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Correlation of the ionization potentials of pseudo-aromatic metal complexes with hammett's substituent constants—II
CORRELATION OF THE IONIZATION POTENTIALS OF PSEUDO-AROMATIC METAL COMPLEXES WITH HAMMElT’S SUBSTITUENT CONSTANTS-II CHROMIUM ACETYLACFiTONATES* L. S. LEvrM’ and B. W. LEWT Department of Chemistry,Collegeof Science, Universityof Texasat El Paso,TX 79968,U.S.A. (Recbd
in USA 29 Ociober
1974;Receivedin UK forpublication
IO March
1975)
Aimtmct-Re first ionization potentials, E,, of substituted chromium(II1)acctylacetonates are found to be a linear function of the sum of Hammett’s o, and O, constants for substitnents in various positions of the pseudo-aromatic rings, relative to the Cr atom. The equation for the regression is E, = 7.40t @380& * 0.02 eV, where 7.40 eV is the interpolated value for the trisCr chelate of malonaldehyde, and the slope of the correlation line is 0.380.From the value of the slopeandother conside~ti~s, it is deduced that the ejected electron is an unpaired d&r electron in a t-MO.
We wish to report the second correlation of molecular ionization potentials of pseudo aromatic org8nometaUic chelate compounds with a set of substituent constants. Recently, we have demonstrated that the ionization energies of the copper acetylacetonates’ and many series of organic compounds’ are linear functions, respectively, of Hammett’s substituent constants’ and of T8ft’s inductive’ substituent const8nts, 01. This permits one to calculate by means of a simple single-parameter equation, to a good ~prox~ation, values of the ionization potentials of compounds which yield a low abundance of the primary cation radical, or are diflicult to obtain in a purified state, or vaporize only with decomposition. We show here that the ioni~tion ener@es of substituted tris (B-keto enolate) complexes of Cr(In),
\ C/
R’\C - 0 *-CC
Cl/3
R/c-o are a linear function of Hammett’s substituent constants.3 The first ionization energy of these chelates corresponds
to the removal of an electron from the highest occupied
molecular orbitat,’ presumably from the n-system or one of the three unpaired electrons in the d,, -type orbi& of Cr, remaining after formation of its hybrid d’sp” orbitals. It is quite obvious that electron releasing alkyl groups should increase the electron density of the hetero~om8tic ring and thereby facilitate electron removal, whereas electron-withdrawinggroups such as -NoZ and -Cl should increase the requisite ionization energy, as well 8s decrease the basic&y and gas-phsse proton afBnity.7 It is seen from the position of the substituents in the above formulation that the appropriate substituent constants for R and R’ should be the a,,, parameters derived originally for meto substi~ents on the benzene ring,’ and the corresponding uP constants for X. It should be noted that CT,.,, like PI, is a measure of the inductive influence of a substituent.’ Table 1 presents the Za values (summed over all nine substituents in the three rings) together with the experimentally observed’ electron impact ionization potentials (IQ for a series of the chromium (III) chelates. Despite contentions to the contrary for the coppe? and c~omium” acetyl8~etona~s, a piot of the & vafues us &r results in an excellent straight line, the equation for which is: EC,+ = EC,+ t aXa. (1) The siope, a, is found to be 0.380 eV and the value &_a,
*Part I should be considered as Ref. 1.
Table 1. Ring substituents and their correspndii 20 values, along with experimental and calculated ionization potentials of various substituted ~tyl~~tonat~~mi~(II~ derivatives
2
QI3
8, nr SO2
M.60
7.61
7.63
3
a3
8, *)2'II62
+l.33
?.91
7.92
4
=3
"D2*m2. mD2
+2.29
0.22
6.22
3
4
cl, Cl, Q
W.b9
__=
7.39
6 7 6
II f-Pt i_?C
I, I, II 8. I* 1 m2* Bs2*m2
0 -0.42 il.92
-e
7.10 7.2b 8.13
-9 -=
*amva2mu 2lwmLr. 3cr It- 0.01k - -0.031fra mat.24: i-h - -0.07; "9 Vet
rra Ed. *Ia-
Irt.6. =Sxphmti
wlw
o.o;mO,-44.7761 cl-40.227. "tdw not amilebl.,
2355
6tpl
2356
L. S.
LEVIlT
and B. W. LEVIIT
which corresponds to the tris-Cr chelate of malonaldehyde, CH#ZHOh, is by interpolation 74OeV, and therefore we have E.+R = 740 + 0.380 Xa 2 0.02 eV.
