Open chains versus closed rings: comparison of binuclear butadiene iron carbonyls with their cyclobutadiene analogues

Abstract

The known butadiene-iron carbonyl derivatives (η4-C4H6)Fe(CO)3, (η2-C4H6)Fe(CO)4, and (η2,2-C4H6)[Fe(CO)4]2 are potential precursors to binuclear derivatives of the type (C4H6)2Fe2(CO)n related to the known cyclobutadiene derivative (η4-C4H4)2Fe2(μ-CO)3. In this, density functional theory predicts viable structures with tetrahapto C4H6 ligands as well as one, two, and three bridging carbonyl groups and formal Fe–Fe single bonds, FeFe double bonds, and FeFe triple bonds, respectively, for (C4H6)2Fe2(CO)5, (C4H6)2Fe2(CO)4, and (C4H6)2Fe2(CO)3. The tetracarbonyl (C4H6)2Fe2(CO)4 is predicted to be unfavorable with respect to disproportionation into (C4H6)2Fe2(CO)5 + (C4H6)2Fe2(CO)3. The carbonyl-richer binuclear derivatives (C4H6)2Fe2(CO)6 and (C4H6)2Fe2(CO)7 are predicted to have structures having one or two dihapto η2-C4H6 ligands, respectively, in addition to formal Fe–Fe single bonds. However, these carbonyl-rich structures are predicted to dissociate into mononuclear (η2-C4H6)Fe(CO)4 and (η4-C4H6)Fe(CO)3 fragments. Natural bond order (NBO) analysis is used to explore qualitative electronic structure considerations.