Synergism in the solvent extraction of alkali metal ions by thenoyl trifluoracetone

J. inorg,nucl.(7hem.,1968.Vol.30, pp. 1025to 1(136. PergamonPress. Prinledin Great Britain

S Y N E R G I S M IN T H E S O L V E N T E X T R A C T I O N OF ALKALI METAL IONS BY T H E N O Y L TRIFLUORACETONE T. V. H E A L Y Atomic Energy Research Establishment, Harwell, Didcot, Berks

(Received 23 August 1967) Abstract-Large synergistic effects for polyvalent metals have been previously reported for the system H.zO/Mn+/HTTA/S/diluentwhere HTTA is thenoyltrifluoracetone and S is a neutral donor solvent. Similar effects for the monovalent alkali metals are reported here. The formula for the extracted species has been found to be M(TTA)S2 (M = Li, Na, K or Cs), where S may be a phosphine oxide, a phosphate, an alcohol, a ketone or an amide. Nitromethane (NM) on the other hand forms Li(TTA)(NM)2 and M(TTA) (NM) 3 (M = Na, K or Cs). Equilibrium constants for these synergistic reactions are reported and are shown to vary with the alkali metal over a wide range.

THE WORD "synergism" was first used in solvent extraction by Blake and coworkers [ l ] to describe an enhancement of divalent metal extraction from aqueous solution by a mixture of an acidic dialkyl phosphate (HA) and a neutral donor extractant (S), the resulting mixture giving greater extraction than either extractant alone. Other workers [2] had previously reported a similar enhancement of extraction of trivalent rare earths using a fl-diketone, thenoyl trifluoracetone (HTTA), as the acidic extractant. These synergistic effects, along with later work by the present author[3] and by Irving and co-workers[4] using HTTA with a series of di-, tri- and tetravalent metals, have recently been reviewed [5]. As regards monovalent metals, synergism has been demonstrated with caesium extracted by an acidic dialkyl phosphate and a complex phenol (B)[7,8]. Although the complex MAB4 (M = metal) is of a different type[7] from the M(TTA)xS~ complexes, there seemed to be no reason why synergism should not occur for a monovalent metal with a fl-diketone and a neutral extractant. The present paper shows that alkali metals can be synergistically extracted in a similar manner to the polyvalent metals, with the aid of HTTA. A series of complexes of formulae M(TTA)~Su has been isolated[6] as solids 1. C. A. Blake, C. F. Baes, K. B. Brown, C. F. Coleman andJ. C. White, Proceedings of the Second International Conference on Peaceful Uses of A tomic Energy, No. 15/P/1550, United Nations, Geneva (19581. 2. J. G. Cuninghame, P. Scargill and H. H. Willis, U.K. Atomic Energy Document, No. C/M 215 A.E.R.E. (19541. 3. T. V. Healy,J. inorg, nucl. Chem. 19,314(1961). 4. H. Irving and D. N. Edgington, J. inorg, nucl. Chem. 15, 158 (19601: ibid20, 314 321 (1961); ibid21, 169(1961). 5. T. V. Healy, Nucl. Sci. Engng 16, 413 (1963). 6. T. V. Healy andJ. R. Ferraro, J. inorg, nucl. Chem. 24, 1449 (1962). 7. W. E. Keder, E. C. Martin and L. A. Bray, Solvent Extraction Chemistry of Metals (Edited by H. A. C. McKay, T. V. Healy, I. L. Jenkins and A. Naylor), p. 343. MacMillan, London (19651. 8. T. V. Healy, Solvent Extraction Chemistry (Edited by D. Dyrssen, J-O-Liljenzin and J. Rydberg), p. 119. North-Holland, Amsterdam (1966). 1025

1026

T.V. HEALY

or oils, which are similar in composition to the extracted species in dilute solution[3,4]. It has, however, not been ascertained how S was attached in the complex. It was hoped that simpler complexes with the monovalent alkali metals, M(TTA)Su, might be obtained as solids for structural examination. Another reason for studying the alkali metal complexes was to separate the alkali metals and possibly also the lithium isotopes. Lee[9] considers the fractionation of lithium isotopes in an ion exchange system to be, in effect, an extreme example of the chromatographic fractionation of the alkali metals, and the author considers that the same concept may apply in solvent extraction. EXPERIMENTAL

