S. Fujita, H. Horii. T. Mori, and S. Taniguchi
960
ly suited for an investigation of these interactions. Kinetic behavior of eaq- in the presence of labeled and unlabeled CTAB micelles indicates that the reaction of eaq- with a molecule on a charged interface may be an extremely rapid process. This would indicate very fast diffusion on the micelle surface. Insights gained for the reactions of eaq- or other negatively charged species in micellar model systems may be found applicable to systems of more complex bioaggregates.
References and Notes (1) (a) The Radiation Laboratory of the University of Notre Dame is operated under contract with the U.S. Atomic Energy Commission. This is AEC Document No. COO-38-972.Also supported in part by the U.S. Atomic Energy Commission under Contract No. AT(l 1-1)-3238with Carnegie-Mellon University. (b) Address correspondence to this author at the Department of Medical Physlcs, Michael Reese Medical Center, Chicago, 111. 60616. (2)(a) K. M. B a n d , L. K. Patterson, E. J. Fendler, and J. H. Fendler, Int. J.
Rad& Phys. Chem., 3, 321 (1971);(b) C. L. Greenstock and 1. Dunlop. i b K 5. 237 119731. (3)S. C.Wallace and>. K. Thomas, Radiat. Res., 54, 49 (1973). (4)J. H. Fendler, N. V. Klassen, and H. A. Gillis, J. Chem. Soc.. Faraday Trans. 1, 70, 149 (1974). ( 5 ) P. P. Infelta, M. Gratzel, and J. K. Thomas, J. Phys. Chem.. 78, 190
(1974). (6)G. A. Infante, J. H. Fendler, and L. K. Patterson, unpublished results. (7)M. Gratzel, J. K. Thomas, and L. K. Patterson, Chem. Phys. Lett., 29, 393 (1974). (8) (a) T. Kajiwara and J. K. Thomas, J. Phys. Chem.. 74, 2290 (1970):(b) L. K. Pattersonand J. Lib, ht. J. Radlat. Phys. Chem.. 6,129 (1974). (9)(a) M. N. Jones and D. A. Reed, Kol/oid-Z.,235, 1196 (1970);(b) A. L. Loeb, J. Th. Overbeek, and P. H. Wlersema, "The Electrical Double Layer Around a Spherical Colloid Particle", MIT Press, Cambridge, Mass., 1961. (IO)L. K. Patterson and E. Vieil, J. Phys. Chem., 77, 1192 (1973);R. R. Hautala, N. E. Schore, and N. J. Turro, J. Am. Chem. Soc., 95,5508 (1973); M. Gratzel and J. K. Thomas, ibid, 85, 6885 (1973). (11)P. Debyeand E. W. Anacker, J. Phys. Chem., 55, 644(1951). (12)L. C. Parratt. "Probability and Experimental Errors in Science", Wiley, New York, N.Y., 1961. (13)P. Mukerjee and K. J. Mysels, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 36 (1971).
Pulse Radiolysis of Mercuric Oxide in Neutral Aqueous Solutions Shin-ichi Fujita,' Hideo Horii, Toshiaki Mori, and Setsuo Taniguchi Radiation Center of Osaka Prefecture, Shinke-cho, Sakai, Osaka, Japan (Received August 22, 1974)
The reduction of HgO in deaerated aqueous solutions was studied by the pulse radiolysis technique and spectrophotometric analysis after electron pulse irradiation. HgO reacts with eaq- with a rate constant of (2.3 f 0.2) X 1O1O M-' sec-l to yield a transient species of HgOH having an absorption maximum at 233 nm and a shoulder at 260-270 nm (Po(5.3 f 0.2) X 103 M-l cm-l). The pK of the equilibrium, Hg+ HgOH, was estimated as 5.1. HgOH dimerizes with a rate constant of 2k = (4.4 f 0.5) X lo9 M-l sec-l at the lower concentration of HgO (5100 p M ) . The dimer, assigned to Hg2O or Hg2(0H)2, has an absorption maximum at 215 nm with a shoulder at 230 nm and further decays according to a first-order rate law. At higher concentrations of HgO, however, the formation of polynuclear mercury oxide having a longer lifetime was taken into consideration from some evidence.
