Complexes of 2-pyridone with MnCl2, CoCl2, NiCl2 and CuCl2

J. inorg, nucl.Chem., 1968, Vol. 30, pp. 1493to 1502. PergamonPress. Printedin Great Britain

COMPLEXES OF 2-PYRIDONE W I T H MnC12, COC12, NiCI2 A N D CuCI2 C. C. H O U K * and K. E M E R S O N C h e m i s t r y D e p a r t m e n t , M o n t a n a State University, Bozeman, M o n t a n a 59715

(First received 14 September 1967; in revised form 6 December 1967) A b s t r a c t - C r y s t a l l i n e c o m p o u n d s of 2-pyridone with MnCI2, COC12, NiCI2, and CuCI 2 have been prepared. F o u r of the c o m p o u n d s are 1 : 1 adducts; in addition COC12 forms a 1 : 3 adduct and CuCI2 forms a 1 : 2 adduct. Infra-red, u.v. and visible spectra and magnetic properties of t h e s e c o m p o u n d s are presented and discussed. Powder diffraction data on the six c o m p o u n d s and a projection of the structure of CuClz. T P are also presented and discussed. T h e reported observations indicate that in solution the cobalt and nickel c o m p o u n d s are octahedrally coordinated; in the solid phase, the four 1 : 1 adducts all contain as a basic structural unit a chlorine bridged dimer. T h e s e dimers are loosely b o u n d together into chains by long C I - M interactions. T h e point s y m m e t r y at the metal atom is distorted octahedral in all these compounds. INTRODUCTION

a general program investigating ligands with co-ordination geometries similar to acetate ion, we have studied the behaviour of 2-pyridone with four divalent transition metals. 2-Pyridone (TP) is the stable tautomer of 2-hydroxypyridine; both prior work and the work reported here clearly indicate that the acidic hydrogen is bound to the nitrogen and not the oxygen. Complexes with three different stoichiometries have been isolated: MCI2.TP, MCI,,.2(TP), and MClz. 3(TP). AS

PART of

EXPERIMENTAL

SECTION

Preparation of compounds. 2-Pyridone was obtained from A c e t o Chemical Co. as a brown powder. Recrystallization from b e n z e n e gave a colourless product, m.p. 106.5-107°C. T h e i.r. s p e c t r u m of this recrystallized material is essentially the same as that reported by M a s o n [ 1]. T h e identity of the material was further confirmed by a nitrogen analysis. Calc. 14-7 per cent; F o u n d 14.5 per cent. T h e metal chlorides used as starting materials had been dehydrated as described by Pray [2]. Ethanol was dried before use by refluxing over M g turnings for 12 hr and then distilling. All c o m p o u n d s were prepared by mixing stoichiometric a m o u n t s of the desired metal chloride and 2-pyridone in ethanol. In m o s t cases the adduct precipitated immediately. It was filtered, w a s h e d with ethanol, and dried in air. MnCI2.C.~HsNO. This formed as a white powder as soon as the reagents were mixed. Calc. for MnCI2.CsH~NO: M n 24.9; C132.1 ; N 6.3. F o u n d : M n 24.3; CI 31.9; N 6.5. CoCI2.CsH~NO. A pale blue powder formed on mixing the starting materials. S o m e w h a t larger crystals could be obtained by slow evaporation of an ethanol solution of the adduct. Calc. for COCI2. C.sHsNO: Co 26.2; C1 31.6; N 6.2. F o u n d Co 26.0; C131.5; N 6.4, NiCIz.CsHsNO. T h i s formed as a salmon coloured powder. Calc. for NiCI2.CsHsNO: Ni 26.1; C131.6; N 6.2. F o u n d Ni 25.8; C131-0; N 6-4. * Present address: D e p a r t m e n t of Chemistry, Ohio University, A t h e n s , Ohio 45701. 1. S. F. M a s o n , J. chem. Soc. 4847 (1957). 2. A. R. Pray, Inorganic Syntheses, Vol. V, p. 153. McGraw-Hill, N e w York (1957). 1493

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C . C . H O U K and K. E M E R S O N

