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Unsaturation in Binuclear (Cyclobutadiene)cobalt Carbonyls with Axial and Perpendicular Structures:  Comparison with Isoelectronic Binuclear Cyclopentadienyliron Carbonyls

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journal contribution
posted on 02.04.2020, 16:53 by Hongyan Wang, Yaoming Xie, R. Bruce King, Henry F. Schaefer
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)2Co22-μ-CO) structure with a four-electron-donor bridging CO group and a cobalt−cobalt distance suggestive of a Co⋮Co triple bond.