122
Ind. Eng. Chem. Fundam. 1903, 22, 122-126
Extraction of Copper with 2-Hydroxy-5-nonylbenzophenone Oxime and the Catalytic Role of Bis( 2-ethylhexyl)phosphoric Acid Isao Komasawa’ and Tsutao make Department of Chemical Engineering, Osaka University, Toyonaka 560, Japan
The equilibrium performance and kinetic behavior of copper extraction from nitrate media were studied with two organic solutions, one containing 2-hydroxy-5-nonylbenzophenoneoxime and the other containing the oxime together with bis(2-ethylhexyl)phosphoric acid (HDEHP). HDEHP, added in quantities up to 10 mol/m3in xylene and 1 mol/m3 in n-heptane diluents, has been found to have little influence on the extraction performance of the oxime. The addition of small amounts of HDEHP results in a considerable increase in both extraction and stripping rates, but the data in xylene and n-heptane show different trends. A scheme for the reaction between the copper and the oxime containing HDEHP is proposed which interprets HDEHP as a phase-transfer catalyst.
Introduction Since the successful commercial application of hydroxyoxime chelating extractants for copper recovery from acidic leach liquor in the mid-l960’s, a number of investigations have been published concerning the equilibria and the kinetics of this extraction reaction. The papers on the kinetics up to 1979 were reviewed and discussed by Danesi and Chiarizia (1980) and Inoue and Nakashio (1980). Although there has been general agreement on the interfacial nature of the rate-controlling reactions with the hydroxyoximes (Flett, 1977), there still remain some discrepancies in the rate expressions. The reaction order for the oxime, for example, is reported as varying from second order to one-half order (Flett et al., 1973; Atwood et al., 1975; Whewell et al., 1975; van der Zeeuw and Kok, 1979; Cox et al., 1980; Komasawa et al., 1980b; Danesi et al., 1980; Hummelstedt et al., 1980). The hydroxyoxime extractant is known to be rather slow in its extraction rate of copper compared with other extractants such as long-chain alkylamine. The rate is also known to be increased by the addition of small amounts of such compounds as a-hydroxyoxime (Moore and Partridge, 1972; Flett et al., 1973; Atwood et al., 1975; Whewell et al., 1976; Fleming et al., 1978; Hummelstedt et al., 1980), alkylphosphoric acid (Moore and Partridge, 1972; Hazen and Coltorinari, 1975), and some other compounds (Morin and Peterson, 1975; Goren and Coltorinari, 1975). Actually, the familiar extractant, LIX64N (Henkel Co. Ltd.), is basically a mixture of two compounds in kerosene type diluent, Le., a mixture of LIX65N (P-hydroxybenzophenone oxime) and LIX63 (aliphatic a-hydroxyoxime). Flett et al. (1973) were the first to investigate this combination and proposed a mechanism to account for the catalytic enhancement of the rate. On the basis of the work of Flett et al., however, other different mechanisms were also proposed (Ashbrook, 1975a; Miller and Atwood, 1975). For this combination, several experimental results have also been published which differ from each other, and accordingly several mechanisms have been proposed to account for each worker’s own individual results (Hummelstedt et al., 1980; Fleming et al., 1978; Atwood et al., 1975; Whewell et al., 1976). In most of these studies, commercial reagents as delivered were used, and their concentrations were limited to those of commercial importance. This extraction reaction is interfacial in nature, and thus some impurities in the commercial extractants may have an influence on the
kinetics, as discussed by Danesi and Chiarizia (1980) and Hanson (1980). Some impurities might give data which are unrepresentative of the compound when present in small amounts as an accelerator. A systematic knowledge concerning the extraction equilibria and kinetic behavior obtained over a wide range of concentrations based on chemically pure reagent is needed to lead to a reasonably complete picture of this reaction. In the present study, 2-hydroxy-5-nonylbenzophenone oxime isolated and purified form LIX65N was used as an oxime, and bis(2-ethylhexy1)phosphoric acid (HDEHP) was used as an accelerator. This combination was chosen for this study, since the equilibrium relation and kinetic behavior of copper with either this oxime or HDEHP have been studied previously by the present authors (Komasawa et al., 1980a,b,c, 1981). The extraction equilibrium formulation for copper and the kinetic behavior were studied using two organic solutions, one containing only the oxime, and the other containing the oxime and varying amounts of HDEHP. Based on the kinetic data and the nature of the oxime and HDEHP, a possible reaction scheme was proposed to account for the involvement of HDEHP.