(2)
This correlation (r = 0998) indicates that the effect of substituents on the electron density in the chelate ring and at the Cr atom is primarily an inductive one, and lends support to the view that these chelates behave as heteroaromatic compounds.” In Table 1 we show a comparison between the experimentally obtained ionization energies and those values of & calculated by means of eqn (2). The agreement is seen to be very good. Also included in Table 1 are the interpolated Ei values for the trisCr malonaldehyde complex (cpd. 6) and the calculated & values for trisdiisopropylacetylacetonatochromium(III) (cpd. 7), tris-3, Sdiisopropyl4nitroacetylacetonatochromium(HI) (cpd. 8), and the tris-4-chIoroacetylacetonato complex (cpd. S), for none of which experimental data are available t . Although the u constant has units of energy (eV), it is analogous to the reaction constant, p, and reflects and susceptibility of the reaction site, the n-system or the d&J chromium electron to substituent effects. The u value of 0.380 obtained here may be compared to the corresponding u value of the E,‘s of the bisacetylacetonates of Cu(I1) which was found’ to be I.00 and of alkylbenzenes” and alkylpyridines” which we have found to be 4.41 and 49OeV, respectively. Substitution of Na for H in benzene leads to an increase of 068 eV (9X2-9.24),” whereas replacement of the three p-H atoms of the Cr chelates by three p-N02 groups results in a raising of the & by 0.83 eV (8.22-7.39) or 0*28eV/Na group. This fact, in addition to the much smaller a value for the Cr series compared to the Cu series and the benzene and pyridine series, indicates that the Cr chelates are considerably less sensitive to inductive effects than any of the latter. The unpaired d,, Cr electron, in a 1%MO, is the one almost certainly expelled from its orbital, which was thought to be delocalized” over the entire ?r-system, encompassing the Cr atom and all three chelate rings. Loss of the ionized electron by the n-system of the rings would be in accord with Barnum’s HMO calculations” and with the view expressed by Pearson et 01.‘~that ionization of acetylacetonates involves an orbital spread over the ligand. There is also evidence from NMR data on other paramagnetic acetylacetonates in which the spin density of the unpaired electron is delocalized and finds its way to an extent of around 10%into the spin density of the para H atoms, shifting the resonance frequency compared to the diamagnetic complexes.” Since there are sixteen atoms in the 3-ring system of the Cr chelate, one might assume that approximately 56% of the spin density of the 3 unpaired to electrons (d,,, d,,, dy2)might reside at each of the atoms in the rings. Gn the other hand, there is much stronger, indeed unequivocal, evidence that the site of electron loss is from an orbital largely dominated by the Cr atom*: the fact that the correlation is obtained with the appropriate mefa and tNot included in Table 1is the value for the tris-tritluoromethyl substituted complex for which the calculated & value is 8.5 eV, whereas the value reported” is 9.5 eV. which is a vertical $ and was obtained by a different method (PES).
