Reactivity of alkynes toward M-η2-CS2 metal complexes. 2. A theoretical discussion on the coupling products obtainable with iron fragments and their relative stability

Complexes of the type (CO)₂(PR₃)₂Fe-η²-CS₂, known to yield different coupling products with alkynes, are studied. In particular two products, the metallocycle L₄Fe-C(S)C(R)C(R)S and the 1,3-dithiol-2-ylidene L₄Pe-CSC(R)C(R)S, isomerize easily and, for an appropriate choice of the phosphine ligands, are in equilibrium. Extended Hückel calculations and qualitative MO theory are used to analyze the electronic features of these compounds and their possible interconversion pathways. The study illustrates the major orbital effects and, indirectly, the electrostatic effects that may become selective at a certain point of the reaction.