-
Introduction
It has been reported by many workers that the radiolytic reduction of the mercuric ion leads to the formation of Hg+ as the initial reduced species in acidic aqueous solution1s2 or acidic ice From pulse radiolytic study of the solution neighboring pH 4,2 the stable mercurous ion, Hg22+, was formed via an additional step after the disproportionation reaction of the initial species: Hg'
+
Hg'
Hg'
+
-
Hgo + Hg(I1)
Hg(I1) + Hg, 2'
while Nazhat and Asmus5 have demonstrated that the transient, HgC1, formed in the reduction of HgCl2 reacts directly to give Hg2C12 in aqueous solution. It is expected that the Hg+ ion may be hydrolyzed to HgOH and also Hgz2+to Hg20H+ (pK1 = 516 or HgZ(0H)z in solutions of higher pH. However, X-ray diffraction studies7 and some other results8 have shown that the solid mercurous oxide does not exist as HgzO but as a mixture of Hg and HgO. These arguments as well as the concern of enviThe Journal of Physlcal ChemistrypVol. 79, No. 10, 1975
ronmental pollution arouse interest in the behavior of mercury ions in neutral solutions. In the present study, the pulse radiolysis technique has been applied to neutral solutions of HgO. The reaction mechanisms are discussed and compared with the results in acidic solutions.
Experimental Section Experimental details of pulse radiolysis measurements have been described previously.2 Most observations of the long-lived species were carried out in square quartz cells (1.0 X 1.0 X 4.0 cm) with Thermal quartz windows on a spectrophotometer (Shimadzu RS-27). Oscillographicstudies longer than 1msec were performed on the dc connection with the comparison voltage setup of the apparatus (National VP-541A). HgO (Merck, GR grade, yellow), which has rather high solubility in water: was dissolved in hot triply distilled water through a sintered glass filter. The excessively dissolved HgO (0.5-1 mM) thus obtained was found not to precipitate at .room temperature for 1 month. However,
Pulse Radiolysis of HgO in Aqueous Solutions
961
each solution containing the required HgO was prepared by diluting the freshly made stock solution and used within 1 day. The concentration of mercuric oxide was determined spectrophotometrically as its EDTA complex in the acidic solution at 247 nm, similar to previous work.2 The solutions were deaerated by huhbling with argon for periods longer than 2 hr. All materials were used without further purification from the commercial sources. Results Pulse radiolysis of deaerated aqueous solutions of mercuric oxide yielded initially a transient species of Hg(1) having an absorption maximum a t 233 nm and a shoulder a t 260-270 nm. Figure 1 shows the optical absorption spectra immediately, 2.5, and 250 msec after the pulse in a deaerated neutral solution of 200 pM HgO. After the decay of the initial species, the absorption increasing toward shorter wavelength remained. Figure 2 shows the oscillographic traces of the absorption obtained in the same solution. The absorption disappears by a second-order rate law a t 260 nm, but reaches a constant value within ahout 2.5 msec a t 220 nm. Later, the absorption a t 220 nm is found to increase slightly within 250 msec. Essentially similar results were obtained in the solution containing 10 mM tert-butyl alcohol, although the spectrum of its radical slightly overlapped a t an earlier stage. However, little absorption appeared in the solution saturated with N20. The specific rate constant of the hydrated electron with mercuric oxide was determined from the pseudo-first-order decay rate of eaq- a t 500 nm in deaerated neutral solutions of 0-30 &4 mercuric oxide and 10 