To test whether the thermodynamics of ligand binding were controlling the activation of the Ni(0) NHC complexes, seven different fumarate complexes of IMes (1-2, 5-10, Figure ) with varying electronics and sterics were considered. In the model reaction of 4-fluorobenzaldehyde (12) and phenyl propyne (13) using triethylsilane as the reductant, a potential first step for activation of the Ni complex is displacement of the two fumarate ligands with aldehyde and alkyne. We computed the free energy of fumarate ligand exchange with the aldehyde and alkyne reaction components using seven representative fumarate complexes and compared their binding affinities to Cavell's original complex (1) (Table ). Complex 1 was chosen as a reference point for this series, as it is known to be air-stable and was observed to be unreactive in the reductive couplings of aldehydes and alkynes and in Buchwald Hartwig aminations. We anticipated that if the mechanism of catalyst activation simply involves exchange of the fumarate 16 for the aldehyde and alkyne components (12 and 13), then the catalysts with lowest free energy of exchange will most easily reach the active catalyst state. As seen in Table , the complexes examined were found to have similar or higher fumarate binding affinities, relative to The hypothesis of catalyst activation through purely thermodynamic control considered with the relative binding energies from Table suggests that most fumarate complexes (except for 2) would be as inactive as complex 1 in reductive coupling reactions. Regardless, the increased steric bulk of the fumarates compared to 1 and variations in electronics of the aryl groups of 7-10 provided a significant range of binding energies. Therefore, this set could be used to better understand the relationship between fumarate binding and reactivity, and we experimentally tested a representative set of the compounds evaluated by computation. The model coupling reaction (Figure , top) was performed for representative precatalysts and monitored by 19 F NMR. Across the catalyst series 1-2 and 5-10, all of the tested fumarate complexes except for catalyst 10 were found to be more active than the parent dimethyl fumarate complex 1 (see SI).