Destabilizing dπ-pπ orbital interactions and the alkylation reactions of iron(II)-thiolate complexes

For CpFe(CO)₂SR (1) (R = C₆H₄-p-Z; Z = OMe, H, Cl, CF₃, NO₂) the -type interaction between formally occupied metal d orbitals and the sulfur lone pair that is principally 3p in character has been modeled with Fenske-Hall molecular orbital calculations and experimentally investigated by gas-phase photoelectron spectroscopy. A calculation for 1 (R = H) predicts that the highest occupied molecular orbital (HOMO) is metal-sulfur antibonding and largely sulfur in character. The observed HOMO ionization energies of 1 correlate with several chemical properties, including the rate of reaction of the thiolate ligand with alkyl halides. Solvent and substituent effects on the reaction rate favor a mechanism involving nucleophilic displacement of the halide by the coordinated thiolate ligand. The nucleophilicity of the coordinated thiolate ligand of 1 is related to the metal-sulfur dπ-pπ antibonding interactions.