Tellurium( II ) Dialkanethiolates: n p (S)‐σ*(Te−S′) Orbital Interactions Determine the 125 Te NMR Chemical Shift, and the Molecular and Crystal Structure

Tellurium( II ) dimethanethiolate, Te(SMe) 2 , and tellurium( II ) diethanethiolate, Te(SEt) 2 , were synthesized by reaction of TeO 2 and Te(O i Pr) 4 with HSMe and HSEt, respectively. In the solid state, Te(SMe) 2 exhibits a cis ‐conformation of the methyl groups with respect to the TeS 2 plane − an unprecedented situation for nonfunctionalized organotrichalcogenides − whereas Te(SEt) 2 shows a trans ‐conformation. Ab initio calculations performed for Te(SMe) 2 and Te(SEt) 2 show that the cis ‐ and trans ‐conformers represent minima on the potential energy surface and are stabilized by intramolecular π‐type n(S)‐σ*(Te−S′) orbital interactions. In the solid state, the molecules of each compound are associated through two centrosymmetric Te 2 S 2 units with two of their neighbors, resulting in tetracoordinate Te atoms with distorted trapezoidal configurations. While the intermolecular Te ··· S distance increases in the sequence R = Me < Et < i Pr < t Bu, the length of the covalent Te−S bond decreases in the same order, a result attributed to intermolecular σ‐type n p (S)‐σ*(Te−S′) orbital interactions. The 125 Te NMR chemical shift of Te(SR) 2 largely depends on R (R = Me, Et, i Pr, t Bu) and shows a nearly linear correlation with the first ionization energy of the corresponding thiol HSR. Ab initio calculations of the 125 Te NMR shifts for the model compound Te(SH) 2 ( C 2 symmetry) reveal that it also depends strongly on the HSTeS torsion angle. These results can be explained by a model in which π‐type n p (S)‐σ*(Te−S′) and n p (Te)‐σ*(S−H) orbital interactions determine the paramagnetic shielding of the tellurium nucleus. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)