Reversible complexation of ammonia by breaking a manganese–manganese bond in a manganese carbonyl ethylenedithiolate complex: a theoretical study of an unusual type of Lewis acid
The reaction of Mn(CO)5Br with sodium ethylenedithiolate was reported in 1968 to give a dark red binuclear H2C2S2Mn2(CO)6 complex possessing the unusual property of complexing reversibly with ammonia to give a yellow H2C2S2Mn2(CO)6·NH3 adduct. In order to provide some insight into the nature of this adduct, density functional studies were performed on the H2C2S2Mn2(CO)n (n = 4 to 8) systems as well as their relevant ammonia and trimethylphosphine adducts. These theoretical studies support the structure of H2C2S2Mn2(CO)6 originally suggested 50 years ago involving the binding of the ethylenedithiolate CC double bond as well as the sulfur atoms to the Mn2 unit with a bonding Mn–Mn distance of ∼2.8 Å. Complexation of H2C2S2Mn2(CO)6 with NH3 or Me3P preserves the complexed CC double bond of the ethylenedithiolate ligand but lengthens the Mn⋯Mn distance to a non-bonding ∼3.6 Å. Thus H2C2S2Mn2(CO)6 represents a novel type of Lewis acid where reversible complexation with Lewis bases involves the rupture of a metal–metal bond. Carbonyl dissociation energies in the H2C2S2Mn2(CO)n series account for the formation of the hexacarbonyl H2C2S2Mn2(CO)6 as the stable product from the Mn(CO)5Br/ethylenedithiolate reaction.