Nuclear Magnetic Resonance Studies of Complexes Involving β

NMRSTUDIES OF ,&DIKETONE COMPLEXES

2899

Nuclear Magnetic Resonance Studies of Complexes Involving PDiketones

and Some Neutral Organophosphorus Esters

by George Pukanic,l* Norman C.

Li,lb

Wallace S. Brey, Jr.,lc and George B. Savitskyld

Department of Chemistry, Duquesne University, Pittsburgh, Pennsylvania, and Department of Chemistry, University of Florida, Gainesville, Florida (Received March 7 , 1966)

Nuclear magnetic resonance studies of hydrogen bonding between the hydrogen donorsthenoyltrifluoroacetone, hexafluoroacetylacetone, and trifluoroacetylacetone-and the electron donors-tributyl phosphate, diethyl ethyl phosphonate, and tri-n-octylphosphine oxide-have been carried out. Of the three p-diketones studied, hexafluoroacetylacetone gives the most stable complex with a given phosphorus ester, as is expected. With a given P-dilietone, the equilibrium constant of hydrogen bond formation with the phosphorus esters increases in the order tributyl phosphate, diethyl ethyl phosphonate, and tri-noctylphosphine oxide and is in line with the relative basicity of the phosphorus ester. Evidence for the preferred enol form of thenoyltrifluoroacetone is presented and indicates that the preferred form is the one where the carbonyl group is adjacent to the perfluoromethyl group. Frequency assignments for the thiophene ring protons in thenoyltrifluoroacetone have also been made and are based on hydrogen-bonding studies. The modes of interaction of hexafluoroacetylacetone with methanol have been studied by nuclear magnetic resonance methods.

Introduction Three previous papers from the laboratory of Duquesne University2-* have described solvent-extraction studies of iernary metal complexes of p-diketones and neutral organophosphorus esters. This paper presents nuclear magnetic resonance studies of complexes involving the P-diketones-thenoyltrifluoroacetone (HTTA), trifluoroacetylacetone (HTFA), hexafluoroacetylacetone (HHFA)-and the neutral organophosphorus esters-tri-n-octylphosphine oxide (TOPO), (C8H17)3P0,diethyl ethyl phosphonate (DEEP), (CzH60)2(CzH6)P0,and tri-n-butyl phosphate (TBP), (C4H90)3P0. These were chosen because mixtures of the P-diketones and organophosphorus esters have been found to enhance the extraction of metal ions from aqueous to organic phase, the so-called “synergistic eff ectl”2-6and the synergistic extraction of metal ions depends upon the nature of the p-diketone and of the phosphorus ester. I t is well known that P-diketones exhibit keto-enol tautomerism. Burdett and Rogers’ have determined the percentages of enol tautomers for acetylacetone,

HTFA, HHFA, and HTTA in CS2 to be 81, 97, 100, and 100, respectively. Substitution of electron-withdrawing groupsl as the perfluoromethyl group and the aromatic ring in HTTA, for the methyl group in acetylacetone results in an increase in enolization. Rogers and Burdetts have shown that in “inert” solvents, the P-diketones are principally in the enol form, since the internally hydrogen-bonded molecule is less polar than the keto molecule. In CCl, as solvent, two possible enol structures (I and 11) can be postulated for HTTA. ( 1 ) (a) Research assistant on a U. S. Atomic Energy Commission (b) contract, No. AT(30-1)-1922, with Duquesne University; Duquesne University; (c) University of Florida; (d) Clemson University. (2) R. L. Scruggs, T . Kim, and N. C. Li, J . Phys. Chem., 67, 2194

(1963). (3) W. R. Walker and N. C. Li, J . Inorg. S u c l . Chem., 27, 411 (1965). (4) N. C. Li, S.M. Wang, and W. R. Walker, ibid., 27, 2263 (1965). (5) T . V. Healy, ibid., 19, 338 (1961). (6) L. Newman, ibid., 25, 304 (1963). (7) J. L. Burdett and M. T . Rogers, J . Am. Chem. SOC., 86, 2105 (1964). (8) M. T. Rogers and J. L. Burdett, Can. J . Chem., 43, 1516 (1965).