CuCle'CsHsNO. An orange powder formed readily upon mixing the reagents. Larger crystals were obtained by slow evaporation of an ethanol solution. Calc. for CuCI2"CsHsNO: Cu 27.7; CI 30.9; N 6.1. Found Cu 27.7; C130-9; N 5.8. CoCI2"3C~HsNO. When anhydrous cobalt chloride was reacted in ethanol with excess CsH5NO in a I : 3 ratio or when CoCI2-CsHsNO was treated with a solution of CsH~NO in alcohol, a blue compound with a 1:3 mole ratio precipitated. Calc. for CoCI2"3CsH5NO: Co 14-2; C117.1; N 10.1. Found Co 14.6; C117.6; N9.8. CuCI~.2C5HsNO. This complex precipitated as a chartreuse powder from solutions containing a 2-fold on more excess of CsHsNO over CuC12.Calc. for CuCI2"2CsHsNO: Cu 19.6; CI 21.9; N 8.63. Found: Cu 20.0; C122.2; N 8-65. Physical measurements. Electronic spectra were recorded on a Beckman DK-2 spectrophotometer. Vibrational spectra were taken on a Beckman I R-4 i.r. spectrometer; the wavelength scan was checked at intervals using a piece of styrene film as a standard. X-ray powder spectra were recorded on two different instruments: a Phillips powder camera, which gave a photographic record, and a G. E. diffractometer which gave a graphical presentation. The results of the two different methods were in good agreement. Cu K s radiation was used in both cases. Magnetic measurements were made by the Gouy method, using a Harvey Wells 4-in. electromagnet with tapered pole caps. Readings were taken at several different fields to check for ferromagnetic impurities; the maximum field used was 8100 oersteds. Low temperature measurements were obtained using a specially constructed Dewar flask which fitted between the pole faces of the magnet. Samples were packed in 6 mm borosilicate glass tubes; each measurement reported represents an average of at least two different packings of the compound. RESULTS AND DISCUSSION

The 1 : 1 adducts are all very similar in general appearance to the anhydrous chlorides from which they were prepared. They are much more stable in moist air, however; no visible change in appearance occurs even after several weeks of standing exposed to the atmosphere, whereas the corresponding chlorides will absorb water and transform to the hydrate in a few days at most. CoCI2"3CsHsNO and CuCI~.2CsHsNO are more distinctive in appearance. Both of these compounds are also stable in contact with the atmosphere. Infra-red spectra of the six complex compounds were compared with spectra previously reported for 2-pyridone[1, 4]. The most important bands from a structural point of view are listed in Table 1. In interpreting the frequency shifts in Table 1, it should be remembered that crystalline 2-pyridone is a hydrogen bonded Table 1. Infra-red stretching frequencies

Compound

N--H Stretch cm -1

2-pyridone MnCI2.TP CoC12"TP NiCIz-TP

3100 3450 3200 3250

s* s s s

CuC12.TP CoCIz'3TP CuC12.2TP

3200 s 3300 s 3225 m

C---Q Stretch cm -1 1685 s 1685 s 1645 m t 1660 s 1655 m 1680 s 1640 s

*Strong. tMedium strength. 4. J. A. Gibson, W. Kynastor, and A. S. Lindsey, J. chem. Soc. 4340 (1955).

C o m p l e x e s of 2-pyridone

1495

polymer in which the labile hydrogen is strongly bonded to the oxygen of a neighboring molecule [5]. With this in mind, it is clear from the change in frequency of the N-H stretch in co-ordination that the hydrogen bonding is decreased in all four compounds. The frequency of the vibration still suggests some hydrogen bonding; this may be intermolecular H-bonding to chlorine atoms. The carbonyl stretching frequency, on the other hand, is appreciably lowered in all compounds except the manganese one. Co-ordination through the nitrogen would be difficult with the hydrogen attached, and co-ordination through carbonyl oxygen is therefore strongly indicated. The strength of the bond to manganese seems weaker than others, as would be expected in a d 5 system. We have considered whether the 3200 cm -1 bands might in fact represent OH stretches and the 1650 cm -1 band a ring vibration of the pyridine ring. Pyridine shows a vibration at about 1590 cm -1, rather lower than any of the carbonyl stretches reported here. In addition, the fingerprint region provides clear evidence that the ring system is 2-pyridone rather than 2hydroxypyridine. Pyridine shows strong bands at 1218, 1140, 1068, 1030, and 995 cm -1. 2-Pyridone in the same region has bands at 1245, 1160, 1100, 1010, and 920 cm-L The first four of these five bands occur with only very minor shifts in all six of the reported compounds. The u.v. spectral data for the six compounds are summarized in Table 2, together with the data for 2-pyridone itself. The bands in the region 2000-3500/~ are clearly absorptions of the ligand molecule. These bands have undergone only very modest shifts in hmax. and Emax.upon co-ordination, and indicate only that the 2-pyridone has not undergone any important structural modification upon coordination. Considerable dissociation of the complexes in solution in polar solvents such as these would be expected; the spectra presented suggest, however, that this dissociation is not complete. Table 2. Ultra-violet spectral bands of 2-pyridone c o m p l e x e s Compound TP