Experimental Section Reagents and Solutions. 2-Hydroxy-5-nonylbenzophenone oxime (abbreviated as the oxime or simply RH) was isolated and purified from commercial LIX65N, following the procedure of Preston and Whewell (1977). A modification was made in that copper acetate was employed instead of copper sulfate to precipitate the copper complex. The content of the active isomer in the resultant isolated reagent was obtained as 94 w t 70by measurement of the “ultimate loading” of the reagent at pH 4 (Ashbrook, 1975b). Bis(2-ethylhexy1)phosphoricacid (abbreviated as HDEHP or simply LH), kindly supplied by Daihachi Chemicals Ind. Co., Ltd., Osaka, Japan, with a purity of 99%, was further purified by precipitation as a copper complex from ether solution, following the procedure of Partridge and Jensen (1969). All inorganic chemicals and the diluents, n-heptane, toluene, and xylene, were supplied by Wako Chemical LM. as analytically pure reagent grade. n-Heptane and water were purified by simple distillation. The aqueous solutions for the equilibrium study and the extraction process in the kinetic study were prepared by dissolving copper nitrate in distilled water, to which 500 mol/m3 (Na,H)NO, was added. The solutions for the
0 196-4313/83/ 1022-0122$01.50/0 0 1983 American Chemical Society
glass tube I PTFE
-
Ill I statqr Dlat e
I1
-
-
electrode suppxt
1
Ind. Eng. Chem. Fundam., Vol. 22, No. 1, 1983
1' i
'
'
123
io' " / j
"I
sam ling &
r F J n g port
T
E
t.
two-paddle stirrer
,
I Ill
1 t.
lo0
10'
102
[RHil (mol/w)
j
I
Ltj
4
Figure 2. Equilibria of copper extraction with the hydroxyoxime alone.
10-1
16
10'
HDEHP cone
102
CLH ( m l / m s )
103
Figure 3. Effect of varying amounts of HDEHP on the equilibria of copper extraction with the hydroxyoxime.
total (analytical) concentration, CRH,except a t very high concentrations, by the dimerization constant as follows.
-
-
Kex = [CuRz][H+]2 / [CU"] [RH]
where Kexand D are the extraction constant and distribution ratio of copper, respectively. RH is the monomeric species in a diluent and its concentration is related to the
[m]+ 2[(RH)2] = [m]+ 2K2[RHI2
(5) The constants of the present oxime were determined as K2 = 120 L/mol (= 1.2 X lo-' m3/mol) in n-heptane and K 2 = 3 L/mol (= 3 X m3/mol) in toluene diluent (Komasawa et al., 1980a). The constant in xylene diluent was not directly determined but was assumed to have an identical value for toluene. The present results are shown in Figure 2 as a plot of D[H+I2against monomer concentration. The pH value was varied from 0.74 to 2.4 (i.e., [H+] = 4-180 mol/m3). Respective straight lines with a slope of 2 are obtained in the whole range of the concentrations, as expected from eq 3. The extraction constants are determined as 90 for the n-heptane diluent system and 1.5 for the xylene and toluene diluent systems. (ii) Effect of HDEHP on Extraction Equilibria. Both the reagents, oxime and HDEHP, can extract copper. HDEHP has, however, a much lower pH functionality (Komasawa et al., 1981) and the measurements were made under such conditions that the formation of Cu-HDEHP complex was negligible. The ratios of D[H+l2obtained with the solution containing the oxime together with HDEHP and (D[H+I2), with the oxime alone are plotted against HDEHP concentration in Figure 3. HDEHP, added in an amount up to 10 mol/m3 in xylene and up to around 1 mol/m3 in n-heptane, had little influence on the equilibrium performance of the oxime. Above these quantities, the performance starts to be reduced. The degree of reduction is dependent on the concentration of the oxime in n-heptane, but is independent of oxime for the xylene diluent. Hazen et al. (1975) found that HDEHP used in an amount CRH
=
124
Ind. Eng. Chem. Fundam., Vol. 22, No. 1, 1983
lo-'
I
1
1
1
I
i
l
I
I
1 1 i 1
I
1
A/
1
1
10'
/
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1 "
100
/
'
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1 ' '
10'
'
'
1
' 102 1
'
'
1 1 1
l@ (mo~ms) Figure 4. Extraction rate of copper with the hydroxyoxime alone.
