We conducted several experiments in order to shed light on the effect of counter anions on the competitive equilibria of ion pairing and association of 2 with 1′. In addition to compound 1, we measured the equilibrium (dissociation) constants for salts 4 -6, in which the counter anions are -BF4, TFA -, and Cl -, respectively, and we calculated the Gibbs free energy of association (G) and Lewis basicity (LBI) of these anions (Table and Figure ). A comparison of the G and LBI values corresponding to 2, OTf, BF4, TFA, and Cl aligns with our earlier observations regarding the relative reactivity of 1, 4 -6 (Table and). The association constants of 1′ with Lewis bases decrease in the order of Cl -> TFA -> 2 > -OTf ~ - BF4 (Table ). The corresponding G values reveal that ion pairing of 2 is more exergonic than ion pairing of 1′ with OTf or BF4 (cf. entries 1 -3 in Table ). Additionally, association of 1′ with 2 is less exergonic than association of 1′ with Cl or TFA (cf. entries 1, 4 and 5 of Table ). These results also align with the relative Lewis basicity values obtained on the iodonium Lewis acidity scale, such that Lewis basicity (LBI) values decrease in the order Cl > TFA > 2 > OTf > BF4 (Table ). Collectively, these results suggest that more Lewis basic groups (i.e., TFA and Cl) competitively bind 1′ and inhibit the formation of A, the species that undergoes reductive ligand coupling to form the product 3. Additional evidence for this scenario was observed in NMR titration experiments with 2 to obtain Ka(N) values. We observed that when these experiments were conducted at increasing total concentrations of 1 (0.001 -0.05 M) the Ka(N) values decreased from 164 M -1 to 7.4 M -1 and the decrease in Ka(N) values correlates with the mole fraction of 1′ based on Kd(IX) (Figure ). 20 Cl - 39500 -6.2 4.6 a Calculated from G = -RTlnKa (T = 298.15 K). Calculated from logKa = sILAI + LBI (LAI = 0 for 1′). Experimental energy landscape. The ligand coupling step is generally regarded as the rate determining step in nucleophile arylation reactions with diaryliodonium salts. Having established the relative energy of equilibrating species (1, 1′, and A) that precede the rate determining step, we experimentally probed this step to construct an experimentally determined energy landscape. We did this by measuring kobs at a range of different temperatures (50 -90 °C) and plotted ln kobs vs 1/T (Arrhenius plot) to determine the energy of activation (Ea ~ G ‡ ) for this reaction (Scheme 5). The Ea value obtained from the slope of the Arrhenius plot was 26.8 kcal/mol, which is substantially larger than the energies for the equilibrating species that precede the ligand coupling step (Scheme 3-5). A basic energy landscape can be sketched from the energy values obtained from the van't Hoff plots and the Arrhenius plot (Scheme 3c, 4, and 5). The dissociation of ions in 1 is endergonic by 2.8 kcal/mol to generate 1′ and OTf (Scheme 6). Association of 2 to 1′ is exergonic by 3.3 kcal/mol, and therefore intermediate A is -0.5 kcal/mol relative to 1 and the lowest energy species on the energy landscape (Scheme 6). The ligand coupling pathway has an energy barrier of 26.8 kcal/mol and based on literature precedent likely proceeds through a three-member transition state similar to reductive elimination (Scheme 6). Based on our experimental data, the complete mechanism for arylation of DABCO with diaryliodonium salts in acetonitrile involves ion pair dissociation followed by DABCO association and ligand coupling (Scheme 6). Consequently, the observed rate constant (kobs) is the product of the equilibrium constants for ion pair dissociation (Kd(IX)), DABCO binding (Ka(IN)) and ligand coupling k3 and further supports the kinetic relevance of the counter anion. More specifically, Kd(IX) decreases with increasing Lewis basicity of the counter anion and therefore results in a proportional decrease in kobs. Additionally, consistent with our observation that Ka(IN) decreases with increasing total concentration of 1, we observed a similar trend in decreasing kobs values when reactions were conducted at increasing initial concentrations of 1 (Figure ). Computational studies. The general reaction profile and empirical and expanded rate laws were provided through experimental studies including VTNA, NMR titration, conductance, and temperaturedependant thermodynamic and kinetic analysis. However, these methods do not reveal the identity of the frontier orbitals used by iodine to interact with the nucleophile or the bond angles and lengths in reactive intermediates for transition states. We turned to DFT and NBO analysis to probe these aspects of the mechanism by using the M062x/6-31+G(d) with Def2-tzvp for Iodine and SMD18 solvation in acetonitrile to find optimized structures of the reaction components and transition states in Gaussian09. Natural Population Analysis based charges and orbital contributions to bonds were calculated using NBO 7.0. Although our experimental evidence is consistent with the lowest energy pathway to 3 proceeding though intermediates 1′ and A, we cannot definitively rule out the existence of a tetracoordinate intermediate A′ especially at higher concentrations of 1 (Scheme 7). Therefore, to provide a deeper understanding of how counter anion and aryl "dummy" ligand impact transition state energies we considered reductive ligand coupling transitions states arising from three different intermediates. First, as a baseline, we calculated the transition state for reductive ligand coupling from A and obtained a G ‡ value of 22.7 kcal/mol, which aligns well with our experimental Ea value of 26.8 kcal/mol (Scheme 5-7). Second, to account for the impact of the counter anion and the possibility of a tetracoordinate iodine intermediate at higher concentrations, we considered intermediate A′, but found that this had substantially larger transition state energy of 61.5 kcal/mol (Scheme 7). Third, the impact of aryl "dummy" group on the transition state energy was assessed through intermediate B, and we found a G ‡ value of 24.4 kcal/mol (Scheme 7). Overall, the calculated transition state energies agree well with our experimental data; the lower energy pathway is through a three-coordinate intermediate (A) and the mesityl "dummy" group (B) results in a higher barrier for reductive ligand coupling. Moreover, these three calculated transition states share several structural similarities. First, all three transition states appear to be relatively "early" as there is less C-I bond cleavage (cf. a = 2.094 Å and c = 2.347 Å, Scheme 7 and Table ) than C-N bond formation (cf. e = 2.231 Å vs 1.49 Å in 3; Scheme 7 and Table ). Second, the three-membered transition state has the nitrogen nucleophile shifted so that the C-N contact is shorter than the I-N contact (cf. e = 2.231 Å and d = 3.196 Å; Scheme 7 and Table ). Third, the transferring phenyl group in all three transition states "bends" from an approximately linear 180° C4-C1-I axis to ~ 148°, and the CPh-I-CPh/Mes angle contracts (~ 4-5°; Scheme 7). However, there are also key differences in these transition states that provide insight into their relative energies.