Reactions of aquatitanium(II) with hypervalent chromium species.

Titanium(ii) solutions, prepared by dissolving titanium metal in triflic acid and HF, react readily with derivatives of Cr(vi), Cr(v) and Cr(iv). Reductions of Cr(vi) and Cr(iv), carried out with [Co(NH(3))(5)Br](2+) as a scavenger for Cr(ii), yield no detectable Co(2+), indicating that 2e(-) steps, bypassing Cr(v) and Cr(iii), are not operative. Catalysis by Ti(iv) is observed only for reduction of Cr(vi). Rates for reduction by Ti(iii) are remarkably similar to those for the corresponding reductions by Ti(ii). The addition of Ti(iii) to triflic acid and HF results in rapid and quantitative disproportionation to Ti(ii) and Ti(iv).

Reductions by titanium(II) as catalyzed by titanium(IV).

The cobalt(III) complexes, [(NH3)5CoBr]2+ and [(NH3)5CoI]2+ are reduced by Ti(II) solutions containing Ti(IV), generating nearly linear (zero-order) profiles that become curved only during the last few percent of reaction. Other Co(III)-Ti(II) systems exhibit the usual exponential traces with rates proportional to [Co(III)]. Observed kinetics of the biphasic catalyzed Ti(II)-Co(III)Br and Ti(II)-Co(III)I reactions support the reaction sequence: [Ti(II)(H20)n]2+ + [Ti(IV)F5]- (k1)<==>(k -1) [Ti(II)(H2O)(n-1)]2+ + [(H2O)Ti(IV)F5]-, [Ti(II)(H2O)(n-1)]2+ + Co(III) (k2)--> Ti(III) + Co(II) with rates determined mainly by the slow Ti(IV)-Ti(II) ligand exchange (k1 = 9 x 10(-3) M(-1) s(-1) at 22 degrees C). Computer simulations of the catalyzed Ti(II)-Co(III) reaction in perchlorate-triflate media yield relative rates for reduction by the proposed active [Ti(II)(H2O)(n-1)]2+ intermediate; k(Br)/k(I) = 8.