To distinguish between single-electron transfer (SET) and halogenatom transfer (XAT) pathways, a series of intra-and intermolecular competition reactions were designed. In a previous study, Galli and coworkers reported that in-situ generated tributyltin radical (Bu3Sn • ) exclusively abstracts the iodine atom from the more sterically hindered C-I bond in 2,5-diiodo-1,3-dimethylbenzene (6). This counterintuitive selectivity was attributed to the pinching effect of the ortho-methyl groups, which the authors purport leads to a weakening of the neighboring C-I bond. By analogy to an 'effective' BDE of the C-I bond in iododurene (obtained via extrapolation to a LFER plot of other iodoarenes), 77 the BDE of the crowded C-I bond in 6 is assumed by the authors to be 49 kcal mol -1 . This is significantly weaker than the BDE of the unhindered C-I bond, which is assumed to have the same C-I bond BDE found in iodobenzene (69 kcal mol -1 ). In our case, the intramolecular competition between 1a and 6 leads to a 1:1.25 mixture of products (7a and 7b), with a minor side-product arising from double oxidative addition into the diiodoarene (7c, seen Section 6.2 in the Supporting Information) (Figure ). This result reveals an approximately equal preference for oxidative addition into either C-I bond. According to these previously reported assumptions on BDE differences, this lack of a clear preference for oxidative addition into the more crowded C-I bond already serves as evidence against an XAT pathway. However, in contrast with these previously purported energies of the two C-I bonds of 6, optimization of this substrate in silico revealed that both C-I bonds feature almost identical BDFEs of ~62 kcal mol -1 . Therefore, although simple ground-state energies of the bonds are not enough to rationalize the previously reported selectivity with tributyltin radical, it is still possible that the o,o'-dimethyl substituents kinetically favor elongation of the C-I bond in the transition state for XAT, leading to a TS-stabilizing effect that largely outweighs any unfavorable steric clash between the methyl groups and the organotin species. To further solidify this notion, we synthesized 2,5-diiodo-1,3-diisopropylbenzene (8), to exacerbate the difference in BDFEs and steric hindrance between the competing C-I bonds (Figure ). Optimization of this substrate revealed a significantly lower BDFE for the more sterically crowded C-I bond (56 kcal mol -1 vs 62 kcal mol -1 ), which also featured significant out-of-plane bending to alleviate the steric strain with the neighboring isopropyl substituents (see Section 11.3 in the Supporting Information). Remarkably, employing this substrate under standard conditions exclusively afforded 9a, arising from the oxidative addition into the more sterically hindered C-I bond. Indeed, the 1 H NMR spectrum of the only product in the latter experiment featured sharp, distinct signals for magnetically nonequivalent isopropyl groups and arene C-H bonds. In addition, the expected signals for homotopic protons on the pincer backbone were observed, thus indicating highly hindered rotation about the newly forged Bi-C bond. Yet, this result could still arise from either an XAT mechanism that favors a halogen abstraction of the weaker and more sterically hindered C-I bond, or an SET mechanism with a product-determining mesolytic cleavage step that favors fragmentation of the weaker, out-of-plane C-I bond. To conclusively differentiate between these two possibilities, an intermolecular competition experiment between 2,6-diisopropyliodobenzene (10) and 4-iodobenzonitrile (2) was conducted (Figure ). This led to the formation of a 1.25 : 1 mixture of products 11a and 3a, with the reaction only slightly favoring oxidative addition into the former substrate. This result is inconsistent with an XAT pathway, where an almost exclusive oxidative addition into the weaker C-I bond would be expected, as seen in the intramolecular competition in Figure . Instead, a mixture of products is suggestive of an SET-based oxidative addition mechanism featuring a competition between a substrate that is easier to reduce (-2.12 V vs Fc +/0 ) but has a slower rate of mesolytic cleavage (2) and a substrate that is harder to reduce (-2.50 V vs Fc +/0 ) but undergoes rapid mesolytic cleavage (10). This also helps rationalize the results of the intramolecular competition experiment with 8, where the faster rate of a product-determining mesolytic cleavage step from the more sterically crowded C-I bond leads to the exclusive selectivity for product 9a. Indeed, the increased rate of mesolytic cleavage from the radical anions of ortho-substituted aryl halides has been documented in the literature, and was recently exploited very elegantly in a report by Pierson and Hartwig to elucidate the mechanism of oxidative addition of low-valent Ni species into aryl hal-ides. In their report, the authors disclosed increasing rates of oxidative addition of low-valent Ni species into 4-tert-butyl-1-iodobenzene < 2-iodo-1-isopropylbenzene < 2-iodo-1,3-diisopropylbenzene, which was taken as preference for an outer-over an inner-sphere mechanism. Along the same lines, the ability of 1a to perform oxidative addition into more sterically hindered C-I bonds is also evidence against the existence of an inner-sphere SET or XAT mechanism that requires a close interaction of the iodoarene and the bulky excited-state triplet bismuthinidene. Additional circumstantial evidence in favor of the SET pathway between 1a and aryl iodides includes the 7-fold reduction in reaction yield when the reaction is conducted in pentane vs MeCN (see section 10.1 of the Supporting Information). This dramatic difference in rate is more easily attributed to the destabilization of the ion pair formed after an electron-transfer event, since a similar destabilization would not be expected for a neutral, concerted transition state for XAT.