We present first-principles mol. 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 calcns. show that electron transfer and bond dissocn. take place as two distinct consecutive reaction steps. For the reduced complex, C-S bond fission and subsequent release of ethene can be obsd. directly at only slightly elevated temps. Car-Parrinello mol. dynamics of the reactive process demonstrate that for the dissocn. 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 temp. within the limited time scale of our simulations. The activation energies of the dissocn. process calcd. at the gradient-cor. d. functional (BP) level are 21 and 10 kcal/mol for the oxidized and the reduced form, resp. A detailed anal. of the electronic structure in the transition states confirms the presence of a strong p-back-donation from rhenium d-orbitals into antibonding s*-orbitals of the C-S bonds that is responsible for the pronounced weakening of the carbon-sulfur bond upon redn. [on SciFinder (R)]