Theoretical Study of the Pyramidal Geometry around the Sulfur in the S-Bound Mode of Coordination of Thiophene to the [Cp(CO)₂Fe]⁺ Fragment

Density functional calculations have been carried out to study the pyramidal coordination of the sulfur in the thiophene complex [Cp(CO)₂Fe(η¹-T)]⁺ (Cp = cyclopentadienyl; T = thiophene). Total energy calculations showed the optimal value of the angle between the Fe—S bond and the thiophene plane to be 120°. An analysis of the changes in the orbitals brought about by the angular variation reveals that the mechanism by which this process take places is the reduction of the antibonding interaction between the occupied Fe d_π orbitals and the S πCanonical lone pair in free thiophene. The mechanism found is consistent with the idea of sp²→ sp³ rehybridization of the S atom in thiophene. Calculations performed with and without inclusion of the S 3d basis orbitals show a similar mechanism for the pyramidal distortion.