m M tert-butyl alcohol. k(e.,HgO) = (2.3 i 0.2) X 10'"M-' sec-' was thus obtained. Figure 3 shows the second-order decay plots of the transient a t different doses a t 260 nm in a deaerated 200 &4 HgO solution containing 10 mM tert-butyl alcohol. The slopes of the lines equal 2kld where k is the second-order rate constant, c is the molar extinction coefficient, and 1 is the light path in the irradiation cell. The absorption quantity a t 260 nm was linear against doses within 1500 rads per pulse in the solution of 200 pM HgO containing 10 m M tert-butyl alcohol. Taking the G value of the reducing species including a H radical as 3.2, the molar extinction coefficient of the species a t 260 nm is calculated as (5.3 f 0.2) X 1 6 M-' em-'. Hence k can be calculated. All values of 2klc determined in solutions of 100400 &4 HgO by several doses yielded (8.3 i 0.3) X 105 cm sec-'. The result was not affected by any ionic strength in the solution containing NaC104 up to 0.1 M.Then, the rate constant 2k is calculated as (4.4 + 0.4) X lo9 M-' sec-'. As to the remaining absorption a t 220 nm, it became nearly constant during oscillographic observation in a 100 &4 HgO solution. In solutions of higher HgO concentrations, however, the absorption further increased during a longer time, as shown in Figure 2c. Figure 4 shows the changes of the absorption quantities vs. HgO concentrations a t 2.5 and 250 msec after the pulse in solution with and without tert-butyl alcohol. The absorption a t 2.5 msec becomes nearly constant a t concentrations above 200 &4. However, the absorption a t 250 msec further increases with increasing HgO concentration up to 1mM. Figure 5 shows the spectrum of the species remaining a t 2.5 min after irradiation in a 100 &4 HgO solution, where the solution was pulse irradiated repeatedly ten times giv-
+
Wavelength. nm
Flgun 1. Abswptiol specha obtained in a deaerated aquews MIUlion of 200 phi HgO average dose, about 2000 rads; immediately (0).2.5 (0). and 250 msec (A)afler ihe pulse; the dotted C U N ~ shows Uw spectrum of Hg+ normalized ai 236
Flpure 2. Oscillographictraces obtained in a deaerated aqueous so. lution of 200 pM HgO: average dose, about 2800 rads; ordinates. 4.5%lcm; abscissa (a) 20 &sec/cm.ai 260 nm. (b) 0.5 mseclcm. at 220 nm. (c)20 mseclcm. at 220 nm.
I
0 0
IO
20
Time.
30
40
0
prec
Figure 3. The second-order decay of absorption ai 260 nm due io HgOH ai several doses: upper. 1050; middle. 1800 bonom. 2900
rads.
ing approximately the same dose as that used for the oscillographic observations. The characteristics of the spectra under various conditions are listed in Table I. D2s41D215 varies with the concentrations of HgO or t-BuOH, while D2301D215is unchanged within experimental error. Data obtained in solutions of 200 p M HgO were hardly influenced by the number of repetitions of the pulse in the range of 10-40. These results suggest that the spectra origiThe Jwrnal Of mySica1Chemishy. Vd. 79. No. io. 1975
S. Fujita, H. Horii, T. Mori, and S.Taniguchi
962
TABLE I: Characteristics of the Spectra due to the Long-Lived Absorption and Its First-Order Decay Rate in Various Conditions HgO,
t-BuOH,
PM
mM
D254/
~ 2 1 5 ~
~ 2 1 5 ~
min-'
0.36 0.16 0.21 0.24 0.11
2 2 5 10 0.8
0.08
0.4
0 0.66 0 0.60 10 0.64 100 0.60 300 0 0.62 500 0 0.59 a Determined at 2.5 min after the pulse. 100
200
0
. 0.0
0
0
1
0.4
0.2
[HgO)
1 .o
'
mM
Flgure 4. Dependence of the absorption quantity at 220 nm upon [HgO]: in the presence of 10 m M t-BuOH at 2.5 msec (A)and 250 msec (A)after the pulse, and in the absence of the alcohol at 2.5 (0) and 250 msec (0)after the pulse; average dose, 2000 rads.