[RHI
of 4 vol % with LIX64N brought a decrease in the total loading capacity for copper. The HDEHP molecule has a P - 4 group and the oxime molecule an oxime-hydroxyl group. An intermolecular complex between the two molecules is likely to be formed through hydrogen bonding and the resulting complex may have negligible ability for extraction compared with the free oxime (Komasawa and Otake, 1983).
Extraction Kinetics (i) Rate Expressions and Contribution of Diffusional Resistance. The present data were taken over a small fraction of the approach to the equilibrium (less than 5%), and the contribution from the reverse reaction and the change in the concentration of the species were thus both negligible. The slope of the line obtained by plotting the amount of copper transferred against contact time, t , gives the extraction rate, R = (V,/A)(dC,,/dt), or the stripping rate Rb = (V,/A)(dCc,/dt). The contribution of the diffusional resistance to the observed rate was estimated by comparing the observed rate with the maximum possible value of the simple physical mass-transfer rate of the species. The maximum value for the oxime, for example, is expressed as N = where kRH and are the mass-transfer coefficient and the concentration of the oxime, respectively. The value of RRH under the conditions considered is 3 x m/s, as calculated from Figure 3 in the previous publication (Komasawa et al., 1980~).The maximum rate thus calculated should be at least one order of magnitude greater than the observed rate. This requirement was met in the extraction process by making the aqueous phase pH value less than 1.8 for n-heptane and 2.0 for xylene diluent in all runs except for the special case where the pH dependency of the rate was studied. (ii) Rate for Extraction with the Oxime Alone. The dependencies of the rate on the concentrations of copper and hydrogen ion were found to be first and -first order, respectively. These agree with the published data obtained under the conditions of negligible diffusional resistance (Flett et al., 1973; Danesi et al., 1980; Komasawa et al., 1980b). All the data are plotted in Figure 4 as a plot of R,[H+]/ [Cuz+]against monomeric oxime concentration in each diluent. At high concentrations of the oxime, the data fad on respective straight lines with a slope of 1. The value of the reaction order for the oxime is seen to increase above the value of 1 and approach 2 at very low concentration of the oxime.
kRH[m],
[m]
10-1
10'
lo0
HDEHP conc
~ L H
102 (mol/rr+)
103
Figure 5. Effect of varying amounts of HDEHP on the rates of copper extraction with the hydroxyoxime.
Consequently, the extraction rate for the present system is correlated as follows. For very low concentration of the oxime ~
Ro = ~ , [ C U ~ + ] [ R H ] ~ / [ H + ]
(6)
and for normal and high concentration of the oxime Ro = kl'[Cu2+] / [H+] (7) The intersection of the two expressions is 0.7 mol/m3 for n-heptane and 12 mol/m3 for xylene diluent. The values of the constants are kl = 2 X lo4 m4/mol s and kl' = 1.5 X lo4 m/s for n-heptane and k1 = 6 X m4/mol s and k,' = 7 X m/s for xylene diluent. A reaction order higher than unity has been observed in two investigations, using hydroxyoxime reagents diluted in toluene at very low concentrations. Cox et al. (1980) have found the order to vary from 1.85 to 1.0 as their oxime concentration was increased from less than 4 mol/m3 to greater than 10 mol/m3. van der Zeeuw and Kok (1979) have found an order of 2 in the range of their oxime concentration, 5.5-39 moi/m3. (iii) Rate for Extraction with the Oxime and HDEHP. The extraction rate is increased as the amount of HDEHP added is increased, and a slight increase is seen even in the range of high concentration of HDEHP where the ratio, D[H+]2/(D[H+]2)o, in Figure 3 becomes less than unity. For simplicity, however, the data here considered are taken in range such that the ratio is unity. The results are shown in Figure 5, where R, and Ro denote the rates with the oxime in the presence and absence of HDEHP, respectively. A different tendency is seen between the results for n-heptane and xylene diluents. For n-heptane the ratio of the rates, R,/Ro, is independent of the concentration of the oxime, and about a sixfold increase in the rate is attained. For xylene, on the other hand, the degree of the enhancement becomes greater as the concentration of the oxime is decreased. Thus the addition of 10 mol/m3 HDEHP brings a fortyfold increase in the rate for 4 mol/m3 oxime solution but only an eightfold increase for 59 moi/m3 solution. (iv) Rate for Stripping with the Oxime Alone. The stripping rate is considerably fast. The ratio of the observed rate to the maximum possible value of mass-trqsfer rate of the organic copper complex using the value of kCuR = 3 X 10" m/s is between 0.05 and 0.45 for n-heptane and between 0.03 and 0.15 for xylene diluent systems. It is less easy to estimate the effective concentration of hydrogen ion in the present stripping solution, Le., 60-3000 mol/m3 nitric acid. As an approximation, the concentration of hydrogen ion was related to the concentration of nitric
[m]
Ind. Eng. Chem. Fundam., Vol. 22, No. 1, 1983 125
,
&lo-rl 1 0-6 10'
I
102
[H'l
103 h-"o/mY
,
j lo4
Figure 6. Effect of the concentration of hydrogen ion on the stripping rate.