para substituent constants, that all substituents in the rings were given equal weights, and that the only atom equidistant from all the substituents is the Cr atom; the fact that the a value is only one-thirteenth of that obtained’* in the pyridine correlation (a perfectly analogous situation) indicates that the reaction site is relatively distant from the substituents and cannot therefore be the n-system (compare al=28@’ for RIG: but 14@ for R2C= 6:, where the electron in both cases is lost from a non-bonding oxygen lone pair); the fact that substitution of Cr for the enol H atom of acetylacetone (El = 8.87 eV)16 lowers the El, whereas a higher value would be expected if a ligand electron were lost, since C? withdraws electron density from the ring. Additional evidence that the metallic center is most certainly the site of electron loss can be adduced from the oxidation of metallocenes and metal arenes where the ferrocenium, nickelocenium and his -benzene chromium cations are thought to have the positive center at the metal atom.‘*This view is also supported by data and arguments for Cr chelates given by LloydI and by Evans et u/.,~ and for Cu chelates by McGarveJ’; and finally by the analogous reaction of photoionization of chlorophyll where, in the “photoelectric” or “photoionization” theo# of photosynthesis, much evidence has been amassed= for the central Mg atom as the site of initial electron loss. REFERENCES
‘B. W. Levitt and L. S. Levitt, 1. Coord. Chem. 3, 187 (1973); Chem. d! Ind. 302 (1974). “L. S. Levitt and B. W. Levitt, Ibid, 990 (1970):bB. W. Levitt and L. S. Levitt, Experiento 26, 1183(1970);‘B. W. Levitt and L. S. Levitt. Ismell. Chem. 9.71 (1971):“L. S. Levitt and B. W. Levitt, J. &g. Chem. 37, 332 (1972);“‘B. W. Levitt and L. S. Levitt, Israel J. Chem. 9, 711 (1971);‘L. S. Levitt and B. W. Levitt, Chem. & fnd. 132(1973);‘L. S. Levitt and C. Parkanyi, Inf. 1.. Sulj C/rem.,8,329 (1973);‘L. S. Levitt, B. W. Levitt and C. PBrkanyi, Tetrahedron 28.3369(1972);‘L. S. Levitt, C/tern.&? Ind. 637(1973);‘H. Widing and L. S. Levitt, Tetrahedron 30,611 (1974);*L. S. Levitt and B. W. Levitt, Ibid., 29, 941 (1973);‘B. W. Levitt and L. S. Levitt, C/tern. & Ind. I85 (1973);“B. W. Levitt, H. Widingand L. S. Levitt, Con. J. Chem.51,3%3 (1973); “B. W. Levitt and L. S. Levitt, Chem. de Ind. 724(1973);“B. W. Levitt, H. Widing and L. S. Levitt, Ibid, 793 (1973). “L. P. Hammett. Phvsical Orzonic Chemistnt (2nd Edn.). D. 356,McGraw-Hih, New York, (i970); ‘J. Hine,>hysicol O&o& Chemistry, (2nd Edn.), p. 89, McGraw-Hill. New York, (1%2); ‘D. H. McDaniel and H. C. Brown, I. Org. Chem. 23,420 (1958); “S. K. Dayal, S. Ehrenson and R. W. Taft, Jr., J. Am. Chem. Sot. 94, 9113 (1972). ‘R. W. Taft and 1. C. Lewis, Tetrahedron 5, 210 (1959). T. Koopmans, Physica 1, 104(1933);J. C. Lorquet. Rev. Mod Phys. 32,312 (1960); R. S. Mulliken,1. Chem. Phys. 3,564 (1935); Phys. Rev. 74,736 (1948). “J. E. Huheey, Inorganic Chemistry Chap. 8, Harper & Row, New York, (1972). ‘L. S. Levitt and B. W. Levitt, 1. Phys. Chem. 74, 1812(1970); Tetrahedron 27, 3777 (1971). “M. M. Bursey and P. F. Rogerson, Inorg. Chem. 9,676 (1970).
‘H. F. Holtzclaw, Jr., L. Lintvedt, H. E. Baumgarten, R. G. Parker, M. M. Bursey and P. F. Rogerson, J. Am. Chem. Sot. 91, 3774 (1969). “G. M. Bancroft, C. Reichert, J. B. Westmore and H. D. Gesser, Inorg. Chem. 8,474 (1969). “J. P. Collman, R. A. Moss, H. Maltz and C. C. Heindel. 1. Am. Chem. Sot. 83,531(1%1); R. H. Holm and F. A. Cotton, Ibid 80, 5658 (1958).
“L. S. Levitt, B. W. Levitt and C. Parkanyi, unpublished work. “K. Watanabe, T. Nakayama and J. Mottl, 1. Quanr. Specl. Rad. Transfer 2, 369 (1962).
Correlation of the ionization potentials of pseudo-aromatic metal complexes “A. A. Vlcek, Prog. Inog. Chem. 5, 370 (1963). “D. W. Barnum, I. Inorg. NucL Chem. 21, 221 (l%l); 22, 183 (1961). But see also E. A. Magnusson, K. A. Thomson and A. G. Wedd, Chem. Comm. 842 (1%9). ‘“S. M. Schildcrout, R. G. Pearson and F. E. Stafford, J. Am. Chem. Sot. 90.4006 (1968). “M. C. Day and J. Selbin, Theoreficuf Inorganic Chemistry (2nd Edn,) p. 436. Reinhold (1969). “M. Cais and M. S. Lupin, Adu. Orgonomelallic Chem. 8, 211 (1970).