k , lo-'
DZ3'/
tions. The absorption at 254 nm is found to increase in an earlier period of time in the absence of the alcohol. Table I also lists the first-order rate constants for the decay of the absorption calculated from the slopes under several conditions at 215 nm. The rate decreases with increasing HgO concentration, but increases in the presence of t-BuOH. These rates were little affected by aeration or the addition of HzO2 ( ~ mM) 2 after irradiation. The addition of perchloric acid to the irradiated solution changed the spectra to that of the Hgzz+ ion. When the acid was added after most of the long-lived species had decayed, the quantity of Hgzz+ ion observed was much reduced in comparison with the value which should be obtained on the addition of the acid immediately after irradiation. However, the quantity was found to increase slowly ( t 1 / 2 1 hr) to the expected value.
-
Wavelength, nm
Figure 5. Absorption spectra on a spectrophotometer at 2.5 min after irradiation in a solution of 100 p M HgO: total dose, about 2.0 X rads by ten repeated pulses.
lo4
Discussion
A probable mechanism for the formation of Hg(1) in neutral solutions could be described by the following processes:
-
H,O-e,',
1.5
H, OH, and other stable products (1) (2 1
e,,-+ HgO Hg(1) H + HgO -+ Hg(1)
(31
Here, the H radical would also contribute to the formation of the transient in analogy with the results in the acidic solutions of the mercuric ion.1,2.10 The mechanism was supported by the results of scavenger effects. The retardation of the initial absorption by NzO may be caused by the fast reaction 4 (k4 = 5.6 X lo9 M-l secl).ll OH radicals disap-
1.0
0 0 A
5.5
eaa- + N 2 0
2.0
0
25
50
75
100
min
Figure 6. The first-order decay plots of the long-lived absorption in the 200 pM HgO solutions with 10 mM t-BuOH at 215 (0)and 254 nm (A),and without the alcohol at 215 (0)and 254 nm (A).
nate from two species: one has a peak at 215 nm with a shoulder about 230 nm, the other two peaks split around 254 nm. It was found that these absorptions further decayed slowly. Figure 6 shows the first-order decay plots of the long-lived absorptions at 215 and 254 nm in the solutions of 200 pM HgO with and without t-BuOH. The linearities are good at 215 nm, but not so good at 254 nm in both soluThe Journal of Physical Chemistry, Vol. 79, No. IO, 1975
-+
N2
+
OH
+
(4 )
OH-
peared by their recombination without further oxidizing HgO. In solutions of HgO alone, however, the radical would consume some of the a,- or Hg(1) formed in reactions 2 and 3 to lower the yield of the reduced species. When the absolute yield of Hg(1) was in question, therefore, t-BuOH was added to scavenge OH radicals: OH
+
t-BuOH
-
t-BuOH radical
+
H,O
(5)
where 125 = 2.5 X lo8 M - l sec-l.12 The reaction of the H radical with t-BuOH can be neglected.l3 Although the tBuOH radical has an absorption spectrum with a peak at 225 nm 900 M-l cm-l),14 the contribution may be neglected a t wavelengths higher than about 250 nm. Because of these considerations, the determination of the absorption coefficient or the decay rate of the transient species was carried out a t 260 nm in the presence of t-BuOH. In addition the contribution of the radical becomes negligible
Pulse Radiolysis of HgO In Aqueous Solutions
963
The Long-Lived Species. Since the absorption quantity I at 2.5 msec after the pulse a t 220 nm is hardly affected by 1
"O 0.9
increasing the HgO concentrations higher than about 200
pM (Figure 4), it may be considered that the absorption
o.6
L
t L I
0.0
I
I
I
I
5
4
-
I
I
7
6
- L o g (HCIO.]
