While synthetically useful amounts were obtained for every single gaseous alkane, we observed reduced efficiency in the coupling reaction as the bond dissociation energies (BDEs) for the corresponding C-H scission increased. Additionally, for propane and butane as coupling partners, we noted a selectivity erosion upon radical capture by nickel compared to the direct reaction of nucleophilic alkyl radicals with Michael acceptors (Figure ). To explain these observed trends and gain further insights into the mechanistic rationale of the metallaphotocatalytic cross-coupling of aryl bromides with gaseous alkanes, we conducted a detailed DFT calculation study and carried out some additional mechanistic experiments to support the computational observations. Specifically, we conducted a parallel kinetic isotope effect (KIE) study using cyclohexane as a substrate (Figure ). We observed a negligible reaction rate difference between cyclohexane and its deuterated substrate, indicating that the HAT process by a photoexcited decatungstate is not involved in the rate-determining step (RDS) in our reaction conditions. With these kinetic data in hand and based on recent computational work of Gutierrez and Molander on a related reaction system with tertiary alkyl radicals, we investigated the reactivity trends with various alkyl radicals from gaseous alkanes using state-of-the-art DFT calculations (Figure and Figure S8 Supplementary Information). The reaction begins with the generation of an alkyl radical by the HAT process between the gaseous alkane and a photoexcited decatungstate anion Int2-R by protonation and alkyl radical addition. Throughout the catalytic cycle, we found that the RDS is the oxidative addition, and more interestingly, the activation energy barrier for this orbital of Ni(I) species is strongly affected by the contribution from the 2p orbital of the adjacent alkyl substituent, where the HOMO-LUMO gap Δε with aryl bromide is larger for the less electron-donating methyl group (Δε = 1.8 eV) and the gap is much smaller and favorable for the bulkier and more electron donating i Pr group (Δε = 1.5 eV) (Figure ). These observations, along with the potential catalyst deactivation pathway with smaller alkyl radicals (See Supplementary Information, Figure ), shed light on the origin of decreased reactivity in the coupling reaction of aryl bromides with methane.