The facile coupling of carbon monochalcogenides to ethenedichalcogenone ligands in binuclear iron carbonyl derivatives: a theoretical study

Abstract

The lowest energy Fe₂(CO)_n(CE)₂ structures (E = S, Se, Te; n = 8, 7, 6) do not have separate CE ligands but instead have coupled C₂E₂ ligands functioning as four to six electron donors to the pair of iron atoms. The exothermicity of such coupling reactions as a function of the chalcogen E increases in the sequence S < Se < Te. However, the monomers Fe(CO)₄(CE) are stable species owing to the high activation energies for the coupling reactions. Thus, the thiocarbonyl Fe(CO)₄(CS) is a stable experimentally known complex. However, the coupled Fe₂(CO)₇(C₂E₂) structures are preferred both thermodynamically and kinetically over isomeric Fe₂(CO)₇(CE)₂ structures with discrete CE ligands. For the CS ligand, the previously optimized lowest energy Fe₂(CO)_n(CS)₂ structures (n = 7, 6) with discrete CS ligands are now found to lie ∼11 kcal mol⁻¹ (n = 7) and ∼30 kcal mol⁻¹ (n = 6) in energy above the lowest energy isomeric Fe₂(CO)_n(C₂S₂) structures with coupled C₂E₂ ligands.