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 Fe-2(CO)(9) prov ided the corresponding tricarbonyl(eta(4)-dimethyl 1,3-butadiene-2,3-dicarb oxylato-O)iron, [Fe(C8H10O4)(CO)(3)], (II), as the main product and the nov el mu-[(1,2-eta,kappa O-3:3,4-eta,kappa O-2)-dimethyl 1,3-butadiene-2,3-dic arboxylato]bis(tricarbonyliron), [Fe-2(C8H10O4)(CO)(6)], (III). In the crys tal structure of (II), the electron-deficient butadiene complexes with one Fe(CO)(3) group, whereas in (III), the butadiene adopts an unusual conforma tion 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 c arbonyl ligands occupying the basal positions, and the keto oxygen of the e ster and the other carbonyl ligand occupying apical positions.