This work has been digitalized and published in 2013 by Verlag Zeitschrift für Naturforschung in cooperation with the Max Planck Society for the Advancement of Science under a Creative Commons Attribution 4.0 International License. Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Creative Commons Namensnennung 4.0 Lizenz. The Hei and HeH Photoelectron Spectra of [Fe*? 3 -C3H5(CO)3X] and [Fe^ 3 -C4H7 (CO) 3X] (X = Cl, Br, I) and [CoC3H5(CO)3] Jaap N. Louwen, Jaap Hart, Derk J. Stufkens, and Ad Oskam* Anorganisch Chemisch Laboratorium, University of Amsterdam, J. H. van't Hoff Instituut, Nieuwe Achtergracht 166, 1018 W V Amsterdam, The Netherlands Z. Naturforsch. 37b, 179-185 (1982); received September 29, 1981 Photoelectron Spectra, Iron, Cobalt, Allyl Complexes By means of UV photoelectron spectroscopy (UPS) the electronic structures of [Fe^3.C3H5(CO)3X], [Fe^3-(2-CH3C3H4)(CO)3X] (X = Cl, Br, I) and [Co(C3H5)(CO)3] have been studied. Detailed assignments are possible and surprisingly low ionization energies (as low as 8.19 eV) are found for iodine lone pair type Orbitals. From the spectra a large difference in n backbonding is found between the cobalt and iron complexes. Introduction Allyl complexes, as a class have been known since the late 1950's. They have been extensively studied, mainly because of their relevance to catalytic problems and the fluxionahty most complexes exhibit. Theoretical efforts, both semiemperical [1, 2] and ab initio calculations [3], have mainly been directed towards the platinum metal bis allyl complexes. A number of UV photoelectron spectroscopy (UPS) studies have been devoted to these systems [4^6]. Definite assignments have been given by Böhm, Gleiter and Batich on the basis of He I/He II intensity ratios, substitution effects and INDO cal- culations, combined with a Greens function approach [2]. Orbitals belonging to ligand type ionization bands are complex combinations of fragment Or- bitals. Other UPS studies on allyl complexes are that of Seddon and Green on some Or and Mo complexes [7], where detailed assignments were not possible due to strong overlapping of inonizations bands, and the work of Clark and Green, as well as Van Dam et al., on some bis cyclopentadiene allyl complexes of Ta and Nb [8]. The latter studies yielded ionization potentials of about 7.8 eV for the non bonding allyl TI ionization but the position of the bonding TI Or- bitals could not be determined with certainty. Thus, as a part of studies performed in this group upon the coordination of unsaturated systems to transition metals [9, 10], we were prompted to investigate a few fairly simple allyl complexes of Fe * Reprint requests to Prof. Dr. Ad Oskam. 0340-5087/81/0200-0179/$ 01.00/0 and Co. With these complexes detailed assignments are possible since the presence of only one allyl hgand excludes mixing of orbitals on different allyl fragments and, moreover, the other ligands either have low intensity ionization bands in Hell (the halogens) or do not appear within the upper valence region (CO). Furthermore, comparing the Co and Fe complexes might be instructive with regard to the 7i backbonding capabilities of the allyl ligand, the subject of some discussion [1]. The results should also be of interest because of the possible chemical consequences. For instance, the FeC3Hs(CO)3 moiety survives reduction from Fe(II) to Fe(I) and hence [Fe(C3H5)(C0)3X] is a good starting compound for synthesizing Fe-metal bonds [11, 12]. Also, from a practical point of view, these com- plexes can easily be sublimed and, at least for the iron complexes, it is known from the literature [13] that no decomposition is to be expected at the tem- peratures applied (~50 °C). Experimental Syntheses were performed by literature methods, for [FeC3H5(C0) 3Cl], [FeC4H7(C0) 3Cl], and [FeC3H5(CO)3Br] by reacting the corresponding allyl halide with [Fe2(CO) 9] [14], for [FeC3H5(CO) 3I], [FeC4H7(CO) 3I] and [FeC4H7(CO) 3I] by stirring one of the above complexes with the appropriate sodium halide in acetone for 40 min [14], and for the cobalt complex by the action of allyl bromide on [NaCo(CO) 4] [15]. Identity and purity were checked by IR and NMR spectroscopy. UP spectra were recorded on a Perkin-Elmer PS 18 photoelectron spectrometer, equipped with a He I/He II light source. Ar and Xe were used as internal calibrants, together with the signal arising from the autoioniza- tion of He by Hell photons. The spectra are given uncorrected for analyzer dependency, which must be taken into account in
Transcript
This work has been digitalized and published in 2013 by Verlag Zeitschrift für Naturforschung in cooperation with the Max Planck Society for the Advancement of Science under a Creative Commons Attribution4.0 International License.
Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschungin Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung derWissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht:Creative Commons Namensnennung 4.0 Lizenz.
The Hei and HeH Photoelectron Spectra of [Fe*?3-C3H5(CO)3X] and [Fe^3-C4H7 (CO)3X] (X = Cl, Br, I) and [CoC3H5(CO)3] Jaap N. Louwen, Jaap Hart, Derk J. Stufkens, and Ad Oskam* Anorganisch Chemisch Laboratorium, University of Amsterdam, J. H. van't Hoff Instituut, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands
Z. Naturforsch. 37b, 179-185 (1982); received September 29, 1981
Photoelectron Spectra, Iron, Cobalt, Allyl Complexes By means of UV photoelectron spectroscopy (UPS) the electronic structures of
[Fe^3.C3H5(CO)3X], [Fe^3-(2-CH3C3H4)(CO)3X] (X = Cl, Br, I) and [Co(C3H5)(CO)3] have been studied. Detailed assignments are possible and surprisingly low ionization energies (as low as 8.19 eV) are found for iodine lone pair type Orbitals. From the spectra a large difference in n backbonding is found between the cobalt and iron complexes.
Introduction Allyl complexes, as a class have been known since
the late 1950's. They have been extensively studied, mainly because of their relevance to catalytic problems and the fluxionahty most complexes exhibit.
Theoretical efforts, both semiemperical [1, 2] and ab initio calculations [3], have mainly been directed towards the platinum metal bis allyl complexes. A number of UV photoelectron spectroscopy (UPS) studies have been devoted to these systems [4^6]. Definite assignments have been given by Böhm, Gleiter and Batich on the basis of He I/He II intensity ratios, substitution effects and INDO cal-culations, combined with a Greens function approach [2]. Orbitals belonging to ligand type ionization bands are complex combinations of fragment Or-bitals.
Other UPS studies on allyl complexes are that of Seddon and Green on some Or and Mo complexes [7], where detailed assignments were not possible due to strong overlapping of inonizations bands, and the work of Clark and Green, as well as Van Dam et al., on some bis cyclopentadiene allyl complexes of Ta and Nb [8]. The latter studies yielded ionization potentials of about 7.8 eV for the non bonding allyl TI ionization but the position of the bonding TI Or-bitals could not be determined with certainty.
Thus, as a part of studies performed in this group upon the coordination of unsaturated systems to transition metals [9, 10], we were prompted to investigate a few fairly simple allyl complexes of Fe
* Reprint requests to Prof. Dr. Ad Oskam. 0340-5087/81/0200-0179/$ 01.00/0
and Co. With these complexes detailed assignments are possible since the presence of only one allyl hgand excludes mixing of orbitals on different allyl fragments and, moreover, the other ligands either have low intensity ionization bands in H e l l (the halogens) or do not appear within the upper valence region (CO). Furthermore, comparing the Co and Fe complexes might be instructive with regard to the 7i backbonding capabilities of the allyl ligand, the subject of some discussion [1]. The results should also be of interest because of the possible chemical consequences. For instance, the FeC3Hs(CO)3 moiety survives reduction from Fe(II) to Fe(I) and hence [Fe(C3H5)(C0)3X] is a good starting compound for synthesizing Fe-metal bonds [11, 12].
Also, from a practical point of view, these com-plexes can easily be sublimed and, at least for the iron complexes, it is known from the literature [13] that no decomposition is to be expected at the tem-peratures applied (~50 °C).
