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Electron spin resonance measurements of dibenzenechromium cation
Volume
3. number
1
CHEMICAL
ELECTRON OF
SPIN
PHYSICS
January
LETTERS
RESONANCE
1969
MEASUREMENTS
DIBENZENECHROMIUM
CATION
R. PRINS and F. J.REINDERS IConinklijke/Shell-Laboraton’um, Amsterdam (Skell Research N. V.) Received
6 January
1969
ESR spectra of liquid and solid solutions of Cr(C H6)$ have been measured. Even in solid solution the spectr+a show a well resolved proton hyperfine s f ructure which is explained by rotation of the Cr(C6Hd2 cation around its main axis. An analysis of the 62Cr hyperfine coupling constants indicates results agree fairly well with that the unpaired electron is in the alg (3ds2) orbital. The experimental the results of an e.xtended Hilckel MO calculation.
1. INTRODUCTION Althou@ the literature mentions on the liquid solution ESR spectrum
some papers of Cr(C6H6);
[l] to our knowledge as yet no ESR spectrum of a solid solution or single crystal of Cr(C6H6); has been published nor a quantitative analysis of the ESR spectra been given. Such an analysis would be of interest not only for the e.xplanation of the proton hyperfine coupling constant of Cr(C6H6); but also for a better understanding of the molecular orbital energy level scheme. In this publication we therefore give an analysis of our ESR data of liquid and solid solutions of Cr(C6H6)& 2. EXPERIMENTAL We have measured ESR spectra of dilute solutions of Cr(C6H6)2I in the solvents acetone, alcohol, dimethyl formamide-chloroform (1:l) and ethylene glycol-water (1:l). The ESR spec; tra of liquid solutions of these solvents are all alike. Fig. 1 shows the X-band spectrum of a liqilid solution of DMF-CHC15. This spectrum can be interpreted straightforwardly in terms of an interaction between the unpaired electron spin and the nuclear spins of the twelve protons of the benzene rings. The satellite lines at low field are due to the 53Cr isotope. The corresponding high field lines are only detectable with a higher amplification due to anomalous relaxation of the hyperfine lines [ 21. The ESR spectrum of a solid solution of
Fig. 1. ESR X-band spectrum of a liquid solution Cr(CgH6)2I in D&IF-CHC12 (1:l) at 200OK.
of
characteristic of an molecule. The somewhat unexpected hyperfine structure due to the protons can be resolved for the x,y peak as well as for the z peak (see fig. 2). The resolution of the proton hyperfine structure depends on the solvent system used. In the mixture DMF-CHCl2 (1:l) the resolution is good and independent of temperature up to 1600K where the DMF-CRC32 glass begins to soften. In solid solutions of acetone, alcohol and a glassy ethylene glycol-water (1:l) mixture the resolution is not so good and Cr(C#ifjj+2 axially
shows
two peaks
symmetric
45
Volume 3. number 1
CHEMICAL PHYSICS LETTEPS
January 1969
around the x, y peak, so that BCr can be evalu-
ated from this hyperfine structure. In the spectra no indication has been found of the axial 5%.Zr
-H
hyperfine structure. The spin-Hamiltonian parameters derived from the ESR spectra, measured with a Varian X-band spectrometer with 100 kHz field modulation, are given in table 1. The values for the liquid solution given in the third row are in good agreement with published values [l]. To determine the sign of the isotropic proton hyperfine coupling constant oH we have measured wide-line NMR spectra of pofycrystalline samples Of Cr(C6H6)$. Water Was USf?das an eXternal reference and the measurements were conducted at 3OOoK on a Varian DP60 spectrometer operating at 56.4 MHz. The peak of the protons in Cr(C6H6)3I is shifted 250 j, 10 ppm to lower field compared with the water peak and the line width is 160 tt 10 ppm. This shows that the sign of eH _- is positive. With the formula AZZ = _a Ye@%+ H YN we caIcuIate SH = +3.44 -i:0.15 G which nicely agrees with the value obtained from the ESR measurements.