Volume YO, Number 9 September 1966

G. PUKANIC, N. C. LI, W. S. BREY,JR.,AND G. B. SAVITSKY

2900

H ,,,'

(2)HC--CH(3)

I/

I/

0

0

/I

I

C-CH=C-C

C---

(1)HC

resonance spectra of HHFA in the presence of water, TOPO, and methanol. The results with methanol are included in this paper.

\

Experimental Section F3

\/ s I H (2)HC--CH(3)

II

0

0

I

I1

I/ \/

(1)HC

C--C=CH-C-CFs

S

I1 Intermolecular hydrogen bonding between HTTA molecules is negligible because HTTA remains monomeric and the proton magnetic resonance frequencies remain constant over a wide range of HTTA concentrations in CC14 a t constant temperature and in the absence of a second solute. The participation of fluorine in intramolecular hydrogen bonding may be ruled out because such a bond would involve a strained five-membered ring. Because of the importance of HTTA as an extractant in solvent extraction, we have carried out nuclear magnetic resonance measurements for the purpose of determining whether I or I1 is the preferred enol form. As a by-product of such measurements, we were able to make frequency assignments of the thiophene ring protons in HTTA: CH(l), (a),and (3). Theresults are presented in this paper. In experiments using HHFA in CC14, great care has to be taken to exclude moisture from the air, since the presence of water changes HHFA from the enol form (111) to the keto hydrate form (IV).9 The keto hydrate, HHFA.ZH,O, itself is insoluble in organic H /'

0'

\

0

I

I/

C

C

/\/\

CFI

C H I11

CF3

OH

OH

OH

OH

I I CF3-C-CH2-C-CF3 I I

Calculation of Equilibrium Constants By keeping the concentration of HTTA in CCh constant and varying the concentration of TBP, keeping the latter in large excess over the complexed HTTA, Scruggs, Kim, and Liz calculated the equilibrium constant for hydrogen bond formation of a 1 :1 complex using the equation 1 v

-

vf

-

1 + KAC d A, 1 1

(1)

IV

solvents. When TOPO is present, however, it dissolves owing to hydrogen bonding. We considered it of interest therefore to obtain the nuclear magnetic The Journal of Physical Chemistry

Mateyials. TBP, a Fisher reagent, was washed with 1 M HC1, then with 1 M NaOH, and finally with distilled water until the organic layer was clear and the washings were neutral. The washed product was distilled under reduced pressure. DEEP, an Eastman Organic chemical, was distilled under reduced pressure. TOPO, also an Eastman Organic chemical, was used without further purification. The p-diketones were purchased from Peninsular Chem Research, Inc. HTTA was recrystallized from benzene, while HTFA and HHFA were redistilled just before use. Nuclear Magnetic Resonance Xeaswements. The preparation of samples and method of proton magnetic resonance measurements, using an A-60 nmr spectrometer, were identical with those described by Takahashi and Li."Jtll Chemical shifts of the signals were measured with respect to tetramethylsilane used as an internal standard and are quoted as cps. The signals are all downfield from tetramethylsilane. In experiments in which the vinyl proton in the 0-diketones was measured, a side band of the tetramethylsilane was placed before and after the vinyl proton resonance peak at a separation of 10-15 cps. The side-band frequencies were measured by means of audiooscillators and a frequency counter. The molarity of the solutions, at temperatures other than room temperature, was calculated from the change in density with temperature of carbon tetrachloride. Fluorine magnetic resonance spectra were obtained with a Varian DP-60 spectrometer system with the use of a 56.4->ICradiofrequency unit. The F19 chemical shifts are relative to anhydrous trifluoroacetic acid used as an external standard.