Figure 7. Dependence of
@55/@3e
formed at this time may originate from the recombination product of HgOH. This is supported by the pH dependence of the absorption quantity illustrated in Figure 8. The inclinations are inverse between the results a t 220 and 236 nm where Hgz2+ has an absorption maximum. Both inflection points are in agreement with the pK value (5.0) of the equilibrium, Hg22+ H ~ Z O H +In. Figure ~ 8, there is no evidence for the equilibrium, Hg20H+ Hg2(0H)2. However, the recombination product of HgOH in the neutral solution was ascribed to Hg2O or Hg2(OH)2, because Hg22+ hydrolyzes to precipitate in such a solution.
upon -log [HC104], [H 01 =
200 NM. The data without the acid are plotted at -log [HC104f= 7.
2HgOH
0.00
5
4
7
6
-Log
(HCIO,)
Flgure 8. Dependences of the absorption quantities upon -log [HC104] in the deaerated aqueous solutions of 200 p M HgO and 10
m M t-BuOH: average dose, 2000 rads; 2.5 msec after the pulse at 236 (0)and 220 nm (A);250 msec after the pulse at 220 nm (0). The data without the acid are plotted at -log [HC104] = 7.
even at 225 nm about 500 psec after the pulse irradiation due to its recombination reaction. Reaction 2 is considered to be a diffusion-controlled process from the rate constant determined by the decay of eaq-. The rate constant of reaction 3 could not be determined in the present study. The Short-Lived Species. The spectrum of the transient a t the end of the pulse is different from that obtained in acidic solutions, particularly as to the shoulder around 260 nm. The spectrum of Hg+ normalized at 236 nm is shown by the dotted curve in Figure 1 for comparison. As mercuric oxide dissolves as HgO the first step in its reaction with eaq- might form HgO-. In neutral solutions, however, the second-order decay rate of the absorption is not affected by ionic strength. The result suggests that the transient may not carry any charge on it. Then, the absorption may originate most likely from the species, HgOH, which corresponds to a hydrolyzed form of Hg+ ion. Noting earl-+ HgO HgOH
+
--
HgOH
H'
+
OH'
Hg(H20)*
(6)
(7) the difference between the two spectra in acidic and neutral solutions, the ratio, D255/D236,was taken as a measure for the compositions of Hg+ and HgOH in equilibrium 7. Figure I shows the dependence of D255/D236upon -log [HClOd]. Approximately regarding -log [HC104] as the pH value of the solution, the pK value is estimated as about 5.1 from the S-type curve in the figure.
-
-
Hg,O
+
H,O o r Hg,(OH),
(8
Considering the stoichiometry of the reaction, the absorption coefficient of Hg2O was calculated as (8.7 f 0.7) X lo3 M-l cm-l at 220 nm from the saturated value obtained in the solution containing 10 mM t-BuOH (Figure 4). Slight increase of the absorption above 500 pM of HgO is appreciated as the influence from further increasing absorption within 250 msec. The shorter part of the spectra in Figure 5 is also assigned to HgzO (or Hg2(OH)2), since the absorption at shorter wavelength obtained in the solution of 100 fiM HgO remains constant during oscillographic observation after the decay of the initial transient. It has been reported2 that in acidic solution Hg+ ions disproportionate to form Hgo and Hg(I1) and later Hgz2+is formed by the successive reaction of Hgo with Hg(I1). On the contrary, dimerization in neutral solutions occurs instantaneously with the second-order decay of HgOH as seen in Figure 2b. The fast recombination of HgOH might be attributable to the fact that HgOH has no charge on it. This is similar with the result5 that HgCl recombines in pulse radiolysis of aqueous HgClz solution. X-Ray diffraction study7 and some other resultss have revealed that solid Hg2O does not exist and that the black powder precipitated from solutions of Hgz2+ ion and alkali consists of fine particles of mercury and mercuric oxide. The unstability of the long-lived species is compatible with this fact. Since the possibility that 0 2 as the impurity or H202 as the minor product from radiolyzed H2O may contribute to the decay process was excluded experimentally as cited in the Result section, then, the first-order decay of the long-lived species may be explained by its decomposition to Hg and HgO. H g 2 0 or Hgz(OH),
