Steric versus electronic effects in the structure of heteroatom (S and O)-substituted free and metal (Cr and W)-complexed carbenes

DFT computational energies for free alkoxy- and thio-substitued free carbenes correlate with experimental steric data (Taft parameters) showing the importance of the steric volume at the expense of the electronic interaction in determining the conformation of free carbenes 1-14. The shorter C-O bond distances in the alkoxy derivatives render alkoxycarbene more sensitive to steric effects than thiocarbenes. This strong dependence of the conformation with the steric hindrance is not applicable to metal-complexed carbenes. Thio-substituted carbene complexes exist exclusively as syn-isomers due to the combination of an interaction of the sulfur lone pair with two CO ligands and the steric repulsion with the CO-wall. The stronger C-carbene-S bond compared to C-carbene-O is responsible for the increased syn-anti rotation barrier observed for the alkylthio-substituted metal carbene complexes compared to their oxygen analogues. The differences of polarity between syn- and anti-isomers of alkylthio-substituted metal carbene complexes explain also the increment of the isomerization barrier with the polarity of the solvent. The effect of the substituent attached to the carbene carbon is not decisive in the conformation of these compounds.