Strong σ-donation from NHC ligands (NHC = N-heterocyclic carbene) is shown to have profoundly conflicting consequences for the reactivity of transition-metal catalysts. Such donation is regarded as central to high catalyst activity in many contexts, of which the second-generation Grubbs metathesis catalysts (RuCl₂(NHC)(PCy₃)([double bond, length as m-dash]CHPh), GII) offer an early, prominent example. Less widely recognized is the dramatically inhibiting impact of NHC ligation on initiation of GII, and on re-entry into the catalytic cycle from the resting-state methylidene species RuCl₂(NHC)(PCy₃)([double bond, length as m-dash]CH₂), GIIm. Both GII and the methylidene complexes are activated by dissociation of PCy₃. The impact of NHC donicity on the rate of PCy₃ loss is explored in a comparison of s-GIIm, vs. u-GIIm, in which the NHC ligand is saturated H₂IMes or unsaturated IMes, respectively. PCy₃ loss is nearly an order of magnitude slower for the IMes derivative (a difference that is replicated, albeit smaller, for the benzylidene precatalysts GII). Proposed as an overlooked contributor to these rate differences is an increase in the Ru-PCy₃ bond strength arising from π-back-donation onto the phosphine ligand. Strong σ-donation from the IMes ligand, coupled with the inability of this unsaturated NHC to participate in significant π-backbonding, amplifies Ru → PCy₃ π-back-donation. The resulting increase in Ru-P bond strength greatly inhibits entry into the active cycle. For s-GII, in contrast, the greater π-acceptor capacity of the NHC ligand enables competing Ru → H₂IMes back-donation (as confirmed by NOE experiments, which reveal restricted rotation about the Ru-NHC bond for H₂IMes, but not IMes). Ru → PCy₃ back-donation is thus attenuated in the H₂IMes complexes, accounting for the greater lability of the PCy₃ ligand in s-GIIm and s-GII. Similarly inhibited initiation is predicted for other metal-NHC catalysts in which a π-acceptor ligand L must be dissociated to permit substrate binding. Conversely, enhanced reactivity can be expected where such L ligands are pure σ-donors. These effects are expected to be particularly dramatic where the NHC ligand has minimal π-acceptor capacity (as in the unsaturated Arduengo carbenes), and in geometries that maximize NHC-M-L orbital interactions.

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