Journal of Nuclear and Radiochemical Sciences, Vol. 2, No.1-2, pp. R5-R8, 2001

Reviews
Atomic and Molecular Motion in Iron Organometallics
Rolfe H. Herber*
Racah Institute of Physics, The Hebrew University of Jerusalem 91904 Jerusalem, Israel
Received: November 2, 2001; In Final Form: November 16, 2001
Temperature-dependent 57Fe Mössbauer effect studies have been carried out on organge needle-shaped crystals ofacetyl ferrocene over the range 90 ≤ T ≤ 350 K in an effort to elucidate the motional behavior of the metal atom up
to temperatures near the melting point (359K). The spectra consist of one well resolved doublet at all temperatures, with an isomer shift of 0.554 ± 0.002 and 0.438 ± 0.001 mm s−1 at 90 and 298 K, respectively. The temperature dependence of the isomer shift corresponds to an effective vibrating mass of 74 ± 2 daltons, and, in
conjunction with the temperature dependence of the recoil-free fraction, to a Mössbauer lattice temperature of 134
K. There is a small but significant vibrational anisotropy of the metal atom motion evident in the spectroscopic
data above ~ 200 K.

Introduction
Gamma ray resonance scattering (the Mössbauer effect) has
proven itself to be an excellent tool for the study of atomic
motion in solids, especially those involving iron and tin. In anumber of recent studies1−3 it has been shown that the motionof the iron atom in organometallics can serve as a sensitive
diagnostic tool for the elucidation of the relationship between
the molecular level architecture involving the metal atom andthe macroscopic properties of these diamagnetic solids. Until
fairly recently, these studies have usually involved lowtemperature investigations, since the recoil-free fraction drops
off sharply with increasing temperature, and thus theanisotropy of the metal atom motion, which can become
pronounced at higher temperatures, has not been extensively
documented in the literature.
Because of its centrality in the development of modern
organometallic chemistry, ferrocene [Fe (η5-C5H5)2] [102-54-5]
has been the subject of numerous Mössbauer effect studies4,
and a recent detailed examination of this compound with reference to the iron atom dynamics at temperatures ranging from90 to 370 K, has been reported.3 Because of the complexity ofthe molecular and crystal structure of this compound, which
involves several phase transitions between liquid nitrogen androom temperature, a large number of X-ray crystallographic
investigations have been undertaken,5−8 and the thermal factors(Ui, j values) derived from such studies have permitted a validation of the Mössbauer data in terms of these parameters atseveral different temperatures.9
In addition to the temperature dependence of the recoil-freefraction (derived from the area under the resonance curve foran optically “thin” absorber), such Mössbauer studies havealso permitted an elucidation of the anisotropy of the metal
atom motion (derived from the intensity asymmetry of the twocomponents of the quadrupole-split spectrum), commonlyreferred to as the Gol’danskii-Karyagin effect (GKE).10 Until
recently, it has been assumed that the GKE in ironorganometallics is small or negligible, but a number of recent
studies of such compounds in the high temperature regime
have shown that this effect is much more ubiquitous than had
previously been suspected. This motional anisotropy is ofparticular interest in connection with the ferrocenoid solids,*E-mail: HERBER@VMS.HUJI.AC.IL. FAX: +972-2-6586347.