Sources of materials Tri-n-octyl phosphine oxide (TOPO), triphenyl phosphine oxide (TPPO) and tributyl phosphate (TBP) were all obtained from Albright and Wilson Ltd., Oldbury, England• T O P O and TPPO were sufficiently pure to be used without further purification and TBP was purified according to a method described previously[l 0]. H T T A (Dow Chemical Co., Midland, Michigan, U.S.A.) was recrystatlized from benzene and had a molecular weight of 222.2. Ethyl hexyl alcohol (EHA) (British Industrial Solvents), NN-dibutyl acetamide (DBA) and nitromethane (NM) (Eastman Organic Chemicals, Rochester, N.Y.) were all used without further purification. All other reagents used were of Analar grade. Where tracer Z4Na and 'arCs were used, they were obtained from R.C.C., Amersham.

Determination of distribution ratios The distribution ratio (D), defined as the metal concentration in the organic phase divided by its concentration in the aqueous phase at equilibrium, was usually obtained after 5-10 min mixing of the phases. The relatively fast attainment of equilibrium in the alkali metal ion/HTTA systems is in marked contrast with that obtaining in the polyvalent ion/HTTA systems, where up to 4 hr was needed. This quick attainment of equilibrium is partially due to the higher pH involved. In many instances of alkali ion extraction, re-equilibration with fresh aqueous or organic phases was carried out and D re-determined to establish that equilibrium had been attained and that no appreciable concentration changes of the reagents had taken place. Where radioactive tracers were not used, lithium, sodium and potassium aqueous concentrations were determined with an Eel Flame Photometer. Organic concentrations were similarly obtained in aqueous solution after back-extraction with dilute acid. All partitions were carried out at 20-+ 2°C. Much of this work involved organic solutions containing low molarities of individual reagents• In order to obtain a reasonably exact treatment of the dependence of D on solvent and other concentrations, a constant aqueous activity was maintained• RESULTS

AND

DISCUSSION

Synergistic extraction of lithium Under conditions used in the synergistic extraction of polyvalent metals with H T T A [3,5, 12] it was found that lithium extraction was negligible. At a higher pH, viz pH 5-8, the extraction of lithium can be enhanced by a factor of up to 10 5 with a mixture of H T T A and donor solvents, compared with its extraction by either extractant alone. To obtain information on the species extracted into the organic phase, the three variables [H+], [HTTA] and [S] in the system H20/Li+/HTTA/S/benzene 9. D.A. Lee andJ. S. Drury, J. inorg, nucl. Chem• 27, 1405 (1965). 10. K. Alcock, S. S. Grimley, T. V. Healy, J. Kennedy and H. A. C. McKay, Trans. Faraday Soc. 52, 39 (1956). 11. J. Kleinberg, W. J. Aergersinger and E. Griswold, Inorganic Chemistry, p. 310. Heath, Boston (1960). 12. T . V . Healy,J. inorg, nucl. Chem. 19, 328 (196 I).

Synergism in the solvent extraction of alkali metal ions

1027

were examined, keeping two of the three variables constant at a time. At constant pH and [HTTA], the synergistic effects of various donor solvents are as illustrated in Fig. 1, which shows on a log-log plot the variation of the lithium partition coefficient (DL0 with concentration of S. The linear parts of the curves all have a slope of two, indicating a dependence on [S]" in all cases. TOPO produces the I0 -I - -

i

~

F

T

~

]

/ IO-2

~4 ~

,,.0

~'

~.

DLi

iO-5

IO.6 IO- 4

iO-3

iO-2 SOLVENT

1(3-1 MOLARITY

iO O

iO I

Fig. 1. Second power dependence on [donor solvent] in 0. I M HTTA/benzene system. (Aqueous phase containsacetatebufferand 0.001 MLiNO:~(pH 5-2)). greatest synergistic effect, in line with its very powerful donor properties. TBP and DBA, which are moderately powerful donor solvents, produce an appreciable effect. E H A and N M have the relatively small effects expected for weak donor solvents, but nevertheless produce synergistic enhancements of DL~ of the order of 103-10 s at high concentrations. Methyl isobutyl ketone, not shown in Fig. 1, is a slightly less powerful donor solvent than EHA[3] and has a correspondingly smaller synergistic effect. The effect of TPPO is less than might have been expected from the results with polyvalent metals[3], with which it is only slightly less powerful than TOPO. This anomaly may be due to steric hindrance caused by the large TPPO molecule relative to the small unhydrated lithium atom in the extracted species. Figures 2, 3 and 4 show the effect of [H +] and of [HTTA] on D~.~in typical systems, indicating a dependence on [H+]/[HTTA]. From these results we can assume that the reaction goes according to equation ( 1), where M represents lithium and org refers to the organic phase: M + + HTTA(org) + 2S(org) = M(TTA)S2(org) + H +