Fig. 2. ESR X-band spectrum of a glassy solidsolution fif fl-(CgHs)21 in D&IF-CHC12(1:l) at 120°K. 3. DISCUSSION depends on temperature. At lower temperatures the hyperfine lines are broadened and the structure on the z peak is no longer observable. A similar behaviour has emerged from the NMR and ESR spectra of Cr(C6H6)z [3], Fe(C5H5)3 (47, benzene [5] and CnHn aromatic radicals {S]. This behaviour in the solid state is explained by a rotation around the 2 axis which freezes out at low temperature. The rotation averages out the anisotropy between the proton Ax and Ay values, with the result that the value &4x+Ay) is observed in the hyperfine structure on the x, y peak. In agreement with this explanation we have frllnd no proton hyperfine structure in the soli> solution spectrum of Cr(C6H5 - CH,$: the methyl groups prevent rotation. For tne rest the ERR spectra of the solid ar?d liquid SI It&ions of CR(C6H5 -CH3)i are equal to tho:;e of Cr(C6H6);. At the high-field sideof the solid solution Spectrum Of Cr(C6H6); Satellite lines belonging to one of the four s3Cr lines and split up by the protons are observed. In the spectrum of Cr(C6D6); we have a&o observed the other three 53Cr lines and from this spectrum it became clear that these four 53Cr lines are centred 46
To obtain an insight into the MO level order:lg Of Cr(C#ifj)$ we have carried Out an ex-
tended Htickel calculation for this cation. For the details of this semi-empirical MO method we refer the reader to a previous publication on metallocenes M(C5H5)3 [7]. The geometry of Cr(C6H6)f was chosen to be that of Cr(C6H6)3 and geometric parameters for the latter were obtained from the structural data of HaaIandr8f. The computed net charges are: Cr co*3g, C +0-03 and H +0.03_ In tabIe 2 we have listed the calculated energies and coefficients of the highest filled and lowest empty orbitdls. The coefficients for the molecular orbitals are expressed in atomic symmetry orbitals cpand x ]‘I]. The outcome of our calculation was that the -unpaired electron is in the alg orbital with predominant 3d,3 character. MO cakuktions on V(C6H6)3 f9], which is isoelectronic with Cr(C6H6ff, and on Cr(C!6H6)2 [lo] and Fe(C5H& [7,9,1x jz, which have one ekCtrOn more than Cr(C6Hg)f2, alSO give the aIg orbital as the highest occupied orbital. The ESR results on Cr(C6H6)i can be ex-
plained as the unpaired electron being in the alg
Volume 3, number 1
CHEMICAL PHYSICS LETTERS
Table 1 Spin-Hamiltonian parameters of Cr(CsH&,
January 1969
obtained from the ESR spectra.
