cm-1 band is quite well resolved from the ~1 4 0 0 -c m -~ envelope and it is possible to measure its depolarization ratio more accurately than for the 1490-cm-l band.However, since it is not possible accurately to measure the depolarization ratio of the 1490-cm-l band, it is not possible completely to rule out the possibility of bidentate nitrate group bonding.(15) H. Brintzinger and R. E. Hester, Inorg. Chem., 6, 980 (1966).The kinetics and stoichiometry of the oxidation of hydroxylamine and 0-methylhydroxylamine by silver( 11) have been determined in acid perchlokate media (1.03 M < [HClOd] < 5.83 M ) a t 22'. The kinetics of the reaction with N-methylhydroxylamine, but not the stoichiometry, has also been determined. In the presence of excess silver(II), stoichiometric measurements indicated that the product of reaction with NHsOH' is NOa-; with NHaOCHa+ the products are NOaand COz, the latter directly detected in stoichiometric amounts by gas chromatographic analysis. No dependence of rate on [Ag(I)] or ionic strength was observed, within experimental error, for hydroxylamine; for 0-methylhydroxylamine, the rate was inversely dependent on acidity and ionic strength. The acidity variation for both hydroxylamines has been ascribed to the difference in reactivity of Ag2+ and AgOH+, which coexist in rapid equilibrium at these hydrogen ion concentrations. For K H = [AgOH+] [H+]/[Ag2+], graphical analysis of the rate data at ionic strength 5.95 M yields K H = (0.32 i 0.14) M . for NHaOH+, k < lo4 M-' sec-'and k' = (1.4 =! = 0.5) X lo6 M-lsec-'; for NHaOCHa+, k < lo3 M-' sec-' and k' ( 2 =k 1) X lo4 M-' sec-'. For CH3NHzOHf, the apparent second-order rate constant, k,,,, determined by the stopped-flow technique is k,,, = (1.6 i 0.6) x 106 M-1 sec-'.Comparison is made with analogous results for oxidation of hydroxylamines by Mn(III), for which the comparable rate constants are approximately one order of magnitude less (NH,OH+), and as much as three orders of magnitude less (NHsOCHs+). In this and other studies the stoichiometric consumption ratio may vary with the ratio of initial moles of reactants present. An explanation is provided for this behavior in terms of competitive intermediate steps.With the primed rate constant designating reaction with AgOH+, the rate constants are:The mechanisms of these reactions are discussed in terms of free-radical intermediates.Xenon(I1) fluoride difluorophosphate, FXeOPOF2, and xenon(I1) bis(difluorophosphate), Xe(OPOF2)2, are obtained in high yield by reactions of xenon difluoride with p-oxo-bis(phosphory1 difluoride), P20aF4, a t 22" in trichlorofluoromethane. The new compounds are pale yellow solids which decompose readily a t 22" forming an unstable material, which is probably the difluorophosphate free radical.
