Unsaturation in Binuclear Benzene Manganese Carbonyls: Comparison with Isoelectronic Cyclopentadienyliron and Cyclobutadienecobalt Derivatives

Abstract

The binuclear benzenemanganese carbonyls (η6-C6H6)2Mn2(CO)n (n = 4, 3, 2, 1) have been studied by density functional theory (DFT) using the B3LYP and BP86 methods. The singlet doubly bridged and unbridged isomers of the saturated (η6-C6H6)2Mn2(CO)4 are nearly degenerate, suggesting a highly fluxional system. The global minimum of (η6-C6H6)2Mn2(CO)3 is a triplet electronic state—a triply bridged isomer analogous to the known isoelectronic triplet Cp2Fe2(μ-CO)3. The doubly bridged singlet structure (η6-C6H6)2Mn2(CO)(μ-CO)2 lies 9 kcal/mol (B3LYP) or 7 kcal/mol (BP86) above this global minimum. The global minimum of (η6-C6H6)2Mn2(CO)2 is a doubly bridged structure with a short manganese−manganese distance suggestive of a Mn≡Mn triple bond. Singlet and triplet structures are found for (η6-C6H6)2Mn2(CO) with a two-electron donor CO group and metal−metal distances suggesting a M−M quadruple bond for the singlet state and a M≡M triple bond for the triplet state.