since the onset of ring rotation and libration has a pronounced
effect on this parameter.
In this connection, the study of acetyl ferrocene [Fe[(η5C5H5)(η5-C5H4COCH3] [1271-55-2] [1] has proven to be ofparticular interest, and a detailed study of this solid over thetemperature range 90 to 348 K is summarized in the present
communication. The reported melting point of this solid is 359
K. It should be noted that an earlier detailed Mössbauer investigation of 1, as well of its crystal structure by X-ray diffraction, has been reported by Sano et al.11−13 and others,14 and thepresent study should be considered as an extension (and reexamination) of the earlier work reported by the Japanesegroup.
Experimental
High purity (>99%) crystals of 1 were obtained commercially (Merck) and showed only a single doublet resonancespectrum at all temperatures. These crystals were ground with
BN, transferred to a plastic sample holder, and examined intransmission geometry, as detailed earlier,1−4 using a 57Co in
Rh source at room temperature. It should be noted that theplastic sample holders used in this study were thoroughly
sealed with hard-setting plastic cement for two reasons. First,
it is well known that iron organometallics of the type examinedin this study readily sublime, especially at temperatures near
T mp . Since sample loss by sublimation on increasing thetemperature would mimic an anomalous reduction in therecoil-free fraction, it is important to guard against sample lossby this process. (Moreover, as will be noted below, it isequally important in this connection to acquire Mössbauer datain both heating and cooling regimes and to monitor the
gamma-ray transmission rate as a function of time). Second,since small, unavoidable temperature gradients in the sample
holder may result in sublimation, and hence in deposition of
crystals preferentially oriented with respect to the gamma ray
axis, and since this would drastically effect any evidence for a
GKE in these samples, it is important to minimize suchchanges in the texture effect. A rigorously sealed sample
container which develops its own vapor pressure is assumed tominimize this problem. Spectrometer calibration was effectedusing a room temperature 18.81 mgm cm−2 α-Fe absorber, andall isomer shifts are reported with respect to the centroid of thisspectrum.

© 2001 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/28/2001

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J. Nucl. Radiochem. Sci., Vol. 2, No. 1-2, 2001

Results and Discussion
The temperature dependence of the isomer shift (IS) issummarized in Figure 1 and evidences a departure fromlinearity at low temperature as has been noted previously. At
high temperatures (above ~ 273 K), the data are well fit by alinear regression with a correlation coefficient of 0.999 for 18
data points. The IS calculated from the high temperature data
is 0.554 ± 0.002 mm s−1 at 90 K and 0.438 ± 0.001 at 298 K.
This latter value is in good agreement with that of 0.40 – 0.42
mm s−1 quoted in the earlier study12 (although the temperatures
for these values are not explicitly stated). The effective
recoiling mass, Meff, calculated from the high temperature d IS /d T data is 74 ± 2 daltons, a value which is significantly
smaller than that previously reported15 for other ferrocenoid
solids. The slope of the d IS / d T curve does not evidence any
significant departure from linearity between room temperatureand the melting point, and there is no evidence from these dataof an experimentally significant change in the electron density
around the metal atom in this temperature regime.
The temperature dependence of the quadrupole splitting(QS) is summarized graphically in Figure 2 and shows the
expected decrease with increasing temperature in the temperature range 175−275 K. due to the thermal expansion of thecovalent solid. The QS observed at 297 K is 2.275 ± 0.001
mm s−1 in modest agreement with the value reported earlier.12
At temperatures above 300 K, there appears to be no definite
variation of QS with temperature, and the relatively large

Herbererrors (on the total QS scale) preclude further interpretation ofthese data.
The temperature dependence of the recoil-free fraction(given by the temperature dependence of the area under the
resonance curve for an optically “thin” absorber), normalized
to it 90 K value, is shown in Figure 3. The data in the interval
90 ≤ T ≤ 320 K are well fit by a linear regression with a correlation coefficient of 0.992 for 11 data points. The calculated
value of - d <x2> / d T is (5.79 ± 0.10) × 10−3 K−1, in quite
reasonable agreement with the value of 5.08 × 10 − 3 K − 1
extracted from Figure 1 of the 1987 data of Iwai et al.13 Thistemperature dependence, together with the d IS/ dT value
given above, leads to a calculated Mössbauer lattice temperature, ΘM, of 134 K, which is significantly higher than lattice
temperatures reported for ferrocene itself, mono-substituted
ferrocenes, and fully ring substituted ferrocenes. At temperatures above ~ 330 K, there is a marked decrease in the recoilfree fraction with increasing temperature, indicating an
increase in the mean square amplitude of vibration (msav) of
the metal atom as the melting point is approached. In thiscontext, it has not been possible in this study to reproduce theearlier data reported in the literature, which suggest a decreasein the msav at temperatures above ~ 320 K. This point will be
discussed further below.
In a further attempt to elucidate the metal atom dynamics in
the high temperature regime, a detailed examination of the line
width of the resonance has been carried out, and these data aresummarized graphically in Figure 4. At low temperatures, theFigure 1. Temperature dependence of the isomer shift of acetyl
ferrocene. The isomer shift is with respect to the centroid of a room
temperature spectrum of α-Fe. The slope data at high temperatures
are used to calculate Meff as discussed in the text.

