The relative reducing and oxidizing natures of these photocatalysts can be understood by considering the ligands electronegativity and the catalyst complexs metal center. More electronegative metals and ligands can stabilize electrons better than their less electronegative counterparts. Therefore, complexes with more electronegative ligands are more oxidizing than less electronegative ligand complexes. For example, the ligands 2,2-bipyridine and 2,2-phenylpyridine are isoelectronic structures, containing the same number and arrangement of electrons. Phenylpyridine replaces one of the nitrogen atoms in bipyridine with a carbon atom. Carbon is less electronegative than nitrogen is, so it holds electrons less tightly. Since the remainder of the ligand molecule is identical and phenylpyridine holds electrons less tightly than bipyridine, it is more strongly electron-donating and less electronegative as a ligand. Hence, complexes with phenylpyridine ligands are more strongly reducing and less strongly oxidizing than equivalent complexes with bipyridine ligands.
Similarly, a fluorinated phenylpyridine ligand is more electronegative than phenylpyridine so complexes with fluorine-containing ligands are more strongly oxidizing and less strongly reducing than equivalent unsubstituted phenylpyridine complexes. The metal center's electronic influence on the complex is more complex than the ligand effect. According to the Pauling scale of electronegativity, both ruthenium and iridium have an electronegativity of 2.2. If this was the sole factor relevant to redox potentials, then complexes of ruthenium and iridium with the same ligands should be equally powerful photoredox catalysts. However, considering the Rehm-Weller equation, the spectroscopic properties of the metal play a role in determining the redox properties of the excited state. In particular, the parameter E is related to the emission wavelength of the complex and therefore, to the size of the Stokes shift - the difference in energy between the maximum absorption and emission of a molecule. Typically, ruthenium complexes have large Stokes shifts and hence, low energy emission wavelengths and small zero-zero excitation energies when compared to iridium complexes. In effect, while ground-state ruthenium complexes can be potent reductants, the excited-state complex is a far less potent reductant or oxidant than its equivalent iridium complex. This makes iridium preferred for the development of general organic transformations because the stronger redox potentials of the excited catalyst allows the use of weaker stoichiometric reductants and oxidants or the use of less reactive substrates.
It is often the case that these photocatalysts are balanced with a counter-ion, as is the case with the example complex tris-(2,2’-bipyridyl)ruthenium which is accompanied by two anions to balance the overall charge of the ion pair to zero. However, there are transition metal photoredox catalysts that exist without a counter-ion such as tris(2-phenylpyridine)iridium (often abbreviated Ir(ppy)). The significance of these counter-ions are dependent on the ion association between the photoredox catalyst and its counter-ion(s) and is dependent on the solvent used for the reaction. Although photophysical properties such as redox potential, excitation energy, and ligand electronegative have often been considered key parameters for the use and reactivity of these complexes, counter-ion identity has been shown to play a significant role in low-polarity solvents. Particularly, it has been shown that having a tightly associated counter-ion increases the rate of electron-transfer when reducing a substrate but significantly reduces the rate of electron-transfer when oxidizing a substrate. This is believed to occur because the counter-ion essentially "blocks" the electron-transfer into the photoredox complex by shielding the more positively charged region of the complex; whereas, having the tight counter-ion association pushes the electron density further from the photoredox catalyst's metal-center, making it easier to be transferred from the catalyst (of course this only applies to the case where the photoredox catalyst is a cation and the counter-ion is an anion). Counter-ion identity thus is an additional parameter to consider when developing new photoredox reactions.