( 1t

1028

T. V. H E A L Y

I01

DLi

IC) -I

~o-2

I0 "3

I

i

i lilill

io

i

i

i lllllJ

,o-B

I

i llillll

,

io-7

, i ,,,,

io-6

Io-s

[H 4-] Fig. 2. First power dependence on [H +] in 0.1 M HTTA/0.1 M TBP/benzene system. (Aqueous phase varies from acetate buffer to bicarbonate).

,o-i

I

L

/

X

X DLI

iO "2

I

10 -3

id4

I

I

I

I i Ill

I

I

I

I

I I llJ

id 3

Id 2 MOLARITY

OF

I

l

i

I

I

i

l I

@

HTTA

Fig. 3. First power dependence on [HTTA] in 0.1 M TOPOlbenzene system. (Aqueous phase contains acetate buffer and 0.001 M LiNO3 (pH 5.2)).

Synergism in the solvent extraction of alkali metal ions

I01

1029

i

i0°

I0"I DM 10-2 Na 10-3 K 10-4 io-3

io-2

10-j

. . . . . . .

to°

MOLARITy OF H T T A

Fig. 4. First power dependence on [HTTA] in 0.2 M TBP/benzene system. (Aqueous phase contains 0.1 M NaHCO3 plus 0.0002 M LiNOs or the appropriate bicarbonate alone (pH 7.3)).

with an equilibrium constant K* = [M(TTA)Sz] [H+]/[M +] [HTTA] [S] 2 = DM[H+]/[HTTA] [S] 2 In the absence orS we have simply M + + HTTA(org) = M(TTA)(org) + H +

(2)

for which K~* = [M(TTA)] [H+]/[M +] [HTTA] = DM[H+]/[HTTA]. It follows that/3 °, the formation constant for the synergistic reaction in the organic phase alone: M(TTA)(org) + 2S(org) = M(TTA)S,,(org) (3) is given by /3o = K*/K~. The nomenclature used for the equilibrium constants is based on that suggested by Irving[ 13]. The superscript "0" in/3 o refers to an equilibrium in an organic phase, and the asterisk in K* and K* indicates that they are constants involving concentrations in two phases. The equilibrium constants for the synergistic reactions involving Li, H T T A and various solvents (S) are given in Table 1.

Comparison of the behaviour of Li, Na, K and Cs Extraction by H T T A alone gives higher partition coefficients for lithium and caesium than for sodium and potassium. The expected order is Li > Na > K > 13. H. Irving, Solvent Extraction Chemistry (Edited by D. Dyrssen, J-O-Liljenzin and J. Rydberg), p. 91. North-Holland, Amsterdam (1966).

1030

T.V. HEALY Table 1. Equilibrium constants for synergistic reactions givingM(TTA)Sx in benzene

log K*

Li(TTA) --10.16

Na(TYA) --11.16

K(TTA) --11.16

Cs(TTA) -10.20

Li(TTA)S2

Na(TTA)Sz

K(TTA)S~

Cs(TTA)S2

Solvent S log K*

log 130

TOPO TBP DBA TPPO EHA

-2"20 -4"20 --4"65 -4"75 -6-80

7.96 5.96 5-51 5"41 3.36

NM

--9.50

0.66

log K*

log fl0

log K*

-5.0 6.16 -6-90 4"26 -6"75 4"41 . . . --8.90 2"26 Na(TFA)(NM)3 --11.44 -0.28

log fl0

log K*

log/30

-6.36 4"80 -6.90 3"30 -8"16 3.00 -8"42 1"78 . . . . . . . --9"60 1"56 --9" 15 1'05 K(TTA)(NM)3 Cs(TTA)(NM)3 --11.22 -0.06 --10.90 --0.70

