Unsaturation in Binuclear (Cyclobutadiene)cobalt Carbonyls with Axial and Perpendicular Structures: Comparison with Isoelectronic Binuclear Cyclopentadienyliron Carbonyls
journal contributionposted on 02.04.2020 by Hongyan Wang, Yaoming Xie, R. Bruce King, Henry F. Schaefer
Any type of content formally published in an academic journal, usually following a peer-review process.
The binuclear (cyclobutadiene)cobalt carbonyls (η4-C4H4)2Co2(CO)n (n = 1−4) have been studied by density functional theory (DFT) using the B3LYP and BP86 methods. The singlet doubly bridged and unbridged isomers of (η4-C4H4)2Co2(CO)4 are nearly degenerate, suggesting a highly fluxional system similar to the experimentally known and closely related (η5-C5H5)2Fe2(CO)4 and (η4-diene)2Co2(CO)4 systems. The global minimum of (C4H4)2Co2(CO)3 is the doubly bridged singlet isomer (η4-C4H4)2Co2(CO)(μ-CO)2, in contrast to the isoelectronic (C5H5)2Fe2(CO)3, where the global minimum is the triply bridged triplet isomer (η5-C5H5)2Fe2(μ-CO)3, analogous to dioxygen. A related triply bridged triplet isomer of (C4H4)2Co2(CO)3 is also found, but at 9.6 kcal/mol (B3LYP) or 5.1 kcal/mol (BP86) above the global minimum. The (C4H4)2Co2(CO)2 system behaves analogously to the corresponding (C5H5)2Fe2(CO)2 system in that its lowest energy structure is the doubly bridged isomer (η4-C4H4)2Co2(μ-CO)2 with a short cobalt−cobalt distance suggestive of the Co⋮Co triple bond required to give both cobalt atoms the favored 18-electron configuration. Both axial and perpendicular metallocene structures are found for (C4H4)2Co2(CO), including a low-energy (η4-C4H4)2Co2(η2-μ-CO) structure with a four-electron-donor bridging CO group and a cobalt−cobalt distance suggestive of a Co⋮Co triple bond.