The absorption, resonance Raman, and EPR spectra of the one-electron oxidation products of (NiTPP)-T-II (TPP = tetraphenylporphyrin) are examined in detail. The data indicate that the pi-cation radical, Ni-(II)(TPP.)(+), which is the oxidation product obtained in solution at ambient temperature, is converted to Ni(III) complexes of the general form [(L)(2)(NiTPP)-T-III](+) upon addition of coordinating ligands such as tetrahydrofuran (THF), pyridine (py), and CH3CN. Contrary to previous reports, the Ni(III) complexes of TPP are readily formed at ambient temperature and are stable for extended periods (up to 24 h) when maintained under inert atmosphere. The electronic ground states of the [(L)(2)(NiTPP)-T-III](+) complexes (L = THF, py, CH3CN) are low-spin (S = 1/2), and the unpaired electron resides in the d(z)(2) orbital of the Ni(III) ion. The addition of coordinating ligands such as (C6H5)(3)P and CO to solutions containing Ni-(II)(TPP.)(+) immediately reduces the complex back to the neutral starting material. The CN- ion also reduces Ni-II(TPP.)(+) to (NiTPP)-T-II; however, an unstable complex of the form [(CN)(2)(NiTPP)-T-III](-) is produced as a minority species. The ground state of this latter complex is S = 1/2; however, the unpaired electron resides in the d(x)(2)-(2)(y) rather than the d(z)(2) orbital. The spectroscopic data obtained for Ni-II(TPP.)(+) and the [(L)(2)-(NiTPP)-T-III](+) complexes (L = THF, py, CH3CN) indicate that the conversion to Ni(III) species is facilitated by mixing of the d orbitals of the metal ion with the a(2u) orbital of the porphyrin ring, the latter of which contains the hole in the pi-cation radical form. This mixing also introduces some metal ion character into the wave function for Ni-II(TPP.)(+). Collectively, the studies indicate that the limiting case descriptors "Ni(III) complex" and "porphyrin pi-cation radical" do not accurately describe the ground states of the oxidation products of (NiTPP)-T-II.