During the synthetic scope investigation, a black precipitate was observed after the reaction, which did not appear under oxidative conditions. It is reasonable to assume that this precipitate originates from the reduced bismuth (II) species which is described as a radical itself. There is little work published on the behaviour of these radicals outside of sophisticated stabilizing ligand systems. To get more inside into the behaviour of bismuth (II) in our system, we performed cyclic voltammetry (CV) studies. Electroreduction serves as a complementary approach to photochemical reduction offering us the opportunity to study bismuth (II) radical behaviour. The cyclic voltammogram of BiCl 5 2- revealed reversible behaviour (Figure ), whereas voltammogram of BiCl 3 showed an additional irreversible reduction peak (Figure -I) indicating some chemical reaction. Gratifyingly, this reaction could be identified by monitoring the bismuth species interconversions during the electrochemical reduction by UV-vis spectroscopy (Figures and). These combined spectro-electrochemical measurements displayed the shift of the absorption maximum of BiCl 3 from 280 nm to 340 nm in the first reduction step (Figure ). This behaviour is mimicking the observation of the UV-vis titration studies (Figure ) making evident that the newly formed species is Bi III Cl 5 2-. Finding this newly formed bismuth species, still in oxidation state III is counterintuitive at the first sight. However, this suggests that the unstable bismuth (II) radical rapidly undergoes disproportionation to bismuth (III) and to a further reduced bismuth species without any absorption features. Hereby, the released chlorine is transferred to bismuth (III) to eventually form BiCl 5 2-as observed by UV-Vis. Indeed, the absorption and electrochemical behaviour of in situ generated BiCl 5 2-and preformed BiCl 5 2-is the same. Further reduction of BiCl 5 2-results in a complete disappearance of all absorption features (Figures 3B-II and 3C-II), likely indicating its complete reduction. Upon oxidation, the absorption features of BiCl . Upon further oxidation, the concentration of Bi 3+ ions increases, and in the absence of free chloride ions, Bi 3+ ions start taking chloride atoms from BiCl 5 2-. As a consequence, we observed reappearance of the BiCl 3 signal at 280 nm (Figure -IV). Interestingly, during these investigations we observed the black deposit again, this time on the electrode. To better characterize the formed black solid, we successfully conducted a direct photochemical synthesis by irradiating concentrated BiCl 5 2-solution without any reactants (Scheme S3). The analysis by powder diffraction identified the black precipitate as elemental bismuth (Figure ). In the context of our C-H activation protocol this disproportionation to bismuth (0) poses a loss of active catalyst and therefore termination of the reaction. However, judging on the high product yields (up to 98%) it seems that disproportionation does play not an important role in our reaction. Therefore, the productive pathway is expected to outcompete disproportionation. Reaction kinetic monitoring within the first hour gave a fast linear profile showing six catalyst turnovers (Figure ). To determine whether the reaction is light initiated or light-driven, we measured the quantum yield (Table ). The obtained quantum yield of 1.5%, makes a radical chain pathway to the coupling product unlikely. These two measurements suggest, that the active catalytic species continuously catalyses the reaction and remains intact as long as reactants are present. A plausible explanation for the efficiency of this catalytic system and neglectable disproportionation is an interaction of the open shell bismuth (II) radical fragment with carbon centred radicals. Such a rebound mechanism would stabilize the highly reactive bismuth (II), and at the same time assemble reaction partners for a fast turnover. Similar effects have been postulated for bismuth polymerisation catalysts. Additionally, in a seminal report on the nature of such bismuth (II) -carbon radical equilibria have been studied in detail in SET oxidative additions of redox active esters to bismuth (I). Based on our mechanistic experiments and previous abovementioned studies on low valent bismuth, the following mechanism is proposed (Scheme 2). The addition of two equivalents of TBACl to BiCl 3 generates the active BiCl 5 2-catalyst. Under 385 nm irradiation this catalyst undergoes bismuthchlorine bond homolysis releasing an electrophilic chlorine radical. This radical abstracts a hydrogen atom from the C-H bond precursor. The resulting carbon centred radical can recombine with bismuth (II). After radical addition of the cyclohexyl radical to an electron-deficient alkene, a reduction event gives the desired coupling product and regenerates photoactive bismuth (III). This can be either facilitated by an innersphere protodemetallation step or by an outer-sphere SET event followed by protonation. The regenerated bismuth (III) then perpetuates the bismuth (III/II) catalytic cycle.