(9) B. G. Schultz and E. M. Larsen, J . Am. Chem. Soc., 73, 1806 (1949). (10) F. Takahashi and N. C. Li, J . Phys. Chem., 6 8 , 2136 (1964). (11) F. Takahashi and N. C. Li, ibid., 69, 2950 (1965).

NMRSTUDIES OF @-DIKETONE COMPLEXES

2901

~

~~

Table I: Proton Magnetic Resonance Frequency of Vinyl Proton in HTTA, HHFA, and HTFA (0.300 M in C C 4 )as a Function of Concentration of DEEP 100

210

379

CDEEP

CPS

CDEEP

CDEEP

CPS

0 0.615 1.193 1.716 2.364 2.976

382.3 391.5 398.2 402.6 407.6 411.9

0 0.627 1.217 1.749 2.411 3.034

(a) HTTA 382.9 0 393.0 0.635 400.5 1.233 405.4 1.772 410.6 2.443 415.6 3.074

0 0.594 0.885 1.192 1 ,494

381.6 372.4 370.5 309.5 368.9

0 0.605 0.903 1.215 1.523

382.2 372.0 370.2 369.2 368.7

0 0.599 0,908 1.200 1.531 1.790

352.1 356.6 359.3 361.1 3ti2.9 364.2

0 0.610 0.925 1 ,224 1.561 1.825

(c) 352.6 357.4 360.6 362.5 364.2 365.3

20

CPS

CDEEP

CPS

383.4 394.6 402.6 408.0 413.3 417.8

0 0.644 1.250 1.798 2.478 3.118

383.8 396.4 404.8 409.7 415.6 420.7

0 0,627 0.951 1.258 1.605 1.875

353.8 358.8 362.3 364.6 366.3 367.9

(b) HHFA

where Ac,the hydrogen bond shift, is the difference between the vinyl proton frequencies of the free and complexed HTTA as hydrogen donor, and d is the total concentration of TBP, which acts as electron donor. Equation 1 was first derived by Mathur, Becker, Bradley, and Li,'2 and applied to hydrogen-bonding equilibria with the following assumptions: (a) the equilibria involve very rapid reactions, so that the observed frequency, V , is the weighted average of the characteristic frequencies of the free and complexed hydrogen donor (respectively, v t and vc),and (b) the concentration of the electron donor is in large excess over the complexed hydrogen donor, so that the concentration of the free electron donor is equal to the total concentration, d. From eq 1, a plot of 1/(v - YY) us. l/d yields a straight line, from which the values of Ac and K can be determined separately. In the present investigation, the concentrations of the hydrogen donors HTTA, HTFA, and HHFA in CCI, were kept constant at 0.300 M , while the concentrations of the electron donors, DEEP and TBP, were varied and kept in large excess. The vinyl proton frequency in the p-diketone was measured, and eq 1 was used in obtaining Ac and K . The equilibrium constant of the reaction HTTA TOPO = HTTA.TOP0 was obtained from a plot of eq 1 using FI9 resonance frequencies, rather than the vinyl proton frequencies. This is because, with the

+

0 0.613 0.915 1.231 1.543

382.8 371.5 369.8 369.0 368.4

HTFA 0 0.619 0.938 1.240 1.582 1.849

353.2 357.9 361.4 363.5 3ti5.1 366.6

latter, the intercept, l/A,, was too small (of the order of 0.005) for an accurate value of K to be obtained. Mixtures of TOPO and HTFA and of TOPO and HHFA were prepared; however, the F19nmr spectra indicated that side reactions in addition to simple complex formation occurred in these systems.

