Binuclear butadiene chromium carbonyls: Comparison with their trimethylenemethane isomers

Butadiene is known to form a chromium carbonyl complex (η4-C4H6)Cr(CO)4, which has been structurally characterized by X-ray crystallography. The structures and energetics of the butadiene chromium carbonyl complexes (C4H6)Cr(CO)m (m = 5, 4, 3, 2) and (C4H6)2Cr2(CO)n (n = 7, 6, 5) have now been investigated by density functional theory. The lowest energy (C4H6)2Cr2(CO)7 structure is a singlet singly bridged structure with a Cr–Cr single bond distance of ∼3.1 Å. This structure is predicted to be disfavored with respect to dissociation into mononuclear fragments. The lowest energy singlet (C4H6)2Cr2(CO)6 structure is an unbridged structure with a CrCr double bond distance of ∼2.7 Å. However, a doubly bridged isomeric singlet (C4H6)2Cr2(CO)4(μ-CO)2 structure with two bridging CO groups and a similar CrCr double bond lies only slightly higher in energy. Related triplet (C4H6)2Cr2(CO)6 structures with longer Cr–Cr distances of ∼3.0 Å corresponding to formal single bonds are comparable in energy to the singlet structures. The hexacarbonyl (C4H6)2Cr2(CO)6 is predicted to be energetically disfavored with respect to disproportionation into (C4H6)2Cr2(CO)7 + (C4H6)2Cr2(CO)5. The lowest energy (C4H6)2Cr2(CO)5 structure has all terminal CO groups and a short CrCr triple bond distance of ∼2.4 Å. A higher energy (C4H6)2Cr2(CO)5 structure by ∼4 kcal/mol has a four-electron donor η2-μ-CO group and a CrCr double bond distance of ∼2.7 Å. These (C4H6)2Cr2(CO)n (n = 7, 6, 5) structures are similar to the isomeric [(CH2)3C]2Cr2(CO)n structures that are previously reported except for some differences in the arrangements of the bridging carbonyl groups.