10-7b
j
loo
10-1
[cZFG]
10' (mo1/m3)
Figure 7. Effect of the concentration of copper complex on the stripping rate.
acid, using the literature values of the mean activity coefficient of the acid (Dobos, 1975). The effect of the hydrogen ion and the organic copper complex on the rate are shown in Figures 6 and 7, respectively. The data scatter considerably, and the data in Figure 7 are seen to deviate from the straight line at high stripping rates, which may be due to the mass-transfer limitation involved inherently in the observed rates. The effect of the free oxime (not bound to copper) seems to be negligibly small since the rates obtained with full loaded organic solutions are not systematically different from those obtained with partly loaded solutions. In spite of the above limitations in the stripping process, the rate may be correlated as
-
= k_l[CuR2][H+] (8) for n-heptane and The value of the constant is 2 X 5.8 X m4/mol s for xylene diluent system. This rate expression agrees with other determinations. Whewell et al. (1975) have found that the rate can be correlated only by the product of the concentrations of copper complex and hydrogen ion. van der Zeeuw and Kok (1979) have found that the rate is first order in the concentrations of copper complex and hydrogen ion and basically zero order in the concentration of the free oxime. The change from xylene to n-heptane diluent brought about a 3.3 times increase in stripping rate. This value is lower compared with the increase found for the extraction rate and extraction constant. A similar tendency has been found for a hydroxyoxime extractant by van der Zeeuw and Kok (1979). The mass-action constant, kl/k1, is calculated at 100 and 1.0 for n-heptane and xylene diluent, respectively, and Rb
the extraction constants as 90 and 1.5. The agreement is surprisingly good, in spite of the difference in the ionic strength of the aqueous solutions and some limitations involved in the interpretation of the data in the stripping process. (v) Rate for Stripping with the Oxime and HDEHP. The rate for the stripping process is also enhanced by the addition of HDEHP, as shown in Figure 5. The observed rate ranges up to the same order of magnitude with the maximum possible value of mass transfer of the species. The situation is very complicated in this enhanced rate system, and no further consideration is made in the present study. Discussion It has been found that HDEHP is much more interfacially active than the oxime and HDEHP is preferentially adsorbed at the interface in a mixed solution system (Sonoda, 1981). The extraction of copper with the present oxime and HDEHP has the following features: (1)The kinetic mechanism must involve HDEHP in a catalytic role. (2) Two parallel reactions, one catalytic and the other uncatalyzed, must be considered. "Uncatalyzed" indicates the reaction in which only the oxime is involved. A simple reaction scheme for the "uncatalyzednreaction has been presented (Komasawa et al., 1980b) and discussed (Cox et al., 1980). This is now extended to the catalyzed reaction. In the case of uncatalyzed reaction, hydrated copper ion reacts with the oxime molecule at the interface to form the species C U ( H ~ O ) ~ ~ +This R - . then replaces its coordinated water molecules with the oxime to form the final species. The overall equilibrium formulation is thus split as Cu(Hz0)d2++ (RH),d * Cu(HzO)?+R- + H+
- -
Cu(H20)d2+R-+ RH
A
C U R+ ~ H+ + 4H20
where the subscript ad denotes the interfacially adsorbed species. With the latter part being rate-controlling step, the reaction rate is expressed as
Ro = I~[RH],~[RH][CU~+]/[H+] - k_i[CuR2][H+]
(9)
In the range of oxime concentrations of very low levels, a direct - proportional relation may hold between [RH],d and [RH]. Ro = It1[RH]2[C~2+]/[H+] - ~ - ~ [ C U R ~ ] [ H(10) +] In the range of normal and high bulk concentrations of the oxime, the condition of virtually constant value of [RHIad is met for the present oxime. Thus
Ro = ~ ~ ' [ R H ] [ C U ~ + ] / [-Hk_i[CuR2][H+] +] (11)
