Oxidative chemistry of nickel porphyrins

The oxidative chemistry of nickel(II) porphyrins is reviewed. Whether electron abstraction occurs from the metal to yield Ni(III) or from the porphyrin to yield Ni(II) pi cation radicals is discussed in terms of the relative energy levels of the metal and porphyrin orbitals. The effects of axial ligands in further modulating this ordering as well as the orbital occupancy of Ni(III) are also reviewed. Structural considerations, based on existing stereochemical data for Ni(I), high spin Ni(II) and related Ni(III) tetraaza complexes, are used to predict the metrics of Ni(III) porphyrins for which no structural data are available.