Experimental Syntheses were performed by literature methods,
for [FeC3H5(C0)3Cl], [FeC4H7(C0)3Cl], and [FeC3H5(CO)3Br] by reacting the corresponding allyl halide with [Fe2(CO)9] [14], for [FeC3H5(CO)3I], [FeC4H7(CO)3I] and [FeC4H7(CO)3I] by stirring one of the above complexes with the appropriate sodium halide in acetone for 40 min [14], and for the cobalt complex by the action of allyl bromide on [NaCo(CO)4] [15]. Identity and purity were checked by IR and NMR spectroscopy. UP spectra were recorded on a Perkin-Elmer PS 18 photoelectron spectrometer, equipped with a He I/He II light source. Ar and Xe were used as internal calibrants, together with the signal arising from the autoioniza-tion of He by H e l l photons.
The spectra are given uncorrected for analyzer dependency, which must be taken into account in
180 J. N. Louwen et al. • The He I and H e l l Photoelectron Spectra of [Fer?3-C3H5(CQ)3X] 180
comparing intensity of bands separated by a large energy interval.
General Considerations Concerning the Spectra Before discussing the spectra, an inventory of the
possible ionizations and several assignment criteria may be set out.
o c - ^ f e *
Fig. 1. Structure of the iron compounds.
The molecular geometry of the iron complexes is presented in Fig. 1. The compounds formally exist of an iron 2 + ion (a 3d6 closed shell configuration) with a halide, an allyl univalent negative ion and three CO ligands within its coordination sphere.
From the iron three 3d-type ionizations are to be expected. It is well known that ionizations from transition metal 3d orbitals show considerable enhancement on going from He I to He II ionization [16]. The halogens will yield three lone pair p type ionizations, two perpendicular to the F e - X bond being nearly degenerate, and one along this bond. In the case of high localization one expects these three ionizations to have high intensity in He I and hardly any in H e l l .
The higher CO ionizations will not appear in the valence region, but will be visible, however, in the same region as the higher allyl sigma orbitals (between 12 and 17 eV). Because of their oxygen 2p participation, ionizations from higher CO orbitals will be enhanced in intensity in the H e l l spectra, especially relative to the CTCH ionizations which are predicted to have lower H e l l intensity. Of utmost importance are the n orbitals on the allyl part and the corresponding ionizations. The three n orbitals are depicted schematically in Fig. 2. The top one, the antibonding orbital, is essentially unfilled and only of importance in connection with possible backbonding towards the allyl moiety.
The other two are both formally doubly occupied. Their corresponding ionization bands are charac-terized by a H e l l intensity which is considerably lower than that of metal 3d type ionizations, although mixing with metal d orbitals could result in some enhancement of H e l l intensity.
Two other assignment criteria can be used to discriminate between the bonding and nonbonding allyl TI ionizations. Most important is the effect of substitution of a methyl group at the middle C atom of the allyl. This will cause the bonding n orbital to shift considerably, due to the fact that this orbital has its main density on this central C atom while the nonbonding orbital (having a node at this atom) is only inductively affected (as are the other orbitals in the molecule).
Fig. 2. A plot of the three allyl 71 orbitals. From top to bottom: the antibonding 71* orbital, the non-bonding n and the bonding ji orbitals.
Vibrational coupling might also be of some help to dicriminate between the two allyl n ionizations. The TI bonding orbital should have more vibrational coupling with the allyl system than the nonbonding one and this will result in a broader ionization band.
The cobalt complex with a formal Co+ (3d8) configuration on the metal atom can be treated analogously and needs no further discussion.
One further remark has to be made. It is known that these complexes exhibit fluxional behaviour in solution and that throughout the iron series the rates of equilibrium between the endo and exo forms in solution differ to a large extent [17]. In the gas phase, a corresponding behaviour could possibly be reflected in the UP spectra. However, no abnormal broadening of bands, nor differences throughout the series have been observed. Probably, as has been
J. N. Louwen et al. • The He I and Hell Photoelectron Spectra of [Fer?3-C3H5(CQ)3X] 181
noted in similar cases [8], such energy differences are too small to be detected in UPS.
The Spectra Ionization potentials of the iron and cobalt com-
plexes are given in Table I.