The functional forms predicted from the scheme agree with observed rate expressions for both extraction and stripping processes. The condition of a virtually constant concentration is met at concentrations as low as 0.7 mol/m3 for the n-heptane diluent system and 12 mol/m3 for xylene diluent. In the case of the catalytic reaction, the hydrated copper ion reacts at first with a HDEHP molecule to form the species, C U ( H ~ O ) ~ ~ +This L - . replaces coordinated water molecules with the oxime molecule to form the neutral species, CURL,which is followed by a fast ligand exchange. The overall equilibrium formulation is thus split as CU(H~O),~+ + Cu(HzO)>+L-+ H+
m+
Cu(H20)>+L-+-
-W
L
+ H+ + 4Hz0
C~RL+RH=C~R~+LH
Ind. Eng. Chem. Fundam., Vol. 22, No. 1, 1983
126
10’c
1
I ]
- 10’ 10-2
I
I l l
I
I l l
I l l
I
I l l
I
I l l
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I I
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I
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I,
I
14
&
/ A
I
1
I
I
lo-’ loo HDEHP conc. CM
10’
lo2
h“/m3)
Subscripts
Figure 8. Correlation of the enhancement of the extraction rates according to the scheme proposed. Keys as in Figure 5.
This scheme is very similar to that suggested by Ashbrook (1975a) to interpret the data of Flett et al. (1973) for the combination of LIX65N and LIX63. When the dehydration process is the rate-controlling step, the rate is expressed as
R, =
k,[Cu2+][E] [MI/[H+] - k , b [ m 2 ][H+][ m ]/
[m] (12)
The ratio of the extraction rate with the oxime together with HDEHP to that with the oxime alone is thus expressed R,/Ro = (R, + Ro)/Ro= 1 + k,[=]/k[RH],, (13) In the range where [RH],, is proportional to becomes
(R,/Ro - 1)[-]
=
[RH],eq 13
k,’[LH]
(14)
In the range of a virtually constant value of [RH],,, eq 13 is simplified to Rt/Ro - 1 = k,”[LH] (15) In n-heptane diluent systems the kinetic runs using the oxime together with HDEHP were made in the concentration range of the oxime such that the corresponding concentration of the adsorbed species, (RH),d, can be regarded as virtually constant. In the xylene diluent, on the other hand, the concentration of the oxime was varied from 4 to 59 mol/m3, as shown in Figure 5. The corresponding concentration of the adsorbed species ranged from the region where [RH],, is proportional to [RH] up to the threshold of the region of a virtually constant of [RH],,. The present results in xylene diluent and n-heptane diluent are plotted, respectively, according to eq 14 and 15, in Figure 8. A straight line with a slope of is seen for each diluent. The dimerization constant of HDEHP in the present diluents has been found to be as great as 10-100 m3/mol (Komasawa et al., 1981) and therefore the concentration of the monomeric form, is proportional to the square root of the total (or analytical) con-
[m],
centration, CLH. When plotted against the monomer concentration, the results will yield straight lines with a slope of unity. Thus, the kinetic behavior is likely to be expressed by the simple reaction scheme proposed. Acknowledgment The authors gratefully acknowledge the financial support of the Grant-in-Aid for Scientific Research of the Ministry of Education, Science and Culture, Japan (No.56,550,684-1981),and the experimental assistance of Mr. T. Sonoda. Nomenclature C = total (or analytical) concentration, mol/m3 D = distribution ratio of copper k = reaction rate constant, m/s or m4/mol s K,, = extraction constant K z = dimerization constant, m3/mol R= reaction rate, mol/m2 s [CUR,] = concentration of organic copper com lex, mol/m3 [H+] = concentration of hydrogen ion, mol/m! [RH] = concentration of monomeric oxime, mol/m3 [LH]= concentration of monomeric HDEHP, m0i/m3 b = stripping process c = catalyzed reaction 0 = uncatalyzed reaction, or observed rate with the oxime alone t = sum of catalyzed and uncatalyzed reaction, or observed rate with the oxime spiked with HDEHP RH = oxime LH = HDEHP Cu = copper Superscript
_ -- organic phase species or organic phase concentration Registry No. Cu, 7440-50-8; HDEHP,298-07-7; oxime, 37339-32-5.
