A NEW LOOK AT AN OLD LIGAND - SURPRISES WITH THIOETHERS - A DENSITY FUNCTIONAL-STUDY

Using approximate density functional theory (DFT), the electronic and geometrical structures of thioether-containing d3-d3 face-sharing bioctahedral complexes of the type [Mo2Cl9-n(SH2)n]n-3, with n = 2, 3, 4, 5, and (SH2)Cl2Mo(mu-Cl)2(mu-Sr2)MoCl2(SH2), with R = H, F, CH3, are studied. All structures have been partially optimized and are in good agreement with the experiment. The fact that a thioether possesses one lone pair less than chloride decreases the repulsive interaction within the bridge. This and the availability of an empty sigma* orbital on the thioether ligand are largely responsible for a remarkable shortening of the Mo-Mo bond. All (mu-Cl)3 complexes exhibit a high-spin configuration with a long Mo-Mo distance (282 pm-268 pm), whereas the systems with one or more SR2 ligands in the bridge has a low-spin configuration with a short Mo-Mo bond (256 pm-246 pm). The spin-coupling constant J(ab)Ia of the antiferromagnetic complex [(SH2)Cl2Mo(mu-Cl)3MoCl2(SH2)]-(Ia) has been calculated to be -385 cm-1, in close agreement with experimental coupling constant for related systems. Fragment analysis shows that the largest contribution to bonding clearly stems from a donation of electron density from the sulfur's lone pairs (p(x) and sp-hybrid). The sigma-donor strength increases as follows: S(CH3)2 > SH2 > SF2. However, back-donation from the metal centers to a vacant sigma* orbital of the bridging thioether represents a sizable portion of the overall bonding energy. Expectedly, the pi-acceptor ability of thioethers increases upon an increased electronegativity of the substituent on the thioether. Back-bonding is of lesser importance for terminal thioethers.