Binuclear allyliron carbonyls: Fragile dimers and diverse types of allyl groups

The known (η3-C3H5)2Fe2(CO)6 is an interesting unbridged “fragile dimer” having an unusually long ∼3.1 Å Fe–Fe bond, which readily breaks to form paramagnetic (C3H5)Fe(CO)3 monomers. Theoretical studies confirm this experimental (η3-C3H5)2Fe2(CO)6 structure but also predict higher energy (η3-C3H5)2Fe2(CO)4(μ-CO)2 structures with two bridging carbonyl groups and Fe2(η3-μ-C3H5)2(CO)6 structures with two bridging allyl groups. For the pentacarbonyl (C3H5)2Fe2(CO)5 low energy structures with four-electron donor bridging CO groups and structures with FeFe formal double bonds are both found. Both types of structures satisfy the 18-electron rule. The lowest energy structures for the tetracarbonyl (C3H5)2Fe2(CO)4 have Fe–Fe single bonds and obvious gaps in the iron coordination spheres consistent with an iron 16-electron configuration. However, a higher energy (C3H5)2Fe2(CO)4 structure is found with the short (∼2.3 Å) formal FeFe triple bond predicted by the 18-electron rule. Carbonyl-rich (C3H5)2Fe2(CO)n (n = 7, 8) structures are found having monohapto allyl ligands. However, such structures have relatively low carbonyl dissociation energies to give the experimentally known (η3-C3H5)2Fe2(CO)6 and thus are not likely to be viable candidates for experimental realization under normal conditions.

Theoretical studies confirm the experimental “fragile dimer” (η3-C3H5)2Fe2(CO)6 structure but also predict higher energy structures with two bridging carbonyl groups and structures with two bridging allyl groups. Low-energy (C3H5)2Fe2(CO)n (n = 5, 4) structures are found having four-electron donor η2-μ-CO groups, iron–iron multiple bonds, or coordinately unsaturated iron atoms. Low-energy carbonyl-rich (C3H5)2Fe2(CO)n (n = 7, 8) structures are found with monohapto allyl groups. However, these structures have low CO dissociation energies to give (C3H5)2Fe2(CO)6.Figure optionsDownload as PowerPoint slide