glt = 2.0023 + 0.0005
/AzH] = 3.1 k 0.1
g-t = 1.9785 * 0.0005 g
= 1.9865 f 0.0005
Energies
and coefficients Orbital tvve
Symmetry
eag
?T,
= 26.9 zt 0.1 G
aCr
= 18.1 f 0.1 G
31%H
'Ei (eV) 4.34
Wd) -0.70
empty molecular
(p(4S)
x (2%)
-
0.85
* e%
3d
4.81
ezu
77
5.80
ab
3d
1
7.64
1.01
-0.12
0.15
e2g
3d,a
4
9.19
0.73
-
0.53
orbital, since the g values are close to the free electron value so that the ground state configuration of Cr(C6R6)$ must be orbitally non-degenerate. With orbital coefficients and energy differences obtained in the extended Hiickel calculation we calculate glt = 2.0023 and gl = 1.97 for the 2Alg(alg) configuration, which is in good agreement with experiment. The 53Cr hyperfine coupling constants provide quantitative information on the metal character of the alg orbital. lf we write the alg orbital as (alg;)
= crl/3dz2 > + a2l4s)
+ o31(4lig)
the following equations can be derived for the 53Cr hyperfine coupling constants ]12] A ‘Ais
+ $ ~rfP - +(g-L - 2.0023) P
B =Aiso
- $ CYfP + E(g I - 2.0023) P
cz = f(A+
2B) =Aiso
+ (g - 2.0023) P
where P = 2.0023g~j3~p~ < r-3>, pe and &,I are the Bohr and nuclear magnetons, gN is the nuclear g factor and Aiso is the isotropic contact term. For the evaluation of “1 we should have to consider four possibilities for the signs of a and B since ESR experiments only give the absolute values of hyperfine coupling constants. From the observed anomalous relaxation of the 53Cr lines, however, information can be obtained
1.02
+A,,H( = 3.64 I 0.05 G IoHI =3.46iO.O5G
Table 2 of the highest filled and lowest
No. of eiectrons
3d
jBCrj
G
-
-0.52
1.00
orbitals
of CrfC
x (Woo)
XW,)
-6);
. X(lSH)
0.10
-0.01
0.29
-0.12
0.06
0.01
-0.00
-0.01
-0.06
0.13
-0.01
0.03
-0.08 0.15
-0.01
about the relative signs of a and B. The condition for broadening of the high-field lines compared with the low-field lines is a(a -B) V X (911 -gI)< 0 [Z]. With the experimental g. la1 and 1BI values (see table 1) this condition can only be fulfilled if a and B have the same sign.
ln that case we derive from the equations given above that (cy: + 0.034) P = *30.8, where the plus sign holds for botha and B negative and the minus sign for both a and B positive. But since P is negative the only possibility is that both a and B are positive. With a = +18.1 G and B = s26.9 G we calculate A = +0.5 G. The axial 53Cr hyperfine structure will thus be situated near the top of the z peak in the ESR spectrum and will certainly be hidden under the z peak. This explains our inability to observe the axial 53Cr hyperfine structure. To be able to evaluate arl from the equation (~4 t 0.034) P = -30.8 we must know the value of P. Values for P for different valence state= of the chromium atom have been tabulated b) McGarvey [12]. Guided by the extended Hifckel calculation given above we estimate the net charge on the chromium ,atom to be within the range +0.05 to +l.O. For the !-ordering values we calculate: cr+O. 5 cl!1 = 1.02 P = -28.9 G Cr+l.O
P = -31.5 G
crl = 0.98
The outcome of our analysis is that acl is \ery 47
Volume
3, numbx
CHEMICAL
1
PHYSICS
near 1.00 and that the alg orbital is essentially a chromium 3d,2 orbital. This result does not exclude the possibility that the coefficients of the ligand part of the alg MO have small non-zero values (see table 2). Information on these ligand coefficients can be obtained from an analysis of the proton hyperfine coupling constants. The isotropic hyperfir?e coupling constant A iso originates from an exchange polarization contrlb&on and a contribution of the unpaired spin density in the 4s atomic orbital. The value of A iso = 17.6 G does net deviate much from the value for chromium in an octahedral environment [12], where no 4s contribution can occur. We therefore conclude that the 4s contribution to the alg orbital is small ((~2 < 0.1). For vanadocene, too, it has been found that the 4s contribution to the alg orbital is small [13,14] and these results show that the type of hybridization between the 3dZ2 and 4s atomic orbitals as assumed by Moffitt 1151does not occur in sandwich compounds. The calculated energy level order given in table 2 shows two interesting characteristics. First
the energy
difference
between
Che alg
and
orbitais is calculated to be 1.6 eV, which agrees qualitatively with the value of 2.1 eV obtained in a ligand field analysis of the lowest energetic band in the absorption spectrum of Cr(C6H6); il6]. The energy difference of 2.1 eV is much larger than that found in metallocenes: 0.6 eV [14] and indicates a large participation of the e2 ligand orbitals in the bonding of the complex, which is also reflected in Cr(C6a6); the e2g orbital coefficients (see table 2). This result confirms the curreni idea that back donation via the e2g orbital is more important in the dihenzene complexes than in the dicyclopentadienyl complexes [l?]. The second characteristic point in table 2 is t$at the e2u orbital has a iower energy than the elg orbital contrary to the result for metallocenes [‘7]. In the extended Hiickel method, however, not too much confidence should be placed in the result of a comparison of two orbitals connected with a different charge distribution such as the e2u (ligand) and e;g (metal) orbital. e2g
li****
LETTERS
January
1969
Therefore the energy order calculated does not necessarily imply that in the electronic absorption spectrum of Cr(C6H6); charge transfer bands occur at a lower energy than the d-d bands. No unequivocal analysis of the electronic absorption spectrum of dibenzenechromium complexes has been published and in view of the uncertainties in the assignment cf the charge transfer bands in the electronic absorption spectrum of ferrocene [18] the value of such an attempt seems doubtful.