Solvent ethanol acetonitrile

MnCI2.TP

ethanol

CoCI~.TP

ethanol

CoCI.2,3TP

ethanol

NiCI2.TP

ethanol

CuCI2,TP

acetonitrile

CuCI.~.2TP

acetonitrile

5. B. R. Penfold, Acta Crystallogr. 6, 591 (1953).

h . . . . (,~) 2260 2980 2270 3020 2260 2980 2270 2980 2250 3050 2270 2980 2260 3050 2270 3030

Emax.(llmole_cm~ 8.32 x 5.23 x 7-57 x 4.85 x 8-53 x 5.76 x 7.42 x 6.22 × 2.33 x 1-48 x 8-53 x 5.58 x 9.33 x 5.29 x 1.66 x 1.13 ×

103 103 103 103 10'~ 103 103 103 104 104 103 103 103 103 104 104

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C . C . H O U K and K. EMERSON

The visible spectra for the five colored compounds are summarized in Table 3. Assignments of the observed bands to specific transitions are indicated where they seem unambiguous. The general spectrum of both the cobalt compounds is similar to that observed for CoX4 = tetrahedral species; the extinction coefficient is lower than one would.expect, however, and the transition energies are somewhat higher. Moreover, similarly structured bands have been observed by other workers for octahedrally co-ordinated cobalt [6, 7]. Table 3. Visible spectral bands of 2-pyridone complexes Compound CoCIz.TP

CoCI2-3TP

Solvent ethanol

ethanol

hmax.(fl~)

E.... (llmole-em)

5750 (sh) 6120 (sh) 6380

168

4T~:g--~ 4A2g

5750 (sh) 6000 (sh) 6400

152

4T2g ~ 4Azg SA2g --~ STlg

NiCI2-TP

ethanol

4130

11

CuCI2"TP

ethanol

8800

164

acetonitrile

4500

931

acetonitrile

4500 8750

775 75

CuCI2-2TP

Assignment

We feel, therefore, that these complexes are probably octahedral in solution. The similarity in the two spectra suggests that the same cobalt species may predominate in both cases. We did not attempt to investigate this possibility further. The case of NiCI2.TP seems clear cut; the one observed absorption is in the proper range for the transition assigned, and the extinction coefficient is suitably low for an octahedral geometry. The other transitions which are characteristic of octahedrally co-ordinated nickel have lower extinction co-efficients and could not be observed in the available concentration range. The two copper complexes exhibit the characteristic bands of copper-chloride complexes, one at 4500 ,~ and one at about 8800 ~,. There is at present considerable disagreement in the literature as to how these bands should be assigned; our study does not contribute anything to the resolution of this problem, and we have not, therefore, made any assignments for these two cases. The magnetic moments of the six compounds are given in Table 4. The experimental susceptibilities were corrected for atomic diamagnetism; the contribution of chloride was estimated using Pascais constants[8] and that of 6. G.J. Janz and A. E. Marcinowsky, USAEC, NYO-8525, (1961). 7. H . N . Ramaswamy and H. B. Jonassen, lnorg. Chem. 4, 1595 (1965). 8. B. N. Figgis and J. Lewis, Techniques of Inorganic Chemistry, (Edited by H. B. Jonassen and A. Weissberger), Vol. IV, p. 142. lnterscience, New York (1965).

Complexes of 2-pyridone

1497

Table 4. Magnetic moments Compound

T(°K)

t~en.(B.M.)

