First Hydrolysis Constants of Hexaaquacobalt(III) and -manganese(III): Longstanding Issues Resolved

The acid dissociation constants, Kh, of the hexaaquacobalt(III) and -manganese(III) ions are reported under conditions which minimize the problems of instability of these species at modest acidities (pH 1-4). A rapid and convenient preparation of aqueous Co(III) has been used, and the latter has been reacted with aqueous Mn(II) to form aqueous Mn(III). The noncoordinating buffer H2PIPPS (1,4-bis(3-sulfopropyl)piperazine) has been used to react with H+ and provide some control of the pH. Spectrophotometric changes, after stopped-flow mixing for Co(III) and conventional mixing for Mn(III), were used to determine the Kh values. The pKh values are as follows: Co(III), 3.10 +/- 0.07 (6.0 degrees C) and 2.92 +/- 0.04 (20.0 degrees C) in 1.0 M H+, Na+, ClO4-; Mn(III), 0.22 +/- 0.04 (7.6 degrees C) and 0.08 +/- 0.04 (20.0 degrees C) in approximately 4 M H+, Mn2+, ClO4-. Global fits of the data for each metal have been used to estimate the DeltaH degree and DeltaS degree values for Kh. It is suggested that the unusually high acidity of Mn(OH2)6(3+) may be due to a Jahn-Teller distortion effect.