REFERENCES 1. R. D. Feltham. P. Sogo and M. Calvin, J. Chem. Phys. 26 (1957) 1354: S. I. Vetchinkin. S. P. Solodovnikov and V. M. Chibrikin. Ont. Suectrosc. 8 (19601 71: K. H. Fia&ser: Naturwiss.‘48 (i961) 426,666. 2. R.N. Rogers and G. E. Pake. J. Chem. Phys. 33 (1960) iio7: E. De Boer and E. L.Mackor. J. Chem. Phys. 38 (1963) 1450. J. 3. L. N. Mulay. E.G. Rochow and E. 0. Fischer, Inorg. Nucl. Chem. 4 (1957) 231. J. Chem. Phys. 30 4. C.H. Holm and J A.Ibers. (1959) 885. 5. E. R. Andrew and R. G. Eades, Proc. Roy. Sot. A218 (1953) 537. D. E. Wood and H. M. McConnell. 6. H. J. Silverstone, J. Chem. Phys. 41 (1964) 2311. R. Prins and P. Ros, In7. J. H. Schachtschneider. org. Chim. Acta 1 (1967) 462. 8. A. Haaland. Acta Chem. Stand. 19 (1965) 41. Theoret. Chim. Acta 1 (1963) 418. 9. R. D. Fischer, 10. E. ill. Shustorovich and M. E. Dyatkina. J. Struct. Chem. USSS 2 (1961) 40. D. G. Carroll and S.P. McGlvnn. 11. A.T. Armstronn. J. Chem. Phys.-k7 (1967) 1104. 12. B. R. RIcGarvey, J. Phys. Chem. 71 (1967) 51. P. Biloen and J. D. W-Van Voorst, J. 13. R. Prins. Chem. Phys. 46 (1967) 1216. J. Chem. Phys. 14. R. Prins and J. D. W. Van Voorst. 49 (1968) 4665. J. Am. Chem. Sot. 76 (1954) 3386. 15. W.Moifitt. J. Phys. Chem. 69 16. D. R-Scott and R.S. Becker, (1965) 3207. Bull. Acad. Sci. USSR 989 (1959). 17. M.E.Dyatkina, F. Smith, E. Elder and S. P. 18. A. T. Armstrong. McGlynn. J. Chem. Phys. 46 (1967) 4321.
3. number
1
CHEMICAL
ELECTRON OF
SPIN
PHYSICS
January
LETTERS
RESONANCE
1969
MEASUREMENTS
DIBENZENECHROMIUM
CATION
R. PRINS and F. J.REINDERS IConinklijke/Shell-Laboraton’um, Amsterdam (Skell Research N. V.) Received
6 January
1969
ESR spectra of liquid and solid solutions of Cr(C H6)$ have been measured. Even in solid solution the spectr+a show a well resolved proton hyperfine s f ructure which is explained by rotation of the Cr(C6Hd2 cation around its main axis. An analysis of the 62Cr hyperfine coupling constants indicates results agree fairly well with that the unpaired electron is in the alg (3ds2) orbital. The experimental the results of an e.xtended Hilckel MO calculation.