[Feri*-CzHb(CO)*Cl] and [Fer)*-CJh(CO)sCl]
The spectra depicted in Figs. 3a and 3 b are very much alike, with the exception of the extra a type ionizations around 13.5 eV in the spectrum of [FeC4H7(CO)3Cl] and the band at 11.63 eV in the spectrum of [FeC3H5(CI)3Cl] which merges with the band at 11.17 eV in the spectrum of the 2-methyl derivative.
At the low energy side of the spectrum one first discerns two ionization bands which clearly enhance in intensity on going from He I to H e l l . In view of the criteria outlined in the previous section, these bands have to be assigned to the three 3d type ionizations. Because of its intensity the first band is assigned to ionizations from a degenerate pair of orbitals. The second one, belonging to a single ionization exhibits, in these and the other spectra, stronger enhancement than the first band. Ap-parently, it is connected with the more loealized-d-orbital.
To the higher energy side of these bands are found two sharp intense bands which almost disappear in the H e l l spectrum. Undoubtedly these bands belong to ionizations of the halogen lone pair orbitals. From the spectra the band at lower energy seems to belong to the single ionization (metal-chlorine bond), a deduction supported by the fact that this band does have some H e l l intensity.
The two remaining bands in the upper valence region can now easily be assigned to the two allyl
J FeCsH8(CO)sCl
He(I)
He( II)
10 12 14 18 -I 20 eV
Fig. 3a. The Hel/Hell spectrum of [FeC3H5(CO)3Cl].
FeC4H7(C0)3Cl
H«KII)
—i—i—r
3b. The Hel/HeH spectrum of [FeC4H7(CO)3Cl].
Tab. I. Ionization potentials in electron volts (d = double).
182 J. N. Louwen et al. • The He I and H e l l Photoelectron Spectra of [Fer?3-C3H5(CQ)3X] 182
n ionizations. The two bands can be distinguished by methyl substitution.
In [Fe^a-CaHsCCOJaCl] the band at 11.02 eV, which is hardly affected upon methyl substitution (11.17 eV), must be connected with the nonbonding 7i orbital while the other one at 11.63 eV is clearly derived from the n bonding orbital because of its large shift to 11.17 eV in [ F e ^ - O ^ C O ^ C l ] . The remainder of the spectra is complicated, though some features can be discerned. At ~ 13.5 eV some bands are observed which lose much intensity in H e l l . These are assigned to C - H a type ionizations, an assignment which is supported by the He I/He II intensity ratio and by the fact that methyl sub-stitution seems to add extra ionizations to this region. Next to these can be observed carbon monoxide localized ionizations which are strongly enhanced in intensity in H e l l , due to the oxygen 2p participation. These have a maximum around 15 eV.
The spectra of [FeC3Hb(CO)zBr] and [FeCJI^CO^Br]
In many respects these spectra, as shown in Figs. 4 a and 4b, are similar to those of the chlorine compounds. However, in the spectra of the methyl substituted derivative the allyl TI ionization bands do not coincide since the separation between the corresponding ionization energies of the C3H5 com-plex has now increased, mainly due to destabiliza-tion of the non bonding Tt orbital.
Fig. 4a. The Hel/Hell spectrum of [FeC3H5(CO)3Br].
4b. The He I/He II spectrum of [FeC4H7(CO)3Br].
In contrast, the metal 3d-region and the halogen region are now less separated and the single Fe 3 d ionization band is now visible as either a shoulder or even a slope on a Br ionization band.
In the H e l l spectrum this band again exhibits the most dramatic enhancement which results in a considerable shift of the overall maximum of the second band on going from He I to H e l l .
Because of the strong overlapping the Br lone pair type band at highest energy can only be tentatively assigned to an ionization from the single orbital; this would infer a reverse ordering with respect to the Cl compounds.
The other main features, i.e. the strong shift of the bonding allyl ionization band upon methyl sub-stitution and the position of the a and CO type ionizations are the same as in the spectra of the Cl complexes.
The spectra of[FeC3H5(CO)zI] and [FeC*Hi(CO)zI]
These spectra are to be found in Figs. 5 a and 5 b. With respect to the spectra of the Cl and Br compounds one now observes a dramatic change in shape. The halogen lone pair ionization bands identified, as are the 3d orbitals, by their He I/He II intensity behaviour, have moved to a very low energy indeed (8.36/8.60 eV and 8.19/8.49 eV for the allyl and methyl allyl compounds respectively). The Fe 3d ionizations are now found at higher en-
J. N. Louwen et al. • The He I and H e l l Photoelectron Spectra of [Fer?3-C3H5(CQ)3X] 183
ergies, in contrast to what might have been ex-pected from the inductive effect of iodine compared to that of bromine and chlorine.
