J. Phys. Chem. 1988, 92, 4319-4323
4319
dol Ferrocene Solid StateInfrored Spectrum
Nickelocene
130G+
p=3k,bo,r
L
A
r
0
coo
cco 1000,
L 43 ooo
LBSBV
I
AAA
0
A y 1 t
20
40
60
/
I
,
/
,
800 Wavenumbers
900
850
I
/
750
,
,
~
000
Figure 7. Solid-stateinfrared spectrum of ferrocene-dloat two pressures; the arrows point out the increase in intensity of the band assigned to v5.
r
0
/
950
80
Pressure (kbar)
100
120
Figure 6. Pressure shifts of the bands appearing in the IR spectrum of
Ni(CP)z. symmetry axis is approximately 4 5 O . Coincident with v33 is v2, (E2g (1 ring deformation). The band initially appearing at 833 cm-' at 51 kbar in ferrocene extrapolates to approximately 820 cm-' at atmospheric pressure. A reasonable assignment for this band is v2 (Ai, I C H bend), which is seen in the Raman spectrum at 815 cm-l. The two remaining bands in Table I1 may be assigned to v14 (El, I CH
bend), which is coincident with v19. Finally, all of the arguments presented for the changes occurring in ferrocene are completely consistent with those observed in both nickelocene and ruthenocene. Although this model is rather simple, it appears to consistently account for the changes seen in the solid-state infrared spectra of Fe(cp),, Ni(cp),, and Ru(cp), with pressure. Two particular experiments that could provide additional information that would complement the results shown here are (a) the IR spectra of ferrocene-dio below 800 cm-' and (b) the Raman spectra of ferrocene, nickelocene, and ruthenocene as a function of pressure. Both of these could further show the utility of the coupling model; in particular, changes in the Raman spectra with pressure could determine if the ungerade modes couple to become Raman allowed.
Acknowledgment. This work was supported in part by the Materials Science Division of the Department of Energy under Contract DE-AC02-76ER01198.
Pressure Tuning of the Electronic Energy Levels of Ferrocene, Cobaltocenium Hexafluorophosphate, and Nickelocene R. T. Roginski, A. Moroz, D. N. Hendrickson, and H. G. Drickamer* School of Chemical Sciences, Department of Physics and Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 (Received: November 19, 1987)
The effects of pressure on various d-d transitions for ferrocene, cobaltocenium hexafluorophosphate, and nickelocene in the solid state are presented here. The bands are found to shift in a manner such as to increase the symmetry-imposedsplittings between the d orbitals, which is consistent with a qualitative molecular orbital picture. These splittings are found to increase between 2 and 8% in 100 kbar, as compared with 7-15% for simple ionic compounds. The Racah parameter B is almost independent of pressure, in contrast to the results seen for simple ionic compounds. These results are discussed in terms of the large degree of covalency for these compounds.
Introduction
The electronic structure of d6 and d8 metallocenes has long been the object of both experimental and theoretical considerations, the bulk of which dealt with ferrocene. The electronic structure of ferrocene has been studied by qualitative molecular orbital
treatment^,'^ crystal and ligand field treatments (combined with qualitative molecular orbital consideration^),^-^ Wolfsberg(1) Jaffe, H. H. J . Chem. Phys. 1954, 21, 156. (2) Dunitz, J. D.;Orgel, L. E. Nature (London) 1953, 171, 121.
0022-3654/88/2092-43 19%01.50/0 0 1988 American Chemical Society
4320 The Journal of Physical Chemistry, Vol. 92, No. 15, 1988
Roginski et al.
TABLE I: Energy Expressions for d-d Transitions of d6 and d8 Metallocenes As Obtained by the Strong Field Approach of Ligand Field Theow d6 'AI,
-
-0.5A1 - B - 3 C - 0.5Y A2 - AI - 9B - 3 C A2 - 0.5Al - B - 3C 0.5Y A2 - O.5Al + 6 8 - C - 0.5X A2 - AI - 9B - C A2 - 0.5AI 6B - C 0.5X
a'h,
'E2,
+
b%, a%, IE2g b'E,,
'A2g
+
+
-
d8
+ +
A2 - 0.5A1 4.5B - 0.5Z A2 - A1 - 3 8 4.5B 0.5Z A2 - 0.5Al
'E26 'E,g
AI = cC(e2,) 4%
X
+
- cc(alg)
= cc(el,) - &,)
= [(A, Y = [(A, 2 = [(A,
+ 6B)2 + 384B2]'/2 + 16B)* + 96B2]1/2 + 3B)2 + 216BZ]1/2
Helmholz and calculations of the S C F type;I3-l5 obviously, the results of these have relevance to the electronic structure of cobaltocenium. There is no paucity of theoretical treatments on the electronic structure of nickelocene; ref 16-1 9 represent a sample of calculations of different degrees of approximation. Reference 20 is an excellent review of the nature of the bonding in metal sandwich complexes, from a ligand field theory point of view. Within the framework of Dsdsymmetry, the d orbitals transform as the irreducible representations e,,, alg. and el,. The ordering of the energies of these levels is generally agreed to be ea (bonding) < alg (essentially nonbonding)