Cs, so that caesium is exceptional. These results are shown in Fig. 5 which also shows the synergistic effect of TBP, when the order of extraction reverts to the expected norm. While the separation factor for these alkali metals is never greater than 10 with H T T A in th absence of TBP, this factor increases to > 500 for Li/Na and > 104 for Li/K and Li/Cs in the presence of 0.1 M TBP in benzene. ~

IOI

I

I

°°

,o-,

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./

L/

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_

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,

, ,,,,,,,

iO-3 MOLARITY

,

iO'2 OF

, ,,,,.I

,

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, ,,,, iO o

TBP

Fig. 5. Second power dependence on [TBP] in 0.2 M HTTA/benzene system. (Aqueous phase contains 0.1 M NaHCOa plus 0.0002 M LiNO3 or the appropriate bicarbonate alone (pH 7.3)).

Synergism in the solvent extraction of alkali metal ions

1031

The slopes in Figs. 4 and 5 indicate a species M(TTA)(TBP).~ in all cases. Apart from a much greater synergistic effect, T O P O gives a similar picture to TBP (See Fig. 6), the species formed being M(TTA)(TOPO)2, while the limited information on D B A (Fig. 7) points to its being very similar to TBP in all respects. E H A in concentrations up to 0.1 M has practically no synergistic effect on Na, K and Cs extraction by H T T A , though it has an effect at higher concentrations (Fig. 8). Once again a dependence on [S] ~' is shown. Methyl isobutylketone (not shown) is a slightly weaker donor solvent than EHA. An even weaker donor solvent, NM, nevertheless still produces an appreciable synergistic enhancement provided that the N M molarity is higher (> 1 M) (Fig. 9). In this instance, while the extracted lithium species appears to be Li(TTA)(NM):., the other alkali metals give a line of slope 3 for N M dependency, indicating the species M(TTA) (NM)3, where M is Na, K or Cs. This is probably due to the weaker bonding.in these complexes, in which water may be incorporated giving hexa-co-ordination. There are many instances[11] of solid alkali metal complexes with fl-diketones where the coordination number of the complex is 6 for the metals Na, K and Cs, e.g. NaB-4H20 and KS.2HS, where HB and HS are benzoyl acetone and salicyl aldehyde respectively. As would be expected, the coordination number of lithium never exceeds 4 in any complex. ioI

I

I

i

I

,oo

i0 1

DM

10.2

10"3

i°4 I

io-5 I io-5

10.4

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10-2

io- I

i00

MOLARITY OF TOPO

Fig. 6. Second power dependence on [TOPO] in 0-2 M HTTA/benzene system. (Aqueous phase contains 0.1 M NaHCOa plus 0.0002 M LiNO:~ or the appropriate bicarbonate alone (pH 7.3)).

1032

T.V. HEALY iO I

/

L,/

/

,/

mo°

io-I

X iO "2

DM

io-3

j j x io-4

, , J,,.llO. 3

, , , ,, ,,110.2

, , t ..... I0"11

MOLARITY OF DIBUTYL

....

~1~10-500

ACETAMIDE

Fig. 7. Secondpowerdependenceon [DBA] in 0.1 M HTYA/benzenesystem.(Aqueous phase contains0.1 M NaHCOzplus0.0002 MLiNOaor 0.1 M NaHCOaalone(ph 7.3)).

Effect of diluents in synergistic extraction of alkali metal ions There is a factor of about 30 in DNa in the systems H20/Na÷/HTTA/S/diluent, where S is TOPO or TBP, on changing the diluent from benzene to cyclohexane (Fig. 10). Similarly, when lithium is substituted for sodium in these systems, the factor is about 10. These factors are relatively small and are similar to those obtained with calcium and uranyl extraction into H T T A / T B P systems [ 12]. They are, of course, much smaller than those obtained with trivalent rare earths and thorium, which are > 103. The reason for these effects in different diluents is not yet established, but the author has suggested[14] that changing from one inert diluent to another affects the keto-enol equilibrium of the/3-diketone. Change of diluent could change the activity of the enol form. The enol concentration is bound up with the water content of the organic phase, decreased water content favouring its formation. The synergistic effect decreases in the diluent order C6H12 > C6H8 > CC14 > C6H6 > CHCI3, which is also the order in which the solubility of water in these diluents increases. Dyrssen[15] has suggested that association occurs between TBP and the diluents, thus affecting the TBP activity coefficient and its partition coefficient. 14. T.V. Healy, D. F. Peppard and G. W. Mason, J. inorg, nucl. Chem. 24, 1429 (1962). 15. D. Dyrssen, Proceedings of the Symposium on Co-ordination Chemistry, p. 707. Tihany, Hungary (1964).