1. INTRODUCTION Althou@ the literature mentions on the liquid solution ESR spectrum
some papers of Cr(C6H6);
[l] to our knowledge as yet no ESR spectrum of a solid solution or single crystal of Cr(C6H6); has been published nor a quantitative analysis of the ESR spectra been given. Such an analysis would be of interest not only for the e.xplanation of the proton hyperfine coupling constant of Cr(C6H6); but also for a better understanding of the molecular orbital energy level scheme. In this publication we therefore give an analysis of our ESR data of liquid and solid solutions of Cr(C6H6)& 2. EXPERIMENTAL We have measured ESR spectra of dilute solutions of Cr(C6H6)2I in the solvents acetone, alcohol, dimethyl formamide-chloroform (1:l) and ethylene glycol-water (1:l). The ESR spec; tra of liquid solutions of these solvents are all alike. Fig. 1 shows the X-band spectrum of a liqilid solution of DMF-CHC15. This spectrum can be interpreted straightforwardly in terms of an interaction between the unpaired electron spin and the nuclear spins of the twelve protons of the benzene rings. The satellite lines at low field are due to the 53Cr isotope. The corresponding high field lines are only detectable with a higher amplification due to anomalous relaxation of the hyperfine lines [ 21. The ESR spectrum of a solid solution of
Fig. 1. ESR X-band spectrum of a liquid solution Cr(CgH6)2I in D&IF-CHC12 (1:l) at 200OK.
of
characteristic of an molecule. The somewhat unexpected hyperfine structure due to the protons can be resolved for the x,y peak as well as for the z peak (see fig. 2). The resolution of the proton hyperfine structure depends on the solvent system used. In the mixture DMF-CHCl2 (1:l) the resolution is good and independent of temperature up to 1600K where the DMF-CRC32 glass begins to soften. In solid solutions of acetone, alcohol and a glassy ethylene glycol-water (1:l) mixture the resolution is not so good and Cr(C#ifjj+2 axially
shows
two peaks
symmetric
45
Volume 3. number 1
CHEMICAL PHYSICS LETTEPS
January 1969
around the x, y peak, so that BCr can be evalu-
ated from this hyperfine structure. In the spectra no indication has been found of the axial 5%.Zr
-H
hyperfine structure. The spin-Hamiltonian parameters derived from the ESR spectra, measured with a Varian X-band spectrometer with 100 kHz field modulation, are given in table 1. The values for the liquid solution given in the third row are in good agreement with published values [l]. To determine the sign of the isotropic proton hyperfine coupling constant oH we have measured wide-line NMR spectra of pofycrystalline samples Of Cr(C6H6)$. Water Was USf?das an eXternal reference and the measurements were conducted at 3OOoK on a Varian DP60 spectrometer operating at 56.4 MHz. The peak of the protons in Cr(C6H6)3I is shifted 250 j, 10 ppm to lower field compared with the water peak and the line width is 160 tt 10 ppm. This shows that the sign of eH _- is positive. With the formula AZZ = _a Ye@%+ H YN we caIcuIate SH = +3.44 -i:0.15 G which nicely agrees with the value obtained from the ESR measurements.