Results and Discussion Thermodynamic Data on Hydrogen Bonding. Table I lists the proton magnetic resonance results for HTTA, HTFA, and HHFA bonding to DEEP, while Table I1 gives the FIgresults for HTTA bonding to TOPO. An example of plots of eq 1 at different temperatures is given in Figure 1 for the system HTTA-DEEP. Values of A, and K are tabulated in Table 111. Figure 2 contains plots of log K us. 1/T for the various systems investigated. The values of AH were calculated from the slopes and are included in Table 111. From Table I it is seen that in the absence of DEEP, the vinyl proton frequency in HTFA lies at higher field than HHFA or HTTA. This is reasonable since there is only one electron-withdrawing group in HTFA, whereas two electron-withdrawing groups are present in HHFA and HTTA. The vinyl proton in HTFA, (12) R.Mathur, E. D. Becker, R. B . Bradley, and N. C. Li, J . Phys. Chem., 67, 2190 (1963).

Volume 70,Number 9 September 1066

G. PUKANIC, N. C. LI, W. S. BREY, JR.,AND G. B. SAVITSKY

2902

Table I1 : Fluorine Magnetic Resonance Frequency of HTTA (0.350 M in CClr) as Function of Concentration of TOPO, 34'

TOPO, fk".

0 0.622 0.808 0.991 1.183 1.306

CPS, downfield from CFJCOOH

0.10

155.5 169.3 173.4 174.9 176.3 177.7

0.08

0.09

2 0.07 I

5 0.06 0.05

0.04

Table 111: Summary of Results of HTTA, HTFA, and HHFA Hydrogen Bonding

0.03

Temp, OC

- 4H.

4c,

K

kcal/mole

cps

37 21 10 -2

0.20 0.22 0.23 0.25

0.9

48

HTFA-DEEP

37 21 10 -2

0.27 0.30 0.31 0.32

0.7

37

HTTA-TBP

37 21 10 -2

0.17 0.19 0.21 0.23

1.3

91

HTTA-DEEP

37 21 10 -2

0.24 0.27 0.29 0.33

1.3

72

HTTA-TOP0

34

0.34

HHFA-TBP

37 21 10

1.39 1.81 2.38

3.4

HHFA-DEEP

37 21 10

2.16 2.73 3.79

3.7

System

HTFA-TBP

0.02 0.011

I

I

I

I

I

I

I

I

0.5

I

I

I

I

I

I

I

I

1.5

1.0

Ild.

Figure 1. HTTA-DEEP system. Plots of 1/(Y - V I ) us. l / [ D E E P ] a t the following temperatures: 0 , 37'; 0 , 21'; 0, 10'; 8 , -2".

1.0

L-

12

l

17

,

0.1

4

1 37

21 10 Temp., "C.

-2

Plot of log K us. 1/T for the following systems: 4,HTTA-TBP; 0, HTTA-DEEP; 0 , HHFA-TPP; (3, HHFA-DEEP. Figure 2.

therefore is less deshielded than in HHFA and HTTA. The value of vf, the vinyl proton frequency in the absence of DEEP for the three fl-diketones, is slightly temperature dependent, with an average dvr/dT = 0.04 cps/'deg over the range -2 to 37". This small variation indicates that the P-diketones are predominantly monomeric, thus justifying our initial implicit assumption that v f is the frequency of the monomer. The small temperature effect on v f has been taken into account by using for the determination of K a t each The Journal of Physical Chemistry

8, HTFA-TBP; 0, HTFA-DEEP;

temperature the value of v f found at that temperature. This treatment has been described by RSathur, et a1.,12 for benzenethiol. Table I11 shows that with TBP and DEEP as hydrogen acceptors the equilibrium constant and enthalpy change of hydrogen bond formation are greater for HHFA than for HTTA and HTFA. The greater