The relaxation spectra of aqueous molybdate has been determined by temperature jump at 25°and 1.0 M ionic strength. The pH ranged from 5.50 to 6.75; monomer concentration (i.e., NazMoCh concentration) from 0.01 to 0.25 M. The observed spectrum consisted of one or two relaxation effects, depending on pH and concentration. The longer, more concentrationdependent effect (1-180 msec) is most sensitive to heptamer formation; formally, 7MoC>42~+ 8H+ fi MojO»*-+ 4H20. The shorter, less concentration-dependent effect (200-500 Msec) is most sensitive to octamer formation; formally, M07O246"" + M0O42-+ 4H + <± MosChe4-+ 2H20. Rate constants for these two equilibria were varied until a set of constants yielding the best agreement between measured and observed relaxation time was found. The interpretation of these results depends heavily on the equilibrium constants used to describe the system. The best results were found with the equilibrium data of Aveston, Anacker, and Johnson.2 The rapidity of condensation to form the heptameric isopolymolybdate species is explained by a mechanism involving reactions of protonated monomeric molybdate, which is then assumed to have octahedral coordination. These species, presumably of composition OMo(OH)s-and Mo(OH)i, have the coordination appropriate to units of the polymer structure. The dissociation rate constant for the octamer (yielding heptamer and monomer) is larger than that for the heptamer (complete breakup to monomer), reflecting the enhanced stability of the heptamer.Upon addition of acid to alkaline molybdate solutions, isopolyanions are formed. Equilibrium is established between monomeric molybdate and, predominantly, heptameric molybdate polymers. Despite the relative complexity of these polymolybdate ions, the steps leading to their formation are complete within a few milliseconds at moderate concentrations.8Unlike rapid mixing studies, which are generally independent of any associated equilibrium constants, temperature-jump investigations usually depend on a sound knowledge of these constants for successful interpretation of the results. Thus, the first measurement on these systems, carried out in connection with a temperature-jump investigation of molybdenum (VI)tartaric acid complexatjon using optical rotation detection, yielded relaxation spectra indicative of polymerization, which were not, however, fully interpreted, due to a lack of precise knowledge of the species present.4 A temperature-jump study following further equilibrium acidity measurements on these systems confirmed the validity of the earlier kinetic measurements, and were in essential agreement with the stabil-
Aqueous molybdate-EDT A solutions were studied by temperature jump in the pH range 7.25-8.25 at 25°and ionic strength 0.1 M (NaN03). The upper time limit of the apparatus was extended to allow accurate measurement of relaxation times in the 1-3 sec range by interfacing the signal to an on-line averager. The rate constant for the reaction HMoOr + AH22--Mo03AH3-+ H20 is k¡ = (2.26 ± 0.23) X 10s M-1 sec-1, where AH22-is the diprotonated form of EDTA. The composite rate constant for the reactions Mo03A4-+ HMoOr -03MoAMo034-+ OH-(k6) and M0O3AH3-+ Mo042--03MoAMo034-+ OH-(k7) is A:67 = (3.26 ± 0.88) X 10s M-2 sec-1. Upper limits for the individual rate constants are ke S 3.26 X 104 and k-S 1.03 101 M-1 sec-1.Comparing k5 with rate constants for chelation of HMoOr by 8-hydroxyquinoline and catechol shows the EDTA reaction to be slower. Bimolecular steps in the polymerization of molybdenum(VI) are also more rapid. The nature of the rate-limiting step appears to depend on the ligand; for EDTA, the process of forming molybdenumligand bonds may be rate limiting.Until recently, there have been only a few detailed studies of molybdate complex formation equi-libria2 and none on their kinetics. Protonation of reactants and products as well as polymerization of molybdenum(VI) below pH 7 served to hinder progress in solving equilibria problems. The rapidity and complexity of these reactions, likewise, hampered the kineticist. Improved equilibrium data and the application of fast reaction techniques have led to firm conclusions on the number and identities of the species in equilibrium and their kinetics, for example, for molybdate polymerization,3 formation of 8-hydroxyquinoline complexes4 and formation of catechol complexes.5Anionic molybdenum(VI) forms ethylenediaminetetraacetic acid complexes,6 which have been studied extensively with regard to their equilibria and structure.7-10 Variously protonated 2Mo(VI): A and Mo-(VI): A (i.e., 2:1 and 1:1) complexes are present in aqueous solution below pH 9.9,10 The structure of the 2:1 complex has been determined by X-ray analysis.7It was shown that, with respect to M0O42-, an oxygen atom is lost on complexation. The coordination is octahedral around each Mo(VI), i.e., M0O3 moieties with A4being linked to each Mo(VI) by one nitrogen and two carboxylate oxygen linkages. The formula of the 2:1 complex can be given as O3M0AM0O34-.The structure of the 1:1 complex is analogous.9•10We have carried out temperature-jump relaxation studies on Mo(VI)-EDTA complexes in order to study(1) The authors gratefully acknowledge support from Public Health
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