BUTA-1,3-DIENYLIDENE VERSUS CYCLOPENTYLIDENE BRIDGING LIGANDS IN THE REACTION OF TETRACYANOETHYLENE WITH SUBSTITUTED-ALKENYLIDENE-BRIDGED DIIRON COMPLEXES - X-RAY CRYSTAL-STRUCTURE OF [(ETA5-C5H5)FE(CO)]2(MU-CO)[MU-CCH2C(CH3)2C(CN)2C(CN)2]

Substituted-alkenylidene-bridged diiron complexes [CpFe(CO)]2(μ-CO)(μ-C═CHR) (R = CH3 (2), CH(CH3)2 (4)) react with tetracyanoethylene to give different types of products depending on R. When R = CH3 (2), the tricyanovinylation of the alkenylidene ligand affords the μ-3,4,4-tricyanobuta-1,3-dienylidene complex [CpFe(CO)]2(μ-CO)[μ-C═CCH3C(CN)═C(CN)2] (3). The IR and NMR data indicate that the bridging ligand is a powerful π-acceptor. When R = CH(CH3)2 (4), a formal [3 + 2] cycloaddition accompanied by a 1,2-hydrogen shift occurs, leading to the novel tetracyano-substituted μ-cyclopentylidene complex [CpFe(CO)]2(μ-CO) [formula omitted] (5), characterized by an X-ray analysis. Crystals of 5 are monoclinic, space group P21/n, with a = 8.880 (3) Å, b = 17.112 (4) Å, c = 14.882 (4) Å, β = 105.33 (3)°, and Z = 4. The structure was refined to R = 0.041 and Rw = 0.035 for 2505 reflections with I > σ(I). An elongated Fe–Fe bond (2.538 (3) Å) and Fe–(μ-C) bonds (Fe1–C14 = 2.024 (3) Å; Fe2–C14 = 2.030 (3) Å) associated with an Fe–(μ-C)–Fe angle of 77.5 (1)° reveal a relatively weak bonding between the μ-cyclopentylidene ligand and the bimetallic moiety, largely attributed to steric interactions and cycle strain within the bridging five-membered ring. The availability of a tertiary allylic hydrogen in 4, and not in 2, together with steric demands due to the bulkiness of the isopropyl group in 4 may explain the different reactivities of these complexes. © 1990, American Chemical Society. All rights reserved.