OXIDATION OF NICKEL(II) TETRAPHENYLPORPHYRIN REVISITED - CHARACTERIZATION OF STABLE NICKEL(III) COMPLEXES AT ROOM-TEMPERATURE

The absorption, resonance Raman, and EPR spectra of the one-electron o xidation products of (NiTPP)-T-II (TPP = tetraphenylporphyrin) are exa mined in detail. The data indicate that the pi-cation radical, Ni-(II) (TPP.)(+), which is the oxidation product obtained in solution at ambi ent temperature, is converted to Ni(III) complexes of the general form [(L)(2)(NiTPP)-T-III](+) upon addition of coordinating ligands such a s tetrahydrofuran (THF), pyridine (py), and CH3CN. Contrary to previou s reports, the Ni(III) complexes of TPP are readily formed at ambient temperature and are stable for extended periods (up to 24 h) when main tained 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 th e complex back to the neutral starting material. The CN- ion also redu ces Ni-II(TPP.)(+) to (NiTPP)-T-II; however, an unstable complex of th e form [(CN)(2)(NiTPP)-T-III](-) is produced as a minority species. Th e ground state of this latter complex is S = 1/2; however, the unpaire d electron resides in the d(x)(2)-(2)(y) rather than the d(z)(2) orbit al. The spectroscopic data obtained for Ni-II(TPP.)(+) and the [(L)(2) -(NiTPP)-T-III](+) complexes (L = THF, py, CH3CN) indicate that the co nversion 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 la tter of which contains the hole in the pi-cation radical form. This mi xing also introduces some metal ion character into the wave function f or Ni-II(TPP.)(+). Collectively, the studies indicate that the limitin g case descriptors ''Ni(III) complex'' and ''porphyrin pi-cation radic al'' do not accurately describe the ground states of the oxidation pro ducts of (NiTPP)-T-II.