Fig. 2. ESR X-band spectrum of a glassy solidsolution fif fl-(CgHs)21 in D&IF-CHC12(1:l) at 120°K. 3. DISCUSSION depends on temperature. At lower temperatures the hyperfine lines are broadened and the structure on the z peak is no longer observable. A similar behaviour has emerged from the NMR and ESR spectra of Cr(C6H6)z [3], Fe(C5H5)3 (47, benzene [5] and CnHn aromatic radicals {S]. This behaviour in the solid state is explained by a rotation around the 2 axis which freezes out at low temperature. The rotation averages out the anisotropy between the proton Ax and Ay values, with the result that the value &4x+Ay) is observed in the hyperfine structure on the x, y peak. In agreement with this explanation we have frllnd no proton hyperfine structure in the soli> solution spectrum of Cr(C6H5 - CH,$: the methyl groups prevent rotation. For tne rest the ERR spectra of the solid ar?d liquid SI It&ions of CR(C6H5 -CH3)i are equal to tho:;e of Cr(C6H6);. At the high-field sideof the solid solution Spectrum Of Cr(C6H6); Satellite lines belonging to one of the four s3Cr lines and split up by the protons are observed. In the spectrum of Cr(C6D6); we have a&o observed the other three 53Cr lines and from this spectrum it became clear that these four 53Cr lines are centred 46
To obtain an insight into the MO level order:lg Of Cr(C#ifj)$ we have carried Out an ex-
tended Htickel calculation for this cation. For the details of this semi-empirical MO method we refer the reader to a previous publication on metallocenes M(C5H5)3 [7]. The geometry of Cr(C6H6)f was chosen to be that of Cr(C6H6)3 and geometric parameters for the latter were obtained from the structural data of HaaIandr8f. The computed net charges are: Cr co*3g, C +0-03 and H +0.03_ In tabIe 2 we have listed the calculated energies and coefficients of the highest filled and lowest empty orbitdls. The coefficients for the molecular orbitals are expressed in atomic symmetry orbitals cpand x ]‘I]. The outcome of our calculation was that the -unpaired electron is in the alg orbital with predominant 3d,3 character. MO cakuktions on V(C6H6)3 f9], which is isoelectronic with Cr(C6H6ff, and on Cr(C!6H6)2 [lo] and Fe(C5H& [7,9,1x jz, which have one ekCtrOn more than Cr(C6Hg)f2, alSO give the aIg orbital as the highest occupied orbital. The ESR results on Cr(C6H6)i can be ex-
plained as the unpaired electron being in the alg
Volume 3, number 1
CHEMICAL PHYSICS LETTERS
Table 1 Spin-Hamiltonian parameters of Cr(CsH&,
January 1969
obtained from the ESR spectra.
glt = 2.0023 + 0.0005
/AzH] = 3.1 k 0.1
g-t = 1.9785 * 0.0005 g
= 1.9865 f 0.0005
Energies
and coefficients Orbital tvve
Symmetry
eag
?T,
= 26.9 zt 0.1 G
aCr
= 18.1 f 0.1 G
31%H
'Ei (eV) 4.34
Wd) -0.70
empty molecular
(p(4S)
x (2%)
-
0.85
* e%
3d
4.81
ezu
77
5.80
ab
3d
1
7.64
1.01
-0.12
0.15
e2g
3d,a
4
9.19
0.73
-
0.53
orbital, since the g values are close to the free electron value so that the ground state configuration of Cr(C6R6)$ must be orbitally non-degenerate. With orbital coefficients and energy differences obtained in the extended Hiickel calculation we calculate glt = 2.0023 and gl = 1.97 for the 2Alg(alg) configuration, which is in good agreement with experiment. The 53Cr hyperfine coupling constants provide quantitative information on the metal character of the alg orbital. lf we write the alg orbital as (alg;)
= crl/3dz2 > + a2l4s)
+ o31(4lig)
the following equations can be derived for the 53Cr hyperfine coupling constants ]12] A ‘Ais
+ $ ~rfP - +(g-L - 2.0023) P
B =Aiso
- $ CYfP + E(g I - 2.0023) P
cz = f(A+
2B) =Aiso
+ (g - 2.0023) P
where P = 2.0023g~j3~p~ < r-3>, pe and &,I are the Bohr and nuclear magnetons, gN is the nuclear g factor and Aiso is the isotropic contact term. For the evaluation of “1 we should have to consider four possibilities for the signs of a and B since ESR experiments only give the absolute values of hyperfine coupling constants. From the observed anomalous relaxation of the 53Cr lines, however, information can be obtained
1.02
+A,,H( = 3.64 I 0.05 G IoHI =3.46iO.O5G
Table 2 of the highest filled and lowest
No. of eiectrons
3d
jBCrj
G
-
-0.52
1.00
orbitals
of CrfC
x (Woo)
XW,)
-6);
. X(lSH)
0.10
-0.01
0.29
-0.12
0.06
0.01
-0.00
-0.01
-0.06
0.13
-0.01
0.03
-0.08 0.15
-0.01
about the relative signs of a and B. The condition for broadening of the high-field lines compared with the low-field lines is a(a -B) V X (911 -gI)< 0 [Z]. With the experimental g. la1 and 1BI values (see table 1) this condition can only be fulfilled if a and B have the same sign.
