Open chains versus closed rings: comparison of binuclear butadieneironcarbonyls with their cyclobutadiene analogues

Abstract

The known butadiene-iron carbonyl derivatives (η⁴-C₄H₆)Fe(CO)₃, (η²-C₄H₆)Fe(CO)₄, and (η^2,2-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.