Literature Cited Ashbrook, A. W. Coord. Chem. Rev. 1975a, 16, 285. Ashbrook, A. W. J. Chromatogr. 1975b, 105. 141. Atwood, R. L.; Thatcher, D. N.; Miller, J. D. Metall. Trans. 1975, 68, 465. Cox, M.; Hlrons, C. 0.; Flett, D. S. Int. Solvent fxtr. Conf., R o c . 1980; Paper 80-118. Danesl, P. R.; Chiarlzle, R. CRC Cm.Rev. Anal. Chem. Nov 1980. 1. Danesl, P. R.; Chlarlzla, R.; Vandegrlft, G. F. J. Phys. Chem. 1980. 84, 3455. Dobos, D. “Electrochemical Data”; Elsevler: Amsterdam, 1975; Table 129. Flett, D. S.; Okuhara, D. N.; Splnk, D. R. J. Inorg. Nucl. Chem. 1973, 35, 247 1. Flett, D. S. Ace. Chem. Res. 1977, 10, 99. Fleming, C. A,; Nlcol, M.; Hancock, R. D.; Finkelsteln, N. P. J. Appl. Chem. Biotech. 1978, 28, 443. Goren, M. 6 . ;Colhlnarl, E. L. U S . Patent 3927 169, 1975. Hanson, C. Int. Solventfxtr. Conf., R o c . 1980, Paper 80-1. Hazen, W. C.; Coltorinarl, E. L. U.S.Patent 3872209, 1975. Hummelstedt, L.; Paatero, E.; Nyberg, T.; Rosenback, R. Int. Solvent Extr. Conf., R o c . 1980, Paper 80-73. Inoue, K.; Nakashio, F. Kagaku Kogaku 1980, 4 4 , 301. Komasawa, 1.; Otake, T.; Yamada, A. J. Chem. f n g . Jpn. 1980a, 13, 130. Komasawa, I.; Otake, T.; Muraoka, T. J. Chem. fng. Jpn. 1980b, 13, 204. Komasawa, I.; Otake, T.; Yamada. A. J. Chem. fng. Jpn. 1980c, 13, 209. Komasawa, 1.; Otake. T.; Hlgakl, Y. J . Inorg. Nucl. Chem. 1981, 43, 3351. Komasawa, I.; Otake, T. submitted for publicatlon In J . Chem. Eng. Jpn. Mlller, R. D.: Atwood, R. L. J. Inwg. Nucl. Chem. 1975, 37, 2539. Moore, R. H.; Pattrldge, J. A. Abstract, Northwest Mining Associatlon Meeting, Spokane, Dec 1972. Morln, E. A.; Peterson, H. D. U S . Patent 3878286, 1975. Partridge, J. A,; Jensen, R. C. J. Inorg. Nucl. Chem. 1989, 31, 2587. Preston, J. S.; Whewell, R. J. J. Inwg. Nucl. Chem. 1977, 3 9 , 1675. Sonoda, T.; M.S. Dlssertetlon, Osaka Unlverslty. Osaka, 1981. van der Zeeuw. A. J.; Kok, R. CIM Spec. Vol. 1979, 21, 210. Whewell, R. J.; Hughes, M. A.; Hanson. C. J. Inorg. Nucl. Chem. 1975, 3 7 , 2303. Whewell, R. J.; Hughes, M. A.: Hanson, C. J. Inorg. Nucl. Chem. 1976, 38, 2071.
Receiued f o r reuiew October 15, 1981 Accepted October 26, 1982
