The syntheses of pentaammineruthenium(I1) and pentaammineruthenium(II1) complexes of pyridine and analogous aromatic nitrogen heterocycles are reported here. The pentaammineruthenium(I1) complex of each of these heterocyclic ligands has an intense absorption band in the visible region which, on the basis of evidence presented below, is assigned to a metal-to-ligand charge-transfer transition. The pentaamminepyrazineruthenium-(11) complex ion can be protonated in a reversible fashion, presumably at the noncoordinated nitrogen of the 1,4-diazine. The pK, of this complex demonstrates that the dipositive ion is approximately two orders of magnitude more basic than the free ligand. This result and related data are interpreted in terms of ground-state ir back-bonding from the ruthenium(I1) to the aromatic ligand. e undertook the present work in the hope that com-

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The isotopic composition of oxygen liberated in the catalytic decomposition of hydrogen peroxide using MnO2, Fe2O3, colloidal gold, metallic platinum, and soluble catalase as catalysts has been measured. All the catalysts except catalase produced a measurable fractionation of the oxygen isotopes in the case of complete decomposition. A few rate experiments were also carried out, and the kinetics studied from the standpoint of isotope separation as a function of fraction of hydrogen peroxide decomposed. It is demonstrated that none of the liberated oxygen comes from the substrate water or from the potassium permanganate when hydrogen peroxide is oxidized. Observed fractionation factors are somewhat smaller than the maximum values calculated on the basis of Bigeleisen's equations. A minimum in the O18 percentage of the evolved oxygen in the case of the catalase experiments suggests that two mechanisms are operative for this type of catalysis. The initial enhancement of the O18 percentage of the evolved oxygen over that in the peroxide remains unexplained.

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