1033

Synergism in the solvent extraction of alkali metal ions ........

I

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I

-I I 0 0

~

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....

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,,,,l tO-I

~ i LLu~d . i00

* -h ,bIL, I 0 "5 tO l

M O L A R I T Y OF E T N Y L H E X Y L A L C O H O L

Fig. 8. Second power dependence on [EHA] in 0.2 M HTTA/benzene system. (Aqueous phase contains 0.1 M NaHCO~ plus 0.0002 M LiNO~ or the appropriate bicarbonate alone (pH 7-3)).

I

I

00-2 Li

Cs

~-a

DM

1 0. 4

,

io-2

,

~

,,~,ll

L

~ MOLARITY

I

R

JLI,LI

,

,oo

,

,

,,L,,

id s

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OF NITROMETHANE

Fig. 9. Second and third power dependence on [NM] in 0.2 M HTTA/benzene system. (Aqueous phase contains 0.1 M NaHCO:~ plus 0-0002 M LiNO:~ or the appropriate bicarbonate alone (pH 7-3)).

1034

T.V. HEALY joo

I

I

I

iO-I

iO"2 DM

fO'a

iO-4

/

iO-5

iO'4

i

t

i IIRHI

~-a

I

L i j,l,,I

,

io"2 MOLARITY

i 0 "1

i0 0

OF S

Fig. I 0. Effect of "inert" diluents cyclohexane and benzene on alkali metal ion extraction in 0' I ,Xl HTTA/S/diluent system. (Aqueous phase contains acetate buffer and 0.001 M LiNO3 (pH 5.2)).

This possibility of accounting for the diluent effect is further discussed by Irving [ 13]. However, these diluent effects increase with the valency of the metal, being small for alkali metals and large for rare earths and thorium; that is, the effects increase with increasing TTA content of the species, whatever the TBP content, as in Li(TTA)(TBP)~, UOz(TTA)2TBP, Pm(TTA)3(TBP)2 and TH(TTA)4TBP. Further work is needed on these diluent effects, which may indeed be caused by a combination of changes in the activities both of TB P and of the enol form of TTA.

Bonding and structure of the synergistic species Table 1 and the lower sections of Tables 2(a) and 2(b) give the formulae of the extracted alkali metal species and their mixed equilibrium constants, and although the compositions of these species may be established fairly readily, their structures, like those of so many of the mixed species formed by polyvalent metals, are by no means established with certainty. It has been suggested by Newman[16] that the actual synergistic effect as exemplified in equation (3) can best be evaluated by the equilibrium constant for the organic phase reaction. He has drawn up a table based primarily on Healy's results [3, 12] to show that the two-phase equilibrium constants for a given synergistic complex such as M(TTA)x(TBP)~, in a given diluent, are remarkably 16. L. Newman, J. inorg, nucl. Chem. 25,304 (1963).

Synergism in the solvent extraction of alkali metal ions

1035

Table 2(a). Equilibrium constants for synergistic reactions M(TTA) rSu in benzene (.4 is TTA and S is TBP or T O P O ) S = TBP

Species

ThA4S~ UO2A,~S~ ZnA2S1 TmA:~S2

CmA.sS.,_ AmA:~S,, PmA3S~ CaA2S2 LiAS2

NaAS2 KAS~

CsAS.,_

log K~*

log K*

1-00 -2.26

5.70 2.48

--8"64 --6-96 --7.10 --7.46 --7.77 --12.0 --10.16 --11.16 --11.16 --10.20

S = TOPO

log flo

log K*

log/3 0

4.70 4.70

7.70 3.10

6.70 5.36

-4"26 ---0.34 -0.70 -0-96 --1.04

4 ' 3 8 CCI4 6.60 6.40 6.50 6.70

--2.83 2-51 2.43

--9.93 9.97 10.20

--5.27 --4.20 ---6.90 --8.16 --8.42

6.70 5.96 4.26 3-00 1-78

--3.27 --2.20 --5.00 --6.36 -6-90

8.73 7.96 6.16 4.80 3.30

Table 2(b). Equilibrium constants for synergistic reactions giving M ( T T A ) x S u in cyclohexane (,4 is TTA and S is TBP or T O P O ) S = TBP

Species

THA4SI TmA3SI

UO2A2S~ PuO2A2S1

CoA2S~ CuA2S1 TmA:~S2 A m A aS~ CmA:~S2

PmA3S,2 PuA3S~ E u A 3S,,,

CaA2Sz COA2S2 LiAS,~ UO2A2S:~

S = TOPO

log K*

log K*

Iogfl °

log K*

log/3 °

1.67 -5.60 --2.82

7.95 --0.42 3.20

6'28 5.18 6.02

10"58 1.24 3.7

8.91 6.84 6.5

--1.54 --8.37 -2.84 --5.60 --6-80 --6.60 --7.00

3.13 --3.78 0.48 2.62 2.40 2.50 2.30

4.67 4.59 3.32 8.22 9.20 9.10 9.30

--1.14 5.72 5.14 5-44 5.00

--3.98 11.32 11.94 12.04 12-00

--7.22

5.33

12.55

--

--7.66 --11-80

1.78 --3.27

9.44 8-53

---1.99

-9.81

--

--8"37 --10-0 --2'82

--1"10 --3'2 --

7"26 6"80 --

---5'43 12"9

-4"57 15-72

constant over a range of di-, tri- and tetravalent metals. From this he concludes that, as this constant is a function only of the number of ligands per molecule and scarcely varies from metal to metal, the neutral donor solvent most likely forms a bond with one of the T T A molecules, and not directly with the metal itself. The results on alkali metal extraction, added to previous results compiled by Irving [13], are given in Table 2(a) and 2(b). This table shows clearly that/3 °, the formation constant for the synergistic reaction (Equation (3)) which takes place in the organic phase, is not a constant for different metal species, and hence gives no

1036

T . V . HEALY

support to the suggestion that the adduct S is attached directly to the T T A ligand. In a very recent paper on the synergistic extraction of U(VI) by a combination of TBP and various /3-diketones, Newman now concludes[17] that TBP most probably bonds directly to the uranium, replacing a water molecule in the coordination sphere. If this is so, it appears that in such species as UO2(TTA)2S3, Nd(TTA)3S2 and Th(TTA)4S, either the coordination number of the metal increases beyond its usual maximum[3,4], allowing S to bond directly to the metal, or alternatively a chelate ring may be opened, so that one of the TTA molecules becomes monodentate [ 18]. Irving has suggested [4] that the synergistic effect involves displacement of water by S, and there is no doubt that water plays a major role. Healy has shown [ 18] that in the cases of U, Th and Pm, the synergistic extraction involves a large decrease in the water content of the organic phase, supporting Irving's suggestion. In the present work, however, water content measurements by the Karl Fischer method indicate (Table 3) hydrated species on introducing the alkali metal into the organic phase containing H T T A and S, of compositions Li(TTA)Sz(H20) and Na(TTA)S2(H.,O)3. In Li(TTA)S2(H20), the T T A ligand must be monodentate, if all the other ligands are attached to the metal. Table 3. Water content of hexane phase in H20/M+/HTTA/TOPO/hexane system Hexane Phase

Water Molarity

0.10 M.HTTA 0.20 M.TOPO 0.10 M.HTTA~ 0.20 M.TOPO J 0.09 M. Li ] 0.10 M.HTTA~ 0'20 M.TOPOJ 0"07 M.Na ] 0.10 M.HTTA~ 0.20 M.TOPOJ

0.007M 0.18M 0.19M

/

0.12M

0.25 M

The change from a formula Li(TTA)(NM)2 (in which TTA is presumably bidentate), to M(TTA)(NM)3 for the other alkali metals is presumably due to an expansion of the covalency maximum. It has not proved possible to isolate any solids in the monovalent M(TTA)S2 series, only oils being obtained. Infra-red and N M R spectra on these oils may help in the future to elucidate the structure of the relatively simple alkali metal synergistic species. Alternatively, solid alkali metal species formed from other r-diketones, e.g. dibenzoylmethane [ 19], may prove useful in this respect. 17. K. Batzar, D. E. Goldberg and L. Newman, J. inorg, nucl. Chem. 29, 1511 (1967). 18. J. R. Ferraro and T. V. Healy,J. inorg, nucl. Chem. 24, 1463 (1962). 19. T.V. Healy, Unpublished work.