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

摘要

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)center dot 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 C=C double bond as well as the sulfur atoms to the Mn-2 unit with a bonding Mn-Mn distance of similar to 2.8 angstrom. Complexation of H2C2S2Mn2(CO)(6) with NH3 or Me3P preserves the complexed C=C double bond of the ethylenedithiolate ligand but lengthens the Mn. Mn distance to a non-bonding similar to 3.6 angstrom. 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)(5)Br/ethylenedithiolate reaction.