2357
?I. R. Lloyd, Chun. Comm. 868 (1970); Jnf. J. Mass Spectr. Jon P/l& 4, slm (1970). ‘OS. Evans, A. Hamnett and A. F. Orchard, I. Co& Chn. 2.57 (1972). “A. H. Maki and B. R. McGarvey, I. Chem. P/~ys.29,3l(l958). “L. S. Levitt, Science 118, 6% (1953). “L. S. Levitt, Ibid, 128,33 (1954); Bull. Amer. Phys. Sot. II, 4,376 (1959); 1. Chem. Phys. ts, 515 (1958); Erperienlia 15, 16 (1959); Appl. Specfros 14, I61 (1960). “M. Charton, J. Org. Chem. 30. %9 (1965).
in USA 29 Ociober
1974;Receivedin UK forpublication
IO March
1975)
Aimtmct-Re first ionization potentials, E,, of substituted chromium(II1)acctylacetonates are found to be a linear function of the sum of Hammett’s o, and O, constants for substitnents in various positions of the pseudo-aromatic rings, relative to the Cr atom. The equation for the regression is E, = 7.40t @380& * 0.02 eV, where 7.40 eV is the interpolated value for the trisCr chelate of malonaldehyde, and the slope of the correlation line is 0.380.From the value of the slopeandother conside~ti~s, it is deduced that the ejected electron is an unpaired d&r electron in a t-MO.
We wish to report the second correlation of molecular ionization potentials of pseudo aromatic org8nometaUic chelate compounds with a set of substituent constants. Recently, we have demonstrated that the ionization energies of the copper acetylacetonates’ and many series of organic compounds’ are linear functions, respectively, of Hammett’s substituent constants’ and of T8ft’s inductive’ substituent const8nts, 01. This permits one to calculate by means of a simple single-parameter equation, to a good ~prox~ation, values of the ionization potentials of compounds which yield a low abundance of the primary cation radical, or are diflicult to obtain in a purified state, or vaporize only with decomposition. We show here that the ioni~tion ener@es of substituted tris (B-keto enolate) complexes of Cr(In),
\ C/
R’\C - 0 *-CC
Cl/3
R/c-o are a linear function of Hammett’s substituent constants.3 The first ionization energy of these chelates corresponds
to the removal of an electron from the highest occupied
molecular orbitat,’ presumably from the n-system or one of the three unpaired electrons in the d,, -type orbi& of Cr, remaining after formation of its hybrid d’sp” orbitals. It is quite obvious that electron releasing alkyl groups should increase the electron density of the hetero~om8tic ring and thereby facilitate electron removal, whereas electron-withdrawinggroups such as -NoZ and -Cl should increase the requisite ionization energy, as well 8s decrease the basic&y and gas-phsse proton afBnity.7 It is seen from the position of the substituents in the above formulation that the appropriate substituent constants for R and R’ should be the a,,, parameters derived originally for meto substi~ents on the benzene ring,’ and the corresponding uP constants for X. It should be noted that CT,.,, like PI, is a measure of the inductive influence of a substituent.’ Table 1 presents the Za values (summed over all nine substituents in the three rings) together with the experimentally observed’ electron impact ionization potentials (IQ for a series of the chromium (III) chelates. Despite contentions to the contrary for the coppe? and c~omium” acetyl8~etona~s, a piot of the & vafues us &r results in an excellent straight line, the equation for which is: EC,+ = EC,+ t aXa. (1) The siope, a, is found to be 0.380 eV and the value &_a,
*Part I should be considered as Ref. 1.
Table 1. Ring substituents and their correspndii 20 values, along with experimental and calculated ionization potentials of various substituted ~tyl~~tonat~~mi~(II~ derivatives
2
QI3
8, nr SO2
M.60
7.61
7.63
3
a3
8, *)2'II62
+l.33
?.91
7.92
4
=3
"D2*m2. mD2
+2.29
0.22
6.22
3
4
cl, Cl, Q
W.b9
__=
7.39
6 7 6
II f-Pt i_?C
I, I, II 8. I* 1 m2* Bs2*m2
0 -0.42 il.92
-e
7.10 7.2b 8.13
-9 -=
*amva2mu 2lwmLr. 3cr It- 0.01k - -0.031fra mat.24: i-h - -0.07; "9 Vet
rra Ed. *Ia-
Irt.6. =Sxphmti
wlw
o.o;mO,-44.7761 cl-40.227. "tdw not amilebl.,
2355
6tpl
2356
L. S.
LEVIlT
and B. W. LEVIIT
which corresponds to the tris-Cr chelate of malonaldehyde, CH#ZHOh, is by interpolation 74OeV, and therefore we have E.+R = 740 + 0.380 Xa 2 0.02 eV.
