Computational Insights into the Mechanisms of H 2 Activation and H 2 /D 2 Isotope Exchange by Dimolybdenum Tetrasulfide Complexes

The mechanisms for H 2 activation by [Cp*Mo] 2 (µ‐S) 2 (µ‐S 2 ) ( 1‐a , Cp* = pentamethylcyclopentadienyl) and its reaction product [Cp*Mo] 2 (µ‐S) 2 (µ‐SH) 2 ( 2 ) have been investigated by DFT methods. The reaction of 1‐a involves the homolytic addition of H 2 to its µ‐S ligands, followed by the cleavage of the S–S bond of the µ‐S 2 ligand in a subsequent step. Complex 2 can adopt five conformations that only differ in the stereochemistry of the µ‐SH and µ‐S ligands; although an isomer with adjacent µ‐S ligands ( 2‐a ) is formed initially, it then isomerises into the experimentally observed 2‐d . This species promotes H/D scrambling in H 2 /D 2 mixtures, and the mechanism of the process has also been studied. Notably, all of the computed pathways for the addition of D 2 to 2‐d present prohibitive barriers; instead, only those isomers with adjacent µ‐S ligands are able to react further. The homolytic activation of D 2 by these leads to isomers of [Cp 2 Mo 2 (µ‐SH) 2 (µ‐SD) 2 ], the interconversion of which is the rate‐determining step.