NMRSTUDIES OF P-DIKETONE COMPLEXES

2903

hydrogen-donor strength of HHFA is in line with its greater acic! strength (pK, of HHFA, HTTA, and HTFA = 4.35, 5.70, and 6.40, re~pectively'~) and with the smaller stability of its binary metal complex (log kl values for Cu(HFA) +, Cu(TTA) +, and Cu(TFA) + are 2.70, 6.55, and 6.59, respectivelyl3). When one considers the presence of two electron-withdrawing CF3 groups in HHFA, vs. the presence of one CF3 and one CHs group in HTFA, the greater hydrogen-donor strength and greater acid strength of HHFA become completely reasonable. Wang, Walker, and LiI4 report the equilibrium conTOPO stant at 26" of the reaction Zn2+ 2HTTA = Zn(TTA)2.TOP0 2H+ to be 0.1 and of the reaction Zn2+ 2HHFA TOPO = Zn(HFA)2-TOP0 2H+ to be 12. Since HHFA gives a more stable ternary complex, ZnA2.TOP0,than HTTA, one would expect that HHFA is more effective in enhancing extraction (synergism) than HTTA, and this is experimentally o b ~ e r v e d14s15 . ~ ~ Because of the two electron-withdrawing CFs groups in HHFA, the electron density around the central Zn2+ions in the ZnA, complex is decreased, thus enhancing the ability to form an adduct with TOPO. From the above discussion it is seen that the hydrogen-donor strength, acid strength, and solvent-extraction characteristics of HHFA are all interrelated. TBP has one more alkoxy1 oxygen then DEEP, so that the phosphoryI oxygen in TBP would have a lower electron-donating tendency than in DEEP. One may expect therefore that the TBP hydrogen-bonded complex would be less stable than the corresponding DEEP hydrogen-bonded complex. The equilibrium constants listed in Table 111 bear this out, although the values of AH for TBP and DEEP as hydrogen acceptors for a given P-diketone seem to be about the same. Studies on the Preferred Enol Form of H T T A and the Frequency Assignment of the Thiophene Ring Protons in H T T A . Table IV gives the F19 magnetic resonance frequency of enolic HTTA and HHFA in CC14, in the absence and presence of various concentrations of TBP. I t is seen that the presence of TBP does not affect the F19 frequency of HTTA, whereas the effect on HHFA is considerable. The data show that the preferred enol form of HTTA is 11, rather than I. This is because TBP is an electron donor and would bond with HTTA through the enolic -COH. If the preferred enol form of HTTA were I, then the presence of TBP should have affected the F19chemical shift, as it does with HHFA. Since no effect was observed with HTTA, the conclusion is that the preferred enol form is 11, where the fluorine atoms are farther away from the hydrogen-bonding site.

+

+ +

+

+

+

~

Table IV : FIB Magnetic Resonance Frequency (cps Downfield from CFBCOOH)of Enolic HTTA and HHFA in the Absence and Presence of T B P a t 32" -0,322

M HTTA in CClc-

CTBP

0 0.624 0.832 1.04

-0,350 .W HHFA in CCla-

CTSP

CPS

0

155.7 155.8 156.3 155,7

0.728 1.092 1.820 2.184

CPS

105.7 95.1 91.6 84.5 80.6

Hydrogen-bonding studies have been found to be useful in making frequency assignments of the thiophene ring protons in HTTA. I n the absence of an electron donor, the proton magnetic resonance spectrum of a 0.300 M solution of HTTA in CC14 is shown in Figure 3a, with the OH signal omitted. The singlet is ascribed to the vinyl proton. Chemical shifts of the thiophene ring protons are given in Table V. The multiplet a t 7.16 ppm is ascribed to CH(2) (for the proton number, see I) and arises from coupling to CH(1) and CH(3). The protons CH(1) and CH(3) are contained in the lowest field multiplet, and the question is which of the first-order doublets in this lowest field multiplet represents which proton.

