Acidity, d-orbital Participation, and Charge Neutralization

Coordination to thioethers typically stabilizes the lower oxidation states of metal ions, and, where relevant, the lower spin states as well. These recurring themes derive largely from two characteristics of thioether ligands their n-acidity, and their failure to neutralize positive charge effectively. 

Several lines of evidence impute appreciable Jt-acidity to thioethers. Infrared studies of substituted metal carbonyl complexes show that trans thioethers increase the CO stretching frequency more than pyridine or an aliphatic amines, but less than phosphines [2, 19, 20, 21]. To the extent that such changes derive from differences in n-backbonding, these results indicate n-acidity intermediate between that of amines and that of phosphines. 

Magnetic properties further support it-acidity. Quantitatively, g values reflect 7i-effects because deviations from g (the fi-ee electron value) arise from unquenched orbital angular momentum (fi-om circulation of the t2g electrons). Delocalization of d electrons into n levels on the ligand diminishes the unquenched orbital angular momentum (as measured by k, the orbital reduction factor), and thereby Ag. For example, in copper(II) complexes of thioether ligands, g values deviate from 2 by little compared to those of harder ligands such as OH 2 or NH3 [22]. 

Stabilization of low-spin states indicates n-interaction in a more dramatic fashion. Delocalization of t2g electron density into ligand orbitals of n symmetry (with respect to the M-S bond) diminishes electron-electron repulsion and thereby reduces the spin pairing energy. As a consequence, complexes of thioethers, like those of phosphines, typically assume the low-spin state. 

Nephelauxetic ( cloud expanding ) effects of thioethers point to the same conclusion. Like g values, nephelauxetic ratios ( 3, where P = Bcompiex/Bfreeion and B is the second Racah parameter) measure the delocalizing effect of the ligands. Thus smaller nephelauxetic ratios indicate greater delocalization. In this respect 

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