Tricarbonyliron complexes derived from dimethyl 1,3-butadiene-2,3-dicarboxylate: Formation of [Fe(CO)₃]₂-dimethyl 1,3-butadiene-2,3-dicarboxylate

Treatment of dimethyl 1,3-butadiene-2,3-dicarboxylate with Fe₂(CO)₉ provided the corresponding tricarbonyl(η⁴-dimethyl 1,3-butadiene-2,3-dicarboxylatoO)iron, [Fe(C₈H₁₀O₄)(CO)₃], (II), as the main product and the novel μ-[(1,2-η,κO³:3,4-η,κO²)-dimethyl 1,3-butadiene-2,3-dicarboxylato]bis(tricarbonyliron), [Fe₂-(C₈H₁₀O₄)(CO)₆], (III). In the crystal structure of (II), the electron-deficient butadiene complexes with one Fe(CO)₃ group, whereas in (III), the butadiene adopts an unusual conformation and forms a complex with two Fe(CO)₃ groups. In (III), each Fe atom possesses a trigonal bipyramidal geometry, with one olefinic bond and two carbonyl ligands occupying the basal positions, and the keto oxygen of the ester and the other carbonyl ligand occupying apical positions.