Treatment of (meso-tetra-p-tolylporphyrinato)titanium(IV) oxide, (TTP)Ti=O, with (octaethylporphyrinato)titanium(III)chloride, (OEP)Ti-Cl, in toluene-d(8) results in the reversible exchange of axial ligands to form (TTP)Ti-Cl and (OEP)Ti=O. The net result is a formal one-electron redox process. This occurs with a second-order rate constant of (2.4 +/- 0.2) x 10(2) M(-1) s(-1) to form an equilibrium mixture with K = 1.7 +/- 0.4 at 20 degrees C (Delta H double dagger = 10.8 +/- 0.4 kcal/mol, Delta S double dagger = -10.7 +/- 1.2 cal/mol . K). Use of pivalate in place of chloride on the Ti(III) complex causes no significant change in the rate of this one-electron redox process. In addition, excess chloride only partially inhibits the rate of oxygen transfer with chlorotitanium(III) complexes. A complete kinetic analysis indicates that these redox processes proceed through two parallel pathways, both involving an inner sphere mu-oxo intermediate. Competing associative and dissociative (chloride or pivalate loss) mechanisms are in effect. Reversible oxo/chloro exchange also occurs between (TTP)TiCl2 and (OEP)Ti=O in CHCl3 to produce (TTP)Ti=O and (OEP)TiCl2 with K = 47 +/- 13 and k(f) = 5 +/- 1 M(-1)s(-1) at 20 degrees C (Delta H double dagger = 11 +/- 1 kcal/mol, Delta S double dagger = 11 +/- 3 cal/mol . K, Delta H degrees = -3.7 +/- 0.8 kcal/mol, and Delta S degrees = -5 +/- 2 cal/mol . K). In contrast, when (TTP)Ti=O is treated with (TPP)Ti(eta(2)-PhC=CPh), incomplete oxygen atom transfer occurs to produce the mu-oxo complex, [(TTP)Ti]O-2. X-ray quality crystals of this complex can be prepared by the slow diffusion of O-2 into a solution of (TTP)Ti(4-picoline)(2). The structure of the mu-oxo complex, C114H90N8OTi2, was determined by single-crystal X-ray diffraction (crystal data : monoclinic, C2/c, a = 32.044(8) Angstrom, b = 16.908(5) Angstrom, c = 18.525(5) Angstrom, beta = 117.91(2)degrees, V = 8869(4) Angstrom(3), Z = 4, R = 5.88%, and R(w) = 15.38%). Key metrical parameters include a Ti-O distance of 1.7941(8) Angstrom, average Ti-N distances of 2.113(3) Angstrom, and a Ti-O-Ti angle of 170.8(2)degrees. The Ti atom is displaced 0.66 Angstrom out of the mean porphyrin plane toward the oxo bridge.