MnCI2.TP

298 251 210 80

5.97 6.03 5.83 5.86

CoC12"TP

297 273 251 210 196 80

5.34 5.40 5.43 5.46 5.41 5.29

NiCI2.TP

297 273 251 210 196 80

3.28 3.32 3.32 3-45 3.41 3.99

CuCIz'TP

297 273 251 210 196 80

1-80 1-87 1.86 1.87 1.88 1.83

CoCI2"3TP

298 251 210 80

4-85 4.82 4-87 4.59

CuCI~'2TP

298 251 210 80

1.82 2.06 2.06 1.76

2-pyridone was determined by direct measurement of the magnetic susceptibility. Corrections for temperature independent paramagnetism have not been applied. T h e s e c o r r e c t e d s u s c e p t i b i l i t i e s w e r e t h e n u s e d to c a l c u l a t e the m a g n e t i c m o m e n t s from the simplified form of the Curie Langevin Law. Pen. = 2"828V'(XMT). T h e m o m e n t s r e p o r t e d f o r CuC12-TP a n d M n C I 2 - T P a r e w i t h i n t h e n o r m a l r a n g e s r e p o r t e d f o r s p i n f r e e c o m p l e x e s o f t h e s e m e t a l s [ 9 ] a n d s e e m to b e t e m p e r a t u r e i n d e p e n d e n t . T h e m a g n e t i c m o m e n t f o r C o C I 2 . T P c o m p l e x is v e r y high, i n d i c a t i n g a large o r b i t a l c o n t r i b u t i o n to t h e m o m e n t , a n d s u g g e s t i n g s t r o n g l y t h a t t h e c o b a l t ( I I ) is o c t a h e d r a l l y c o - o r d i n a t e d . T h e m o m e n t f o r t h e n i c k e l ( I I ) 9. B. N. Figgis and J. Lewis, Techniques of Inorganic Chemistry, (Edited by H. B. Jonassen and A. Weissberger), Vol IV, p. 166. Interscienee, New York (1965).

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C . C . H O U K and K. EMERSON

complex is high, and also temperature dependent, suggesting tetrahedral coordination, but as has been pointed out by Lever [ 10], similar magnetic properties have been reported for a number of tetragonal nickel complexes. This appears to to be the most likely explanation, as will be seen below. The magnetic moment of CoCI., 3TP is lower than that of CoCI2.TP, but still in the range normally associated with octahedral cobalt Complexes. The magnetic moments of the two copper complexes are typical of those normally observed in magnetically dilute d 9 systems. The CuC12.2TP shows an interesting temperature dependence which we have not investigated; we are not at the moment prepared to ascribe this to experimental error. The X-ray diffraction patterns of the 1 : 1 compounds are presented in Fig. 1. The manganese, cobalt and nickel compounds are isomorphic. There is a striking similarity between the pattern for the copper compound and the other three, but there are significant differences. The powder diffraction pattern of CuC12.2TP and CoCL,.3TP are shown in Fig. 4. These two compounds clearly have differing structures, and have not been further investigated.

I,,l I1,, I ,,,

I

I,l,,li,I II,

I1, ,,I,,ll , h, ,,I,II1,11,,I

I I0

20

Cu CI2.TP

,

I

niCl 2 -TP

I

CoCl2'TP

I

MnCl2 .TP I O(Brogg Angle) 30

Fig. 1. X-ray powder diffraction data for compounds MC12.TP. The vertical height of the line indicates its relative intensity.

In order to learn a bit more about the structures of these compounds a limited X-ray crystallographic study was undertaken. Single crystals of CuC12.TP and CoCI.,.TP were prepared and X-ray photographs collected by Weissenberg techniques with CuK~ radiation. The space group and unit cell dimensions for these two crystals are given in Table 5. Using this data, it can be shown that, approximately,

Aco = Acu- I~cu Bco = fCcu (2co = 2(Acu+ Bcu). I 0. A. B. P. Lever, lnorg. Chem. 4, 763 (1965).

1499

C o m p l e x e s of 2-pyridone

II I111,I, Ih

I

,, , , , ,

J I0

J 20

l ll,rrr,ll' i, J 0

Cu Cl 2 - 2 TP

coc, . 3 rp

J 50

~(Bragg Angle)

Fig. 2. X-ray powder diffraction data for c o m p o u n d s MCI2.TP.

