However, K2CO3 causes a blue-shift in the observed signal and increase in absorption at ~530 nm and ~780 nm (Fig. ) that is consistent with a shift in the equilibria towards increased formation of [Ni(OMebpy)3]Br2. This shift can potentially be explained by carbonate displacing bromide in the outer coordination sphere, inhibiting the dissociation step of K3. If carbonate were bound to Ni(II) in the inner sphere, a change in the UV-vis spectrum would be expected that departs significantly from the signals of the species identified herein. Ni(II) complexes with varying ligation exposed to a carbonate source tend to form multinuclear Ni complexes with bridging carbonate ligands and distinct d-d bands in absorption spectra that are not observed here. Mechanistic Hypothesis. With the speciation of the Ni/ligand mixture known, we elected to study the C-O coupling reaction between alcohols and aryl halides as a model system for our mechanistic study (Fig. ). We propose a general mechanism, similar to that reported by Mirica and coworkers, that begins with the dark speciation equilibria described in the previous section (Fig. ). Next, the PC absorbs a photon of light (b) and is oxidatively quenched by one of the Ni species (c) to form a Ni(I) complex that undergoes oxidative addition of the aryl halide (d). Following oxidative addition, the alcohol substrate binds to the Ni center (e) and is deprotonated by the base (f). Finally, the Ni(III) aryl alkoxy complex undergoes reductive elimination to furnish the product (g), closing the Ni(I/III) cycle. The oxidized form of the PC is converted back to the neutral PC through single electron transfer (h) with either a halide anion or the Ni(I) complex, closing both cycles. Steps (d-g) can repeat, constituting a dark Ni cycle (i) that turns over with a frequency distinct from that of the oxidative quenching cycle.