Mononuclear and binuclear manganese carbonyl hydrides: the preference for bridging hydrogens over bridging carbonyls

Abstract

The structures of the mononuclear derivatives HMn(CO)_n (n = 4 and 3) are shown by density functional theory (B3LYP and PB86) to derive from octahedral HMn(CO)₅ by losses of various combinations of carbonyl groups with relatively little change in the C–Mn–C angles involving the remaining carbonyl groups. The binuclear H₂Mn₂(CO)_n structures are predicted to have bridging hydrogen atoms in preference to bridging carbonyl groups. Thus, two structures are found for the binuclear H₂Mn₂(CO)₉, isoelectronic with Fe₂(CO)₉, in which all nine of the carbonyl groups are terminal carbonyl groups. The lowest lying H₂Mn₂(CO)₉ structure is the dihydrogen complex (OC)₅Mn–Mn(CO)₄(η²-H₂), in which one of the equatorial CO groups of Mn₂(CO)₁₀ is replaced by a dihydrogen ligand. A slightly higher energy H₂Mn₂(CO)₉ structure by ∼6 kcal mol⁻¹ has an Mn–Mn bond bridged by a single hydrogen and all terminal carbonyl groups as well as a single terminal hydrogen. The H₂Mn₂(CO)₈ molecule is predicted to have a structure with a central Mn(μ-H)₂Mn core related to diborane. In this structure the manganese–manganese bond of length 2.703 Å (BP86) can be considered the diprotonated formal double bond required to give both manganese atoms the favoured 18-electron configuration. The more highly unsaturated H₂Mn₂(CO)_n (n = 7, 6) derivatives have similar structures derived from the H₂Mn₂(CO)₈ structure by loss of one or two carbonyl groups. In many cases the MnMn distance in the central Mn(μ-H)₂Mn unit shortens to ∼2.4 Å suggesting a diprotonated triple bond.