Substrate and Metal Control of Barrier Heights for Oxo Transfer to Mo and W Bis-dithiolene Sites

Reaction coordinates for oxo transfer from the substrates Me3NO, Me2SO, and Me3PO to the biologically relevant Mo­(IV) bis-dithiolene complex [Mo­(OMe)­(mdt)2]− where mdt = 1,2-dimethyl-ethene-1,2-dithiolate­(2-), and from Me2SO to the analogous W­(IV) complex, have been calculated using density functional theory. In each case, the reaction first proceeds through a transition state (TS1) to an intermediate with substrate weakly bound, followed by a second transition state (TS2) around which breaking of the substrate X–O bond begins. By analyzing the energetic contributions to each barrier, it is shown that the nature of the substrate and metal determines which transition state controls the rate-determining step of the reaction.