He(I)
He(II)
~T" 10 12
T~ 14 16 18 ~20 eV
Fig. 5a. The Hel/Hell spectrum of [FeC3H5(CO)3I].
FeC4H7(C0)3
He(I)
He(II)
i—i—i—I—T 8 10 12
1 1 1 1 1 1 1 1 1 4 1 6 1 8 2 0 eV
5b. The Hel/Hell spectrum of [FeC4H7(CO)3I].
The possibility that these shifts are due to changes in the molecular structure can be ruled out with some confidence. Crystal structures of the Br and I complex have been published [12] and, although the published structure for the I complex is unfortu-nately not of particularly high quality, it may be concluded that the structures of the bromine and the iodine complexes do not show basic differences.
Rather, the unexpected shifting of the 3d bands and the very low iodine ionization energies are the result of a different interaction between iodine and metal levels. If one considers the molecule [FeC3H5(CO)3X] as consisting of the ions FeC3Hs(CO)3+ and X - we can view the ionizations, especially of Fe 3d and X np electrons, as arising from levels which are combinations of levels on these two fragments. Clearly, upon substituting the halogen the FeC3Hs(CO)3+ levels do not vary, while the X~ levels do. By combining levels, we lower the lowest and raise the upper one. It is now possible to understand the shifts upon iodine substitution by assuming that the I~ levels are above those of FeC3H5(CO)3+ (while the Cl" and Br- levels are below) and this results in extra raising of the I levels, with lowering of Fe 3d levels.
The absence of spin-orbit coupling might be surprising. However, because of the low local sym-metry of the I~ ion quenching is to be expected, furthermore the possibility of accidently coincident I and Fe 3d levels cannot be fully excluded.
The allyl n bands, the a type ionizations and the CO bands are found at the same energies as before, except for the non-bonding allyl n ionization which has again been destabilized.
The Hel and Hell spectra of [CoCzHb(CO)z]
By comparison with the spectra of the iron complexes, that of the cobalt complex can readily
He(I)
He<II)
18 —I 20 «V
Fig. 6. The Hel/Hell spectrum of [CoC3H5(CO)3].
184 J. N. Louwen et al. • The He I and H e l l Photoelectron Spectra of [Fer?3-C3H5(CQ)3X] 184
be assigned. The first four strongly overlapping bands must be considered as mainly due to Co 3 d ionizations because of their intensity behaviour.
Next, one finds the two allyl n bands and the complex grouping of and CO bands. The allyl bands have been considerably destabilized with respect of the corresponding Fe complexes. The maximum of the CO band (at ~ 14 eV) has also shifted about 1 eV, and the CO ionization bands now almost obscure those of the a-type ionizations. Both de-stabilizations are in agreement with the difference in formal charge on the metal, mainly due to the absence of a halogen atom. Clearly there exists more n backbonding towards the CO ligands in this Co complex than in the Fe complexes. It is therefore of interest that the H e l l intensity of the first band of the spectrum is less than that of the other three d-type ionizations. Evidently this points to mixing of orbitals from the coordinating groups to provide backbonding.
There exists, however, no indication concerning the amount of jr-backbonding to the allyl ligand. From the position of the bonding allyl band we can infer that the charge density on the allyl moiety is higher than in the case of the iron complex, though this can be explained by the decrease in a-donation.
An indication of the importance of allyl n back-bonding could have been inferred from a possible shift of the first 3d band upon methyl substitution. Up to now, however, attempts to record the spec-trum of the methylated compound have failed.
Discussion Within the iron series, donation of charge from the allyl towards the metal is apparent. The energy of the nonbonding allyl n orbital is strongly influenced by the electronegativity of the halogen atom in [FeC3H5(CO)3X], whereas the bonding one is not. Apart from the position of the n bonding ionization, the spectrum is not very much influenced by methyl substitution. The shape of the n* orbital, however, would suggest that any orbital containing some
n* character, would be destabilized (see Fig. 2). Thus we may conclude that throughout the iron series the allyl-metal interaction is mainly a donating one.
