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.