-
Hg
+
HgO (or Hg(OH),)
(9)
The spectrum around 254 nm in Figure 5 is similar to that of atomic mercury in water.15 However, the absorption to be found should be by far lower, considering the low solubility of mercury16J7 and the shorter optical path length of the analytical light. Nevertheless, it was concluded that the absorption around 254 nm in Figure 5 originates from mercury dissolved temporarily in excess, which was produced by reaction 9. This may be well explained by the fact that the absorption at 254 nm is relatively high in solutions at a concentration of HgO (200 w M ) containing t-BuOH, where Hg2O decays faster than in the solution without tBuOH (Table I). Mercury dissolved in excess may be regarded as an intermediate in successive reactions. Hg2O (or Hgz(OH),)
Hgo(aq) -% Hgo(liq)
(10)
The Journal of Physical Chemistry, Vol. 79, No. 10, 1975
S.Fujita, H. Horii, T. Mori, and S.Taniguchi
964
HgZO
- / 0 9
[HgOl, mM
Figure 9. Dependence of the increasing rates of the absorption at 220 nm upon [HgO], [t-BuOH] = 10 mM.
The concentration of the intermediate is expressed by the equation
where co is the initial concentration of Hg2O (or Hg2(0H)2). This equation tells us that the concentration of Hgo(aq) has a maximum value depending upon ha and hb. If k b is constant, the larger value of ha leads to the higher maximum concentration of Hgo(aq).The experimental results are in agreement with this expectation as seen in Figure 6. However, the absorption around 254 nm already appears slightly at 2.5 msec after the pulse and is even found to decay slightly, as seen in Figure 1. This might be explained by the fact that a part of HgOH disproportionates to Hg C Hg(0H)z. Reaction 9 is also ascertained by the experiment where perchloric acid is added after irradiation. According to this mechanism, Hgz2+ would be formed with the proton in the irradiated solution and its quantity should be unchanged on addition of the acid before or after the decay of HgzO. When the acid was added after most of Hg20 had decayed, it took long time to reach the value obtained on the addition just after irradiation. However, the final value was in agreement with the expectation. The recovering process seems to be quite slow. Figures 2c and 4 suggest that another species is formed within 250 msec in solutions of higher HgO concentrations, although any meaningful change of the spectrum is not observed except for the increase of absorption a t shorter wavelengths. t-BuOH does not participate in the formation of the species, since the similar trend of the absorption with the HgO concentration is found in the presence or absence of the alcohol. The difference in the yields may be due to the scavenging effect of OH radical by the alcohol. The slow increase of the absorption apparently obeyed a first-order rate law, since plots of -In (D, - D ) VS. t showed good linearities. Figure 9 shows the dependence of the increasing rate of the absorption at 220 nm calculated from the slopes upon HgO concentrations. Taking into consideration that the data a t the lower concentrations of HgO would have rather the large uncertainty due to the smaller increases, the rate is proportional t o the HgO concentrations. Then, the slow increasing process would occur by the pseudo-first-order reaction against HgO. The first-order decay rate of the absorption becomes lower at higher concentrations of HgO as shown in Table I. These facts may be explained by the formation of a polynuclear species of mercury which has a longer lifetime.
