We have applied Density Functional Theory (DFT) at the generalized gradient approximation (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 experimental trends in C-S bond lengths of the different compounds' are reproduced faithfully. Reduction leads to a lowering of the calculated reaction energies by approximate to20 kcal/mol to values of 4, 10, and 44 kcal/mol for M = Re, Tc, Ru, respectively. The corresponding values for the activation energy are 10, 15, and 44 kcal/mol, which is in agreement with the experimental observation that the rhenium and technetium compounds lose an ethene molecule immediately after reduction, while the ruthenium compound is stable toward such a loss. Our calculations suggest that the unique reactivity of the reduced rhenium and technetium complexes is a result of the higher energies of metal t(2g)-orbitals, resulting from the lower overall charge of the complex. pi-Back-donation from t(2g)-orbitals into C-S sigma*-orbitals is another important effect, leading to low activation barriers, as only little electronic rearrangement is necessary upon cleavage of the C-S bonds.