Figure 3. Temperature dependence of the recoil-free fraction for the
Fe atom in acetyl ferrocene extracted from spectral area measurements. The slope data at low temperaturese in good agreement withthose reported in the literature, and are used to evaluate a Mössbauer
lattice temperature of 134 K, as discussed in the text.

Figure 2. Temperature dependence of the quadrupole splitting parameter for acetyl ferrocene. The error bars reflect the precision of theindividual measurements.

Figure 4. Temperature dependence of the line widths observed in the
Mössbauer spectra of acetyl ferrocene. The open and closed circles
reflect the A(+) and A(−) values, respectively.

Atomic and Molecular Motion in Iron Organometallics

line width shows the expected increase with decreasing
temperature due to saturation effects (i.e.: the absorber can nolonger be considered optically “thin”). Above ~ 200 K the line
width reaches a constant limiting value, and above 300 K, thisparameter has a mean value of ~ 0.255 ± 0.004 mm s−1 (15 data
points). The excess of this value above twice the natural line
width of the 14.4 keV 57 Fe gamma ray, 0.194 mm s −1 , isprimarily due to radiation damage effects16 and 57Fe accumulation in the 1.5 year old source matrix (originally 100 mCi).
The data reported in the earlier study12 evidences an increaseof ~ 0.12 mm s−1 in Γ between 273 and 355 K. Since this is
more than 40 times the average line width error observed in thepresent data set, this broadening should have been readily
detected in the data. However, it has not been possible toreproduce those earlier results in the present study, and thereported increase in line width with increasing temperature
has not been observed in the present data.
One additional parameter, related to the intensity asymmetryof the resonance doublet, can be extracted from the present
data set. These data, reported in terms of the intensity ratio R= A(+)/A(−) [where A(+) and A(−) are the intensities of theresonance line at more positive and more negative values than
the spectrum centroid, respectively] are shown in Figure 5
which summarizes only the temperature-dependent part of R.
From this figure it is inferred that there is a small but definite
increase in R with increasing temperature, and that this effectbecomes more pronounced at T> ~ 300 K. The origin of thiseffect is presumed to arise from a motional anisotropy of themetal atom with respect to the pseudo symmetry axis runningnormal to the two Cp ring planes and through the iron atom
position. This motional anisotropy is also readily evident in
the detailed X-ray structure reported by Sano et al.12 in which
the ellipsoid of motion (at the 50 % probability level) shows aclear departure from spherical symmetry. However, additional
study, using 57Fe enriched absorbers (vide infra) are clearlyneeded to show conclusively whether this motional anisotropy
is reversible over the whole temperature range after the sample
has been warmed to a few degrees of Tmp.
On the basis of the foregoing data, it is now possible tounderstand the apparent differences between the results
reported in the earlier Mössbauer study and those observed inthis work. It is well known that many organic andorganometallic solids can be formed in a number of polymorphic crystal structures, and that the rate of interconversion of
one polymorphic form to a more stable one can occur overlarge time and temperature ranges.17 Frequently, the energy
differences between polymorphs of covalent crystals are quite
small, on the order of a few kilocalories per mole.18 Using 1H