Table V: Chemical Shifts (ppm) of Thiophene Protons in 0.300 M HTTA in CCI4 as a Function of Concentrations of TBP, DEEP, and TOPO CTBP

0 0.712 1,072 2.477

CH(2)

CH(1)

CH(3)

7.16 7.19 7.20 7.22

7.70 7.84 7.89 8.06

7.80 7.96 8.03 8.24

7.18 7.19 7.20 7.21 7.24

7.83 7.90 7.96 7.98 8.10

7.96 8.07 8.12 8.15 8.29

7.17 7.18 7.18

7.92 7.99 8.03

8.12 8.23 8.32

CDEEP

0.615 1.193 1,513 1.716 2.976 CTOPO

0.691 0.985 1,190

(13) L. G. Van Uitert, Thesis, Penn State College, Jan 1951; cited by A. E. Martell and M. Calvin, "Chemistry of the Metal Chelate

Compounds," Prentice-Hall Inc., New York, N. Y., 1952. (14) S. M. Wang, W. R. Walker, and N. C. Li, J . I n o ~ g .dVucZ. Chem., 28, 875 (1966). (15) W. R. Walker and N. C. Li, (bid., 27, 2255 (1965).

Volume 70, Number 9 September 1966

G. PUKANIC, N. C. LI, W. S. BREY,JR.,AND G. B. SAVITSKY

2904

I

b.

e.

Figure 3. Proton magnetic resonance spectra of thiophene ring protons of 0.300 M HTTA in CC4: ( a ) in the absence of electron donor; ( b ) in the presence of 0.691 M TOPO; (c) 1.190 M TOPO.

Upon the addition of an electron donor to HTTA in CCl,, the lowest field multiplet in Figure 3a is now resolved into two first-order doublets, and the lower field doublet is shifted further downfield than the higher field doublet on increasing the concentration of electron donor (see Figure 3b, Figure 3c, and Table V). Since the electron donor is bonded to the enolic OH, H(3) is closer to the coordination site than H(1), and one would expect, therefore, that the signal of H(3) would be shifted further downfield. Our reasoning is in line with that of Li, Scruggs, and BeckeP and RIathur and Li,” who account for the order of chemical shifts of metal complexes of amino acids on the basis of the proximity of protons to the binding sites. We therefore conclude that the lowest field multiplet in Figure 3a is due to a superposition of the H(l) and H(3) signals, and that H(3) has the lower frequency of the two. I n the absence of an electron donor, there is intramolecular hydrogen bonding involving OH. Since H(3) is closer to the coordination site also, its signal lies further downfield than H(1). Our frequency assignment is in agreement with the order given by Varian Associates‘s in which H(l) and H(3) are listed as 7.73 and 7.82 ppm, respectively, in CDCI,. I n ref 18, however, The Journal of Phvsical Chemistrv

no reason is given for this assignment. We have shown here a quick method of making frequency assignments by observing the downfield shifts caused by the presence of an electron donor in hydrogen bond formation. From Table V it is seen that TOPO causes a greater downfield shift of H(l) and H(3) than TBP or DEEP. This is as expected, since TOPO is the strongest electron donor of the three phosphorus esters. With a given pdiketone, therefore, TOPO would form the strongest hydrogen bond. Effects of Adding Methanol to Solutions of H H F A in CCC. On adding methanol to a 0.704 M HHFA solution in CC14, it was found that in addition to the vinyl proton (enol) signal a t 382 cps, there is an additional proton signal a t 135 cps, ascribed to the methylene protons in the keto form. After an interval of about 24 hr, equilibrium ratios ( K = (keto)/(enol)) were obtained by integration of the two resonance peaks. At least four integrations were performed. I n the Fig spectrum of this system there appear also two peaks; that associated with the enol form of the HHFA is 5.1 ppm downfield from that associated with the keto form. Table VI contains data from both types of spectra. As is expected, an increase in methanol concentration results in an increase in K . By analogy with keto hydrate of hexafluoroacetylacetone, 9* l 4 the intermolecular hydrogen-bonded species may be a keto methanolate. The value of K as determined from an F nmr spectrum was found to change with time over a period of an ~

~~

Table VI: Keto/Enol Ratio ( K ) for 0.70 M HHFA in CCla Containing Various Concentrations of Methanol CHaOH concn, M

0.49 0.53 0.68 0.71 0.88 0.96 1.11 1.25

K from proton resonance (37O)

K from F resonance (349

0.40

...

...

