The thermochemistry of oxo transfer from coordinated nitrite in the dinitro(5,10,15,20-tetrakis(o-pivalamidophenyl)-porphinato)iron(III) anion, ion-paired with the tetrapropylammonium ion, {[Fe(III)TpivPP(NO2)(2)]-Pr4N+}, has been evaluated in acetonitrile solution. This ore-transfer half-reaction of {[Fe(III)TpivPP(NO2)(2)]-Pr4N+} has been assessed on the basis of the determination of the E-1/2 = +0.54 V vs SHE for the reversible [Fe(II/III)TpivPP(NO)(NO2)]-(/0) couple and the measurement of the formation constants for the association of NO and NO2- with the mononitroiron(III) porphyrin derivative. The formation constant for nitric oxide association, K-NO, has the value (1.21 +/- 0.08) x 10(3). The stability constant, K-2, for association of a second nitro ligand in 0.0100 M tetrapropylammonium perchlorate medium has been estimated as 2.18 x 10(3)., The oxo-transfer half-reaction free energy, Delta G((X/XO))degrees for addition of oxygen to [Fe(II)TpivPP(NO)(NO2)](-) to form {[Fe(III)TpivPP(NO2)(2)]-Pr4N+} has been found to be -50 kJ/mol.