Monoacetylene Complexes.  Molecular Structures of [Mo(CO)(PEt₃)(h³-anti-1-MeC₃H₄)(h⁵-C₉H₇)] and [Mo{P(OMe)₃}₃{s-(E)-CHCHBu^t}(h-C₅H₅)]

Reaction of [MoLL¹(η-MeC₂R)A][BF₄] [R = Me, Et, or Prⁱ; L = L¹ = P(OMe)₃; LL¹ = Ph₂PCH=CHPPh₂; L = PEt₃, L¹ = CO; A = η-C₅H₅ or η⁵-C₉H₇] with NaBH₄ or K[BHBu^s₃] in tetrahydrofuran affords the anti-substituted η³-allyl complexes [MoLL¹(η³-CH₂-...CH-...CHR)A]. The structure of the complex [Mo(CO)(PEt₃)(η³-anti-1-MeC₃H ₄)(η⁵-C₉H₇)] was established by single-crystal X-ray crystallography. Crystals are monoclinic, space group P2₁/n with a = 10.038(2), b = 17.291(4), c = 11.346(3) Å, β = 95.000(18)°, and Z = 4. Using 3 887 reflections measured on a four-circle diffractometer at room temperature, the structure has been refined to R 0.0344, R′ 0.0459. The molecule adopts an exo-conformation with the methyl substituent of the allyl ligand in an anti-position cis to the carbonyl ligand. Treatment of the complex [Mo{P(OMe)₃}₂(η-HC₂Bu ^t)(η-C₅H₅)][BF₄] with NaBH₄ or K[BHBu^s₃] in the presence of excess P(OMe)₃ gives the σ-vinyl complex [Mo{P(OMe)₃}₃{σ-(E)-CH=CHBu ^t}(η-C₅H₅)], structurally identified by n.m.r, spectroscopy and X-ray crystallography. Crystals are monoclinic, space group P2₁/c with a = 11.731(3), b = 12.796(3), c = 18.827(4) Å, β = 92.33(2)°, and Z = 4. Using 7 437 intensity data, measured at 195 K on a four-circle diffractometer, the structure has been refined to residuals R = 0.026, R′ = 0.026. The molecule shows typical four-legged 'piano-stool' geometry with the vinyl group adopting E stereochemistry, σ-bonded to the molybdenum atom. In contrast, the reaction of K[BHBu^s₃] with [Mo{P(OMe)₃}₂(η-HC₂Pr ⁱ)(η-C₅H₅)][BF₄] in tetrahydrofuran at low temperature affords the allyl complex [Mo{P(OMe)₃}₂(η³-1,1-Me₂C ₃H₃)(η-C₅H₅)]. The n.m.r. spectra and mechanism of formation of these compounds are discussed.