0.30 0.80

... 0.80 1.65

... ...

...

... ...

2.2 2.5 3.1

(16) N. C. Li, R. L. Scruggs, and E. D. Becker, J. Am. Chem. Soe., 84,4650 (1962). (17) R. Mathur and N. C . Li, ibid., 86, 1289 (1964). (18) “High Resolution NMR Spectra Catalog,” Varian Associates, Palo Alto, Calif., 1962,Spectrum No. 185.

HYDROGEN-DEUTERIUM EQUILIBRATION OVER PALLADIUM HYDRIDE

hour or two after the solution had been prepared. The peak corresponding to the enol form decreased in intensity while that corresponding to the keto form increased in intensity. Preliminary kinetic study of the enol-keto conversion in carbon tetrachloride showed it to be approximately first order in the concentration of

2905

enol, with a half-time at 34” of about 10 min for a solution containing 1.54 M concentration of methanol. Acknowledgments. This work was done under Contract No. AT-(30-1)-1922 with the U. s.Atomic Energy Commission, and under an NSF Summer Faculty Participation Program.

Hydrogen-Deuterium Equilibration over Palladium Hydride

by R. J. Rennard, Jr., and R. J. Kokes Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland (Received March 7 , 1966)

21218

Equilibration of a 50 :50 mixture of hydrogen and deuterium over palladium hydride has been studied as a function of composition at -195 and -183”. Sorption of hydrogen and deuterium occurs at these temperatures, but exchange of gas phase deuterium with palladium hydride (or vice versa) does not. The rate of equilibration decreases with increasing hydrogen content of the catalyst. It is concluded that once hydrogen adsorption occurs, exchange or incorporation follows and that the relative rates of these two processes are independent of hydride concentration in the palladium.

Introduction Palladium hydride’ is a general term applied to a nonstoichiometric series of compounds ranging in gross composition from roughly H/Pd = 0.0 to H/Pd = 0.8.’ Above 310°, palladium hydride forms a single phase; below 310”. it forms two phases. At room temperature, the pure a phase exists in the approximate range, 0 < H/Pd < 0.06; in the range 0.06 < H/Pd < 0.55, a mixture of a and P phases is found; only the pure 6 phase is found for H/Pd > 0.55. (These boundaries depend somewhat on preparative conditions.) Despite the complexity of this system, the magnetic susceptibility2 is a linear function of concentration that approaches zero at H/Pd = 0.66; similarly, the electrical resistivity3 is a linear function of concentration up to H/Pd = 0.80. These linear relations hold through the two-phase region. Thus, hydrogen behaves approximately like an alloying metal that fills up the holes in the d band at the composition PdH,,,,. Several have observed that hydride

formation affects the activity of the catalysts both for the ortho-para conversion and ethylene hydrogenation. Of these reactions, the former is by far the simpler, but it can proceed primarily by the magnetic conversion and, hence, shed relatively little light on the chemistry of the hydrogen activation process.* Accordingly, we have selected the hydrogen-deuterium equilibration for study as a function of the hydrogen content of palladium hydride. (1) D. P. Smith, “Hydrogen in Metals,” University of Chicago Press, Chicago, Ill., 1948. (2) C. Kittel, “Solid State Physics,” John Wiley and Sons, Inc., New York, N. Y., 1956, p 334. (3) R. E. Norberg, Phys. Rev., 86, 745 (1952). (4) A. Farkas, Trans. Faraday SOC.,32, 1667 (1936). (5) A. Couper and D. D. Eley, Discussions Faraday SOC.,8, 172 (1950). ( 6 ) R.I. Kowaka, J . J a p a n Znst. MetaEs, 23, 625 (1959). (7) R. J. Rennard, Jr., and R. J. Kokes, J. Phys. Chem., 70, 2543 (1966). ( 8 ) G. C. Bond, “Catalysis by Metals,” Academic Press Inc., New I‘ork, N. Y., 1962.

Volume 70, Number 9 September 1966