The 13C and 119Sn NMR spectra of some four- and five-coordinate tri-n-butyltin(IV) compounds

Silver tetrafluoroborate reacts with Cl(bipy)(CO)₂(η³-C₃H₅)Mo in CH₂Cl₂ to give the dimeric cation, (μ-Cl)[(bipy)(CO)₂(η³-C₃H₅)Mo]₂⁺, isolated as the crystalline BF₄^? salt with CH₂Cl₂ of solvation (5). Complex 5 crystallizes in the triclinic system (space group P$\text{1}$, No. 2) with cell parameters: a 11.831(2), b 10.142(3), c 15.618(3) Å; α 93.96(2), β 104.33(2), γ 91.41(2)°, V 1809.5(8) ųZ = 2, ?_calc 1.60 g cm^?3. Full matrix refinement with all but three atoms anisotropic converged at R₁ = 0.057 and R₂ = 0.073 based on 3742 reflections with I > 30(1). The two halves of the dimer are connected by a single chloride bridge (Mo-Cl 2.554(3), 2.519(3) Å, Mo-Cl-Mo 134.0(1)°). The η³-allyl group is oriented so that its open face points toward the two cis-carbonyl groups, a feature common to all L₂X(CO)₂Mo(η³-allyl) structures determined to date. A molecular orbital analysis shows that this rotational preference of the allyl group has its roots in the strong π-bonding character of the carbonyls. The MO analysis also provides a rationale for regioselectivity observed in the reactions of various η³-allyl complexes with nucleophiles. In particular, the factors which determine whether the terminal carbons are attacked (giving olefin) or whether the central carbon is attacked (giving a metallacyclobutane) are exposed.