ln that case we derive from the equations given above that (cy: + 0.034) P = *30.8, where the plus sign holds for botha and B negative and the minus sign for both a and B positive. But since P is negative the only possibility is that both a and B are positive. With a = +18.1 G and B = s26.9 G we calculate A = +0.5 G. The axial 53Cr hyperfine structure will thus be situated near the top of the z peak in the ESR spectrum and will certainly be hidden under the z peak. This explains our inability to observe the axial 53Cr hyperfine structure. To be able to evaluate arl from the equation (~4 t 0.034) P = -30.8 we must know the value of P. Values for P for different valence state= of the chromium atom have been tabulated b) McGarvey [12]. Guided by the extended Hifckel calculation given above we estimate the net charge on the chromium ,atom to be within the range +0.05 to +l.O. For the !-ordering values we calculate: cr+O. 5 cl!1 = 1.02 P = -28.9 G Cr+l.O
P = -31.5 G
crl = 0.98
The outcome of our analysis is that acl is \ery 47
Volume
3, numbx
CHEMICAL
1
PHYSICS
near 1.00 and that the alg orbital is essentially a chromium 3d,2 orbital. This result does not exclude the possibility that the coefficients of the ligand part of the alg MO have small non-zero values (see table 2). Information on these ligand coefficients can be obtained from an analysis of the proton hyperfine coupling constants. The isotropic hyperfir?e coupling constant A iso originates from an exchange polarization contrlb&on and a contribution of the unpaired spin density in the 4s atomic orbital. The value of A iso = 17.6 G does net deviate much from the value for chromium in an octahedral environment [12], where no 4s contribution can occur. We therefore conclude that the 4s contribution to the alg orbital is small ((~2 < 0.1). For vanadocene, too, it has been found that the 4s contribution to the alg orbital is small [13,14] and these results show that the type of hybridization between the 3dZ2 and 4s atomic orbitals as assumed by Moffitt 1151does not occur in sandwich compounds. The calculated energy level order given in table 2 shows two interesting characteristics. First
the energy
difference
between
Che alg
and
orbitais is calculated to be 1.6 eV, which agrees qualitatively with the value of 2.1 eV obtained in a ligand field analysis of the lowest energetic band in the absorption spectrum of Cr(C6H6); il6]. The energy difference of 2.1 eV is much larger than that found in metallocenes: 0.6 eV [14] and indicates a large participation of the e2 ligand orbitals in the bonding of the complex, which is also reflected in Cr(C6a6); the e2g orbital coefficients (see table 2). This result confirms the curreni idea that back donation via the e2g orbital is more important in the dihenzene complexes than in the dicyclopentadienyl complexes [l?]. The second characteristic point in table 2 is t$at the e2u orbital has a iower energy than the elg orbital contrary to the result for metallocenes [‘7]. In the extended Hiickel method, however, not too much confidence should be placed in the result of a comparison of two orbitals connected with a different charge distribution such as the e2u (ligand) and e;g (metal) orbital. e2g
li****
LETTERS
January
1969
Therefore the energy order calculated does not necessarily imply that in the electronic absorption spectrum of Cr(C6H6); charge transfer bands occur at a lower energy than the d-d bands. No unequivocal analysis of the electronic absorption spectrum of dibenzenechromium complexes has been published and in view of the uncertainties in the assignment cf the charge transfer bands in the electronic absorption spectrum of ferrocene [18] the value of such an attempt seems doubtful.
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