The Journal of Physical Chemistry, Voi. 79, No. IO, 1975
-I- BHgO
--+
Hg,O.BHgO
(11)
Some complexes having polynuclear structures have been proposed in solutions of stable mercuric ions. Hayek e t a1.18 suggested the formation of polynuclear ions, Hg(Hg0)z2+, in solutions of concentrated mercury perchlorate. According to Ahlberg,lg Hg2+ ion is hydrolyzed to form successively the polynuclear ions, HgOH+, . . ., Hg2(OH)22+, Hgd(OH)#+, etc. Johansson has proposedz0 that these species contain the bridged structure where mercury atoms are combined with each other through HzO, OH-, or 0z2-. If our assumption here is true, it seems reasonable that the recombination product from HgOH is also of the type HgOHg or H
in place of the molecule directly binding between two mercury nuclei as HgZC12. Then, trinuclear mercury oxide, for example, would have a structure such as HgOHgOHg or
H H Such bridged complexes have generally been considered in many other metal compounds,21 as well as in crystals of mercuric oxide.22 The HgOH radical might well react with t-BuOH radicals forming an Hg-organic compound. However, no essential differences were found between the product spectra in the presence and absence of t-BuOH. The faster decay of HgzO in aqueous alcoholic solutions could not be explained yet. It might be concerned with the ability of alcohols hydrolyzing mercury ions.
References and Notes (1) (2) (3) (4)
M. Farragi and A. Amozig, Int. J. Radiat. Phys. Chem., 4, 353 (1972). S. Fujita, H. Horii, and S. Taniguchi, J. Phys. Chem., 77, 2868 (1973). D. N. Moorthy and J. J. Weiss, Adv. Chem. Ser., No. 50, 209 (1965) R. S. Eachus, M. C. R. Symons, and J. K. Yandell, Chem. Commun., 979 (1969). (5) N. B. Nazhat and K.-D. Asmus, J. Phys. Chem., 77, 614 (1973). (6) W. Forsling, S. Hietanen, and L. G. SiILn. Acta Chem. Scand., 6, 901 (1952). (7) R. Fricke and P. Ackermenn, Z.Anorg. Allg. Chem., 211, 233 (1933). (8) 6 . J. Aylett, "Comprehensive inorganic Chemistry", Vol. 3, J. Bailer, H. J. Emeleus, R. Nyholm, and A. F. Trotmann-Dickenson, Ed., Pergamonn Press, Oxford, 1973,.p 292. (9) J. W. Melior, "A Comprehensive Treatize on Inorganic and Theoretical Chemistry", Voi. 4, Longmann, London, 1960, p 776. (10) M. Farragi and J. DBsalos, lnf. J. Radiat. Phys. Chem., I, 335 (1969). (11) J. H. Baxendale et al., Nature (London), 201, 468 (1964). (12) M. Anbar and P. Neta, h t . J. Appl. Radiat. Isotopes, 18, 493 (1967). (13) (a) A. Appieby, G. Schoies, and M. Simic, J. Am. Chem. SOC., 85, 3891 (1963); (b) J. Rabani, J. Phys. Chem., 66, 361 (1962); (c) A. Appieby, J. Am. Chem. SOC.,87, 1610 (1963). (14) M. Simic, P. Neta, and E. Hayon. J. Phys. Chem., 73, 3794 (1969). (15) H. Reichardt and K. F. Bonhoeffer, Z.Hektrochem., 36, 753 (1930); Z. Phys., 67, 780 (1931). (16) S. S. Choi and D. G. Tuck, J. Chem. SOC., 4080 (1962). (17) A. Stock, Z.Anorg. Allg. Chem., 217, 241 (1934). (18) (a) E. Hayek, 2.Anorg. A/@. Chem., 223, 382 (1935); (b) E. Hayek and E. Schnell, Monatsh Chem., 85, 472 (1954). (19) I. Ahlberg, Acta Chem. Scand., 16, 887 (1962). (20) G. Johansson, Acta Chem. Scand., 25, 2787, 2799 (1971). (21) C. K. Jorgensen, "Inorganic Complexes", Academic Press, London, 1963, chapter 2; R. B. Heslop and P. L. Robinson, "Inorganic Chemistry", Elsevier,Amsterdam, 1967, Chapter 30, etc. (22) D. GrdeniC, Quart. Rev., Chem. SOC., 19, 303 (1965).
