THE MOLECULAR-STRUCTURE OF CYCLOPENTADIENYL MANGANESE TRICARBONYL DETERMINED BY GAS-PHASE ELECTRON-DIFFRACTION

The molecular structure of cyclopentadienyl manganese tricarbonyl, C5H5Mn(CO)3, has been studied by gas-phase electron diffraction at 70-degrees-C. Two models fit the experimental data equally well: model A, which allows for free rotation of the Mn-(CO)3 moiety (that has C3v Symmetry) about the Mn-X axis (X is the centre of the cyclopentadienyl ring), and model B, which has a staggered arrangement of the carbonyl groups with respect to the cyclopentadienyl ring. The final geometrical parameters for model A are: r(g)(Mn-C(ring)) = 2.149(3)angstrom, r(g)(Mn-CO) = 1.808(3) angstrom, r(g)(C-C) = 1.424(2) angstrom, r(g)(C-O) = 1.148(3) angstrom, angle(OC-Mn-CO) = 92.0(0.5)-degrees with an R-factor of 5.55%. The final geometrical parameters for model B are: r(g)(Mn-C(ring)) = 2.149(3) angstrom, r(g)(Mn-CO) = 1.808(3) angstrom, r(g)(C-C) = 1.424(2) angstrom, r(g)(C-O) = 1.148(2) angstrom, angle(OC-Mn-CO) = 91.9(0.4)-degrees, R = 5.55% from which it can be seen that the geometrical parameters show no significant difference from those obtained with model A. No definitive evidence was obtained for the H-C bonds being bent out of the plane of the Cp ring. When the H-C-X angle was allowed to refine, a value of 1.7(3.4)-degrees was obtained. A positive value indicates that the hydrogen atoms are on the same side of the ring as the manganese atom.