Reactions of Low-Valent Transition-Metal Complexes with Hydrogen Peroxide. Are They “Fenton-like” or Not? 1. The Case of Cu⁺_aq and Cr²⁺_aq

A thermodynamic analysis of the Fenton reaction and the “Fenton-like” reactions points out that their detailed mechanisms involve the formation of a transient complex between the low-valent cation ML_mⁿ and hydrogen peroxide of the type ML_m-1ⁿ·O₂H^-. This complex might decompose into ML_mⁿ⁺¹ + ·OH or ML_mⁿ⁺² + H₂O or react directly with a substrate. Experimental data from studies in acidified solutions indicate that (H₂O)₅Cr²⁺·O₂H^- decomposes into Cr(H₂O)₆³⁺+ ·OH in the presence of ≥0.1 M ethanol, 2-propanol, and 2-butanol, whereas (H₂O)_mCu⁺·O₂H^- reacts with the alcohols directly. However, in the absence of an organic substrate, the latter complex in acidic solution decomposes into Cu(H₂O)_n²⁺ + ·OH. The specific overall reaction rates of Cu⁺_aq with H₂O₂, (CH₃)₃COOH, and S₂O₈^2- are 4.1 × 10³, ~4 × 10³, and ≥1 × 10⁷M^-1s^-1, respectively.