Table 5. Space group and unit cell dimensions

Compound CuClz.TP CoCIz-TP

A 9.93/~ 16-78

B

C

a

/3

y

9.94 ~ 8-65

3.95/~ 24.80

90.9 ° -

92-9 ° 94.2 °

113.2 ° -

Space Group Pl Cc or

C2/c We feel that the above relationship accounts for the similarity in the powder diffraction patterns of the two compounds. For CuCI2.TP, Z -- 2 gives a calculated density of 2.1 g/cc for CoC12-TP; Z = 16 gives D -----1.7 g/cc. The measured densities are 2.2 g/cc and 1.7 g/cc respectively. The 3.95 A dimension in the CuC12.TP lattice suggested that a projection of the structure on the ab plane might reveal a great deal. Careful examination of the photographs of CuCI2.TP revealed a number of faint spots which did not fit the basic lattice reported in Table 5. These indicated a larger superlattice, but were so few in number that it seemed likely that the basic structure could be determined by ignoring them. Data for the hkO projection were accordingly collected; 187 reflections within the C u K a sphere of reflection were estimated visually. A Patterson projection was calculated and from it the Cu and CI positions were deduced. A Fourier map revealed the position of the ring, and this projection of the structure was refined to a conventional R factor of 0.20. The values of Fo and Fc at this point are given in Table 6 for the observed reflections. The two formula units of CuC12.TP are related to each other by a crystallographic center. The positions of the atoms in the asymmetric unit are given in Table 7. The basic structural unit revealed by this determination is the dimer shown in Fig. 3. The atom distances in this dimer reveal some additional information. Distances a and c (See Fig. 3) are very short: 1.6 A as compared to 2-3 A expected and observed for distance d. Distance b is 1.9 ,~, the right order of magnitude for a Cu---O bond. This suggests that the dimer is tilted out of the plane of projection. The angle of tilt can be calculated, assuming a Cu---CI distance of 2-3 .A, as 39 °. This places the chlorines of the dimer in the adjacent unit cell 2.9 ,~ from the Cu atoms, and almost exactly above the copper relative to the proposed plane of the dimer. The dimers are, therefore, loosely connected together into chains by long

1500

C . C . H O U K and K. E M E R S O N

Table 6. Observed and calculated structure factors for CuCI2.TP, hkO level (Unobserved reflections have not been included)

h

10Fo

l 2 3 4 5 6 8 9 10 ll

k=0 208 103 97 107 171 145 99 57 86 85

0 1 -1 2 -2 -3 4 5 6 ~6~ 7 --7 8 --8 9 10 11

k= 1 145 164 190 67 277 44 111 141 -, 151 215 55 234 58 155 54 110 93

0 1 -1 2 -2 3 -3 4 ---4 5 --5

k=2 247 121 32 195 49 193 123 272 80 225 70

10Fc

371 121 -51 112 153 148 -31 -46 --63

h

6 -6 -7 8 -8 9 -9 10 -12

10Fo

164 184 206 98 224 88 180 79 38

--103

147 146 204 -43 328 15 -87 --150 --141 263 --8 254 --38 123 --50 --138 --140

--258 -98 -32 -214 -37 -242 81 -368 -49 --234 25

0 1 -1 2 -2 3 -3 4 -4 --5 6 -6 --8 --9 lO --10

0 l --l -2 -3 4 -4 5 -5 6 -6 7 8

k=3 101 138 175 185 297 246 323 192 162 99 145 72 105 58 40 55

k= 4 51 107 147 238 404 90 411 97 284 99 89 115 134

10Fc

-179 144 210 -11 263 -84 184 --103 47

h

-8 9 -9 -10 -12

0 1 -1 2 -2 44 3 66 -3 --140 -4 -219 --5 --294 6 --343 -6 -324 7 -221 --7 --149 8 --10 95 --9 ---42 --10 85 --11 46 --12 40 36 0 J -I 22 2 --77 - 2 89 - 3 -221 4 --456 --4 86 - 5 -383 6 67 -6 -224 7 116 - 7 - 6 9 --I0 125 --11 165 --12

10Fo

136 96 155 79 53

k= 5 128 121 90 146 40 151 71 204 204 79 163 140 53 115 108 243 168 84 k= 6 276 246 186 164 182 159 81 163 133 86 90 81 149 146 140 132

10Ft

-149 115 -167 -76 -2

h

10Fo

0 1 -1 2 -2 3 -3 4 ---4

k=7 242 97 203 94 89 99 138 55 73

ll9 100 -5 77 6 154 -6 20 -7 144 -9 34 -12 --164 --161 99 --136 0 149 1 27 --1 122 2 --94 --2 --197 3 Z153 --3 --86 4 -4 5 295 - 6 285 - 7 182 - 8 140 - 9 168 --12 141 -37 140 82 83 66 69 109 --99 --140 --128

