Speciation of the Ni Catalyst Prior to Irradiation. Upon mixing all the reaction components, the Ni precatalyst (i.e. NiBr2•glyme) reacts with N,N'-dimethylformamide (DMF) solvent and 4,4'-dimethoxy-2,2'-bipyridine (OMebpy) to form a mixture of Ni-OMebpy complexes (Fig. ). We elected to follow the series of dark speciation equilibria through UVvisible spectroscopy, as each complex possesses unique d-d absorption bands that can aid in deconvoluting the complex mixtures that result when one mixes NiBr2•glyme with OMebpy in DMF. As such, we conducted a UV-vis titration experiment (Fig. ) in which the Ni:OMebpy molar ratio was changed systematically while the solution volume was kept constant (for further details, see SI, Section 2.2).