(2)
This correlation (r = 0998) indicates that the effect of substituents on the electron density in the chelate ring and at the Cr atom is primarily an inductive one, and lends support to the view that these chelates behave as heteroaromatic compounds.” In Table 1 we show a comparison between the experimentally obtained ionization energies and those values of & calculated by means of eqn (2). The agreement is seen to be very good. Also included in Table 1 are the interpolated Ei values for the trisCr malonaldehyde complex (cpd. 6) and the calculated & values for trisdiisopropylacetylacetonatochromium(III) (cpd. 7), tris-3, Sdiisopropyl4nitroacetylacetonatochromium(HI) (cpd. 8), and the tris-4-chIoroacetylacetonato complex (cpd. S), for none of which experimental data are available t . Although the u constant has units of energy (eV), it is analogous to the reaction constant, p, and reflects and susceptibility of the reaction site, the n-system or the d&J chromium electron to substituent effects. The u value of 0.380 obtained here may be compared to the corresponding u value of the E,‘s of the bisacetylacetonates of Cu(I1) which was found’ to be I.00 and of alkylbenzenes” and alkylpyridines” which we have found to be 4.41 and 49OeV, respectively. Substitution of Na for H in benzene leads to an increase of 068 eV (9X2-9.24),” whereas replacement of the three p-H atoms of the Cr chelates by three p-N02 groups results in a raising of the & by 0.83 eV (8.22-7.39) or 0*28eV/Na group. This fact, in addition to the much smaller a value for the Cr series compared to the Cu series and the benzene and pyridine series, indicates that the Cr chelates are considerably less sensitive to inductive effects than any of the latter. The unpaired d,, Cr electron, in a 1%MO, is the one almost certainly expelled from its orbital, which was thought to be delocalized” over the entire ?r-system, encompassing the Cr atom and all three chelate rings. Loss of the ionized electron by the n-system of the rings would be in accord with Barnum’s HMO calculations” and with the view expressed by Pearson et 01.‘~that ionization of acetylacetonates involves an orbital spread over the ligand. There is also evidence from NMR data on other paramagnetic acetylacetonates in which the spin density of the unpaired electron is delocalized and finds its way to an extent of around 10%into the spin density of the para H atoms, shifting the resonance frequency compared to the diamagnetic complexes.” Since there are sixteen atoms in the 3-ring system of the Cr chelate, one might assume that approximately 56% of the spin density of the 3 unpaired to electrons (d,,, d,,, dy2)might reside at each of the atoms in the rings. Gn the other hand, there is much stronger, indeed unequivocal, evidence that the site of electron loss is from an orbital largely dominated by the Cr atom*: the fact that the correlation is obtained with the appropriate mefa and tNot included in Table 1is the value for the tris-tritluoromethyl substituted complex for which the calculated & value is 8.5 eV, whereas the value reported” is 9.5 eV. which is a vertical $ and was obtained by a different method (PES).
para substituent constants, that all substituents in the rings were given equal weights, and that the only atom equidistant from all the substituents is the Cr atom; the fact that the a value is only one-thirteenth of that obtained’* in the pyridine correlation (a perfectly analogous situation) indicates that the reaction site is relatively distant from the substituents and cannot therefore be the n-system (compare al=28@’ for RIG: but 14@ for R2C= 6:, where the electron in both cases is lost from a non-bonding oxygen lone pair); the fact that substitution of Cr for the enol H atom of acetylacetone (El = 8.87 eV)16 lowers the El, whereas a higher value would be expected if a ligand electron were lost, since C? withdraws electron density from the ring. Additional evidence that the metallic center is most certainly the site of electron loss can be adduced from the oxidation of metallocenes and metal arenes where the ferrocenium, nickelocenium and his -benzene chromium cations are thought to have the positive center at the metal atom.