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

Abstract

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.