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Solutions of chromium(II), europium(II), or vanadium(II) chloride in hydrochloric acid evolve molecular hydrogen rapidly in the presence of trace concentrations of the cobalt(II) macrocycle Co(dmgBF2)2. The stoichiometry for Cr(II) corresponds to the net reaction Cr2+ + Cl" + H+ = CrCl2+ + '/2H2. The kinetics are described quite adequately by the Michaelis-Menten scheme. Kinetic studies of the reaction were made during the pre-steady-state phase, during which an intensely absorbing intermediate forms, and also at longer times during the steady-state phase when the pseudo-steady-state concentration of the intermediate slowly declined as the substrate was consumed. Arguments are given in support of the intermediate being [(H20)5Cr-C!-Co(dmgFB2)2]+. Its dissociation leads, in acidic solution, to the hydridocobalt complex HCo(dmgBF2)2, which is responsible for H2 formation. Bromide ions, but not perchlorate, also give catalytic H2 production, whereas iodide forms a ternary complex that does not decompose.

The peroxides from methylrhenium trioxide (MTO) and hydrogen peroxide, CH3ReO2(η2-O2), A, and CH3Re(O)(η2-O2)2(H2O), B, have been fully characterized in both organic and aqueous media by spectroscopic means (NMR and UV−vis). In aqueous solution, the equilibrium constants for their formation are K 1 = 16.1 ± 0.2 L mol-1 and K 2 = 132 ± 2 L mol-1 at pH 0, μ = 2.0 M, and 25 °C. In the presence of hydrogen peroxide the catalyst decomposes to methanol and perrhenate ions with a rate that is dependent on [H2O2] and [H3O+]. The complex peroxide and pH dependences could be explained by one of two possible pathways:  attack of either hydroxide on A or HO2 - on MTO. The respective second-order rate constants for these reactions which were deduced from comprehensive kinetic treatments are k A = (6.2 ± 0.3) × 109 and k MTO = (4.1 ± 0.2) × 108 L mol-1 s-1 at μ = 0.01 M and 25 °C. The plot of log k ψ versus pH for the decomposition reaction is linear with a unit slope in the pH range 1.77−6.50. The diperoxide B decomposes much more slowly to yield O2 and CH3ReO3. This is a minor pathway, however, amounting to <1% of the methanol and perrhenate ions produced from the irreversible deactivation at any given pH. Within the limited precision for this rate constant, it appears to vary linearly with [OH-] with k = 3 × 10-4 s-1 at pH 3.21, μ = 0.10 M, and 25 °C. Without peroxide, CH3ReO3 is stable below pH 7, but decomposes in alkaline aqueous solution to yield CH4 and ReO4 -. As a consequence, the decomposition rate rises sharply with [H2O2], peaking at the concentration at which [A] is a maximum, and then falling to a much smaller value. Variable-temperature 1H NMR experiments revealed the presence of a labile coordinated water in B, but supported the anhydride form for A.

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