‘*This view is also supported by data and arguments for Cr chelates given by LloydI and by Evans et u/.,~ and for Cu chelates by McGarveJ’; and finally by the analogous reaction of photoionization of chlorophyll where, in the “photoelectric” or “photoionization” theo# of photosynthesis, much evidence has been amassed= for the central Mg atom as the site of initial electron loss. REFERENCES
‘B. W. Levitt and L. S. Levitt, 1. Coord. Chem. 3, 187 (1973); Chem. d! Ind. 302 (1974). “L. S. Levitt and B. W. Levitt, Ibid, 990 (1970):bB. W. Levitt and L. S. Levitt, Experiento 26, 1183(1970);‘B. W. Levitt and L. S. Levitt. Ismell. Chem. 9.71 (1971):“L. S. Levitt and B. W. Levitt, J. &g. Chem. 37, 332 (1972);“‘B. W. Levitt and L. S. Levitt, Israel J. Chem. 9, 711 (1971);‘L. S. Levitt and B. W. Levitt, Chem. & fnd. 132(1973);‘L. S. Levitt and C. Parkanyi, Inf. 1.. Sulj C/rem.,8,329 (1973);‘L. S. Levitt, B. W. Levitt and C. PBrkanyi, Tetrahedron 28.3369(1972);‘L. S. Levitt, C/tern.&? Ind. 637(1973);‘H. Widing and L. S. Levitt, Tetrahedron 30,611 (1974);*L. S. Levitt and B. W. Levitt, Ibid., 29, 941 (1973);‘B. W. Levitt and L. S. Levitt, C/tern. & Ind. I85 (1973);“B. W. Levitt, H. Widingand L. S. Levitt, Con. J. Chem.51,3%3 (1973); “B. W. Levitt and L. S. Levitt, Chem. de Ind. 724(1973);“B. W. Levitt, H. Widing and L. S. Levitt, Ibid, 793 (1973). “L. P. Hammett. Phvsical Orzonic Chemistnt (2nd Edn.). D. 356,McGraw-Hih, New York, (i970); ‘J. Hine,>hysicol O&o& Chemistry, (2nd Edn.), p. 89, McGraw-Hill. New York, (1%2); ‘D. H. McDaniel and H. C. Brown, I. Org. Chem. 23,420 (1958); “S. K. Dayal, S. Ehrenson and R. W. Taft, Jr., J. Am. Chem. Sot. 94, 9113 (1972). ‘R. W. Taft and 1. C. Lewis, Tetrahedron 5, 210 (1959). T. Koopmans, Physica 1, 104(1933);J. C. Lorquet. Rev. Mod Phys. 32,312 (1960); R. S. Mulliken,1. Chem. Phys. 3,564 (1935); Phys. Rev. 74,736 (1948). “J. E. Huheey, Inorganic Chemistry Chap. 8, Harper & Row, New York, (1972). ‘L. S. Levitt and B. W. Levitt, 1. Phys. Chem. 74, 1812(1970); Tetrahedron 27, 3777 (1971). “M. M. Bursey and P. F. Rogerson, Inorg. Chem. 9,676 (1970).
‘H. F. Holtzclaw, Jr., L. Lintvedt, H. E. Baumgarten, R. G. Parker, M. M. Bursey and P. F. Rogerson, J. Am. Chem. Sot. 91, 3774 (1969). “G. M. Bancroft, C. Reichert, J. B. Westmore and H. D. Gesser, Inorg. Chem. 8,474 (1969). “J. P. Collman, R. A. Moss, H. Maltz and C. C. Heindel. 1. Am. Chem. Sot. 83,531(1%1); R. H. Holm and F. A. Cotton, Ibid 80, 5658 (1958).
“L. S. Levitt, B. W. Levitt and C. Parkanyi, unpublished work. “K. Watanabe, T. Nakayama and J. Mottl, 1. Quanr. Specl. Rad. Transfer 2, 369 (1962).
Correlation of the ionization potentials of pseudo-aromatic metal complexes “A. A. Vlcek, Prog. Inog. Chem. 5, 370 (1963). “D. W. Barnum, I. Inorg. NucL Chem. 21, 221 (l%l); 22, 183 (1961). But see also E. A. Magnusson, K. A. Thomson and A. G. Wedd, Chem. Comm. 842 (1%9). ‘“S. M. Schildcrout, R. G. Pearson and F. E. Stafford, J. Am. Chem. Sot. 90.4006 (1968). “M. C. Day and J. Selbin, Theoreficuf Inorganic Chemistry (2nd Edn,) p. 436. Reinhold (1969). “M. Cais and M. S. Lupin, Adu. Orgonomelallic Chem. 8, 211 (1970).
2357
?I. R. Lloyd, Chun. Comm. 868 (1970); Jnf. J. Mass Spectr. Jon P/l& 4, slm (1970). ‘OS. Evans, A. Hamnett and A. F. Orchard, I. Co& Chn. 2.57 (1972). “A. H. Maki and B. R. McGarvey, I. Chem. P/~ys.29,3l(l958). “L. S. Levitt, Science 118, 6% (1953). “L. S. Levitt, Ibid, 128,33 (1954); Bull. Amer. Phys. Sot. II, 4,376 (1959); 1. Chem. Phys. ts, 515 (1958); Erperienlia 15, 16 (1959); Appl. Specfros 14, I61 (1960). “M. Charton, J. Org. Chem. 30. %9 (1965).