The position of the nonbonding orbital seems to be directly influenced by the charge on the metal, rather than that on the allyl part. This follows from the fact that upon changing the halogen atom in the iron compounds the separation between the non bonding and bonding bands varies. The bonding orbital remains, in fact, at much the same position.
The question might be raised as to whether the nonbonding allyl n orbital can be visualized as a combination of two atomic 2p orbitals. Maybe the actual molecular orbital on the allyl part is more diffuse, which would be in agreement with the results of extended basis GTO-SCF calculations performed on SO2 [18].
As only one cobalt complex has been measured, it is not possible to draw many conclusions. How-ever, it is clear that the actual charge on Co is higher than that on Fe, and backbonding has become an important factor. The allyl orbitals are destabilized with respect to the iron series, due to the excess of charge. It is, unfortunately, not clear whether this results from an increase of backbonding to the allyl part or from a decrease of donation towards the metal.
A comparison with the spectra of the bis allyl platinum metal systems [6] is of interest. The results from those studies agree with this one, especially with regard to the energy of the n bonding ioniza-tion and the shifts of the allyl ionization energies upon methyl substitution. An important difference is the position of the band, in the region 7.6-7.9 eV throughout the platinum triad, assigned to ioniza-tion from a pure combination of allyl nonbonding 71 orbitals. The difference with the ionization energies found for the non bonding Tt orbital in the systems studied can be explained by the interaction of the allyl levels with the metal orbitals of higher energy.
Mr. A. Terpstra is thanked for helpful discussions.
[1] D. A. Brown and A. Owens, Inorg. Chim. Acta 5, 67 (1971).
[2] M. C. Böhm and R. Gleiter, Theor. Chim. Acta 57, 315 (1980).
[3] M.-M. Rohmer, J. De Muynck, and A. Veillard,
Theor. Chim. Acta 36, 93 (1973) and references cited therein.
[4] D. R. Lloyd and N. Lynaugh, in D. E. Shirley (ed.): Electron Spectroscopy, Proc. of an Int. Conf., Amsterdam, North Holland 1972.
J. N. Louwen et al. • The He I and Hel l Photoelectron Spectra of [Fer?3-C3H5(CQ)3X] 185
[5] C. D. Batich, J. Am. Chem. Soc. 98, 7585 (1976). [6] M. C. Böhm, R. Gleiter, and C. D. Batich, Helv.
Chim. Acta 63, 990 (1980). [7] J. C. Green and E. A. Seddon, J. Org. Met. Chem.
198, C 61 (1980). [8] a) J. P. Clark and J. C. Green, J. Less Comm.
Met. 1977, 31; b) H. van Dam, A. Terpstra, A. Oskam, and J. H. Teuben, Z. Naturforsch. 36b, 420 (1981).
[9] H. van Dam and A. Oskam, J. Electron Spectrosc., Relat. Phenom. 13, 273 (1978).
[10] H. van Dam, J. N. Louwen, A. Oskam, M. Doran, and I. H. Hillier, ibid. 21, 57 (1980).
[11] C. F. Putnik, J. J. Wetter, G. D. Stucky, M. J. D'Aniello, B. A. Sosinsky, J. F. Kirner, and E. L. Mutterties, J. Am. Chem. Soc. 100, 4107 (1978).
[12] F. E. Simon and J. W. Lauher, Inorg. Chem. 19, 2338 (1980).
[13] A. N. Nesmeyanow, Ju. A. Ustynyuh, I. I. Kritshuya, and J. A. Shchembelov, J. Org. Met. Chem. 14, 395 (1968).
[14] H. D. Murdoch and E. Weiss, Helv. Chim. Acta 45, 225 (1962).
[15] R. F. Heck and D. S. Breslow, J. Am. Chem. Soc. 83, 1097 (1961).
[16] H. van Dam and A. Oskam, Review Transition Metal Chemistry, M. Dekker Inc. Ed. B. N. Figgis, Volume 1982.
[17] J. W. Faller and M. A. Adams, J. Org. Met. Chem. 170, 71 (1979).
[18] J. N. Louwen and A. Oskam, unpublished results.
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