J. Nucl. Radiochem. Sci., Vol. 2, No. 1-2, 2001

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spin-lattice relaxation NMR experiments at 20 MHz on solid
acetyl ferrocene, over the temperature range 80−359 K, Kubo
et al.19 have reported the activation energy for C5 reorientationas amounting to 5.02 and 5.74 kcal mol−1 in the close packed
crystal structure. There was no evidence for a phase transition
below T mp . In the case of acetyl ferrocene, three different
morphological species have been reported,20 depending on thesolvent of recrystallization and the details of the purification
process.21 In particular, wine-red needles, wine-red leafletsand light orange needle shaped crystals can be isolated from
pentane and pentane-benzene solutions. Microscopic examination of the samples used in the present study show the marked
predominance of orange needle shaped crystals which mostprobably belong to a hexagonal or trigonal crystal system,
although no detailed X-ray study has been attempted in thiswork.
On this basis it is plausible that the sample(s) of brown
needle shaped crystals of acetyl ferrocene examined by Sano etal.12 were in fact of a different morphology than those herereported, and that both the apparent decrease in msav of theiron atom with increasing temperature, as well as the apparent
increase in line width, can be accounted for by a change in the
crystal structure from a meta-stable to a more stable configuration. This interpretation is not at variance with the crystal
structure reported in the earlier study, since these diffraction
data were acquired on a single crystal of dimension 0.30 × 0.25
× 0.35 mm, whereas the Mössbauer data, of necessity,
involved a macroscopic ensemble of crystals.
It is clearly desirable to resolve the apparent differences
between the results of the present and earlier study, and such aresolution may best be effected by examination of well-crysrallized 57Fe enriched samples at temperatures between 300 K
and the melting point, where the recoil-free fraction drops off
sharply with increasing temperature. Using enriched samples
and strong gamma ray sources is expected to reduce the relatively large statistical errors encountered in Mössbauer studies
of organometallics near their melting point. It is hoped thatsuch a detailed examination can be effected in the near future.
Acknowledgments. The author is indebted to Prof. I.
Nowik for help with data reduction using the transmission integral procedure, to Profs. M. Michmann and S. Biali for helpful
discussions concerning the molecular level architecture of thetitle compound, and to Prof. H. Schottenberger for information
concerning the polymorphism of 1. Finally, the author isindebted to Profs. H. Sano and M. Katada for illuminating discussions pertaining to their pioneering study of acetyl ferrocene.
References

Figure 5. Temperature dependence of the intensity ratio[R=A(+)/A(−)] for acetyl ferrocene. A temperature-independent
contribution from preferential crystallite orientation (“texture effect”)
extracted from low temperature data has been corrected for in theseresults.

(1) R. H. Herber, I. Nowik, T. Asthalter, E. Reichel, and H.
Schottenberger, J. Organomet. Chem. (in press).
(2) R. H. Herber and I. Nowik, Hyperfine Interactions, (inpress); Hyperfine Interactions 126, 127 (2000).
(3) R. H. Herber and I. Nowik, Solid State Sciences (R.
Schöllhorn Festschrifft) (in press).
(4) R. H. Herber, K. Temple, I. Manners, M. Buretea, and T.
D. Tilley, Inorg. Chim. Acta 287, 152 (1999), and references therein(5) C. P. Brock and Y. Fu, Acta Cryst. B53, 928 (1997), andreferences therein.
(6) P. Seiler and J. D. Dunitz, Acta Cryst. B35, 1068 (1979).
(7) J. D. Dunitz, L. E. Orgel, and A. Rich, Acta Cryst. 9, 373(1956).
(8) P. Seiler and J. D. Dunitz, Acta Cryst. B38, 1741 (1982).
(9) R. H. Herber and I. Nowik, Hyperfine Interactions (Proc.
12th Int. Conf. on Hyperfine Interactions, Park City, Utak,
2001) (in press).

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J. Nucl. Radiochem. Sci., Vol. 2, No. 1-2, 2001(10) See the extensive discussions in “Mössbauer Spectroscopy”, N. N. Greenwood and T. C. Gibb, Chapman
Hall, London, 1971, and references therein; “Chemical
Mössbauer Spectroscopy”, V. I. Gol’danskii and R. H.
Herber, Eds., Academic Press, New York, 1968, and references therein.
(11) K. Sato, M. Konno, and H. Sano, Chem. Lett. 1982, 817.
(12) K. Sato, M. Katada, H. Sano, and M. Konno, Bull. Chem.
Soc. Japan 57, 2361 (1982).
(13) K. Iwai, M. Katada, I. Motoyama, and H. Sano, Bull.
Chem. Soc. