We have applied D. Functional Theory (DFT) at the generalized gradient approxn. (GGA) level to investigate the C-S bond cleavage in hexathioether complexes of the form [M(9S3)2]n+ (with 9S3 = 1,4,7-trithiacyclononane and M = Re, Tc; n = 1, 2; as well as M = Ru; n = 2, 3). The exptl. trends in C-S bond lengths of the different compds. are reproduced faithfully. Redn. leads to a lowering of the calcd. reaction energies by ~20 kcal/mol to values of 4, 10, and 44 kcal/mol for M = Re, Tc, Ru, resp. The corresponding values for the activation energy are 10, 15, and 44 kcal/mol, which is in agreement with the exptl. observation that the rhenium and technetium compds. lose an ethene mol. immediately after redn., while the ruthenium compd. is stable toward such a loss. Our calcns. suggest that the unique reactivity of the reduced rhenium and technetium complexes is a result of the higher energies of metal t2g-orbitals, resulting from the lower overall charge of the complex. p-Back-donation from t2g-orbitals into C-S s*-orbitals is another important effect, leading to low activation barriers, as only little electronic rearrangement is necessary upon cleavage of the C-S bonds. [on SciFinder (R)]