0 -1 --2 3 -3 4 --4 --5 --7

10Fe

192 49 302 145 57 35

234 62 166 -60 70 -110 -47 --53 70 162 --54 276 110 54 --18

k=8 133 81 80 110 55 93 68

-107 -86 --42 -119 11 -105 ---49

119 97 96 181 216 269 103 86

--146 --89 --122 119 194 203 63' 104

k=9 115 90 146 89 108 114 129 94 98

-104 --71 -140 --102 --107 -123 --129 --89 46

h

-8 -9 -11

10Fo

10Fe

112 63 45

78 57 --31

0 --2 3 --3 --4 --5 -6 --7 --8 --9 --10 --ll

k = 10 151 95 190 -151 55 -52 224 --180 176 --139 55 -68 96 --88 112 --89 123 --87 97 --74 70 -67 27 --70

0 1 -1 -2 -3 ---4 -5 -7 -8 --9 -10

k=ll 86 37 76 46 108 127 112 80 73 132 90

-i -2 -3 --4 -6 -7 --8

k=12 53 71 83 46 72 83 51

95 46 85 51 -66 -99 -74 27 ---62 --122 -108

87 87 78 39 51 87 73

Complexes of 2-pyridone

1501

Cu---C1 interactions. Figure 4 shows a drawing of the structure proposed. A very similar structure has been reported by Willett [11] for CuCI2-CH3CN. The agreement factor of 0.20 is not particularly good by current standards. We feel, however, that the basic structure we have reported is correct. A Fourier map of the entire unit cell at the end of the refinement shows all atoms in their leastsquares positions and no other peaks anywhere in the cell. An examination of the structure factors in Table 6 reveals that only 18 of them have Fc differing from Fo by more than a factor of 2. Of these 18, 15 have IF01 < 10, the remaining 3 have IFol < 15. If the structure were incorrect, one would expect more disagreement than this, and one would not expect it to be confined to the weak reflections. The most probable explanation of the poor agreement factor is that the structure Table 7. Positions in CuCIz.TP hkO projection Atom

x/a

y/b

Cu CII C12 O C1 C2(N?) Cz C4 C5 Cr(N?)

0.035* 0-197 0.139 -0.042 -0-166 --0.306 --0.451 -0.493 -0.374 -0.221

0-170 0.297 -0.001 0-314 0.306 0.176 0.174 0.282 0.399 0.412

*Uncertainties in these quantities are too large to convey any meaningful information. See text.

Cl

0

/C

0

I

Cl

Fig. 3. Structural unit in solid CuCI2.TP. 11. R. D. Willett and R. E. Rundle,J. chem. Phys. 40, 838 (1964).

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C . C . H O U K and K. EMERSON

©°' ©o 0 •

CsHsN

o,,

b

%

p.

Fig. 4. Diagram of porposed packing in CuCI~.TP. The unit cell axes are shown as arrows. The Cu--Cl interactions (2.9 ,~) are shown as dotted lines. The angle of the ring cannot be determined from the available data, and the CsHsN simply indicates that this group is attached to the oxygen atom.

of the superlattice, evidence for which was cited above, becomes important. We feel that this problem would be better investigated with three dimensional data, which we have not obtained. Our bond distances and standard deviations are neither accurate nor precise, and we have not reported them here. We are also unable to specify the position of the ring nitrogen. We have considered whether the CoCI2.TP, NiCI2.TP, and MnCI2.TP can on the basis of the available evidence be assigned the same structure. The relationship of the cell dimensions of CuCI2.TP and CoC12.TP is not so good that it could not be an accident. Two unit cells with these relationships might, indeed, give somewhat similar powder diffraction patterns; however, if the structure were totally different, one would expect the intensity patterns to be quite different. The similarity of intensity patterns in Fig. 1 is quite striking, and we feel that the present evidence indicates that all four structures contain the same basic unit of structure that we have determined for CuCI2.TP. Acknowledgements - T h e authors are grateful to the National Science Foundation for a grant, G P-1949, without which this work would not have been possible. One of us (C.C.H.) is also indebted to the Department of Health, Education and Welfare for an N D E A fellowship. We also thank Dr. Roger Willett of Washington State University for a number of stimulating ~nd helpful discussions.