We present first-principles molecular dynamics studies of the reductive C-S bond cleavage reaction in hexathioether complexes of the form [Re(9S3)(2)](m+) (with 9S3 = 1,4,7 trithiacyclononane and m = 1,2). Our calculations show that electron transfer and bond dissociation take place as two distinct consecutive reaction steps. For the reduced complex, C-S bond fission and subsequent release of ethene can be observed directly at only slightly elevated temperatures. Car-Parrinello molecular dynamics of the reactive process demonstrate that for the dissociation to occur two carbon-sulfur bonds have to be broken quasi simultaneously. For the oxidized form on the other hand, no release of ethene takes place at the same temperature within the limited time scale of our simulations. The activation energies of the dissociation process calculated at the gradient-corrected density functional (BP) level are 21 and 10 kcal/mol for the oxidized and the reduced form, respectively. A detailed analysis of the electronic structure in the transition states confirms the presence of a strong A-back-donation from rhenium d-orbitals into antibonding sigma*-orbitals of the C-S bonds that is responsible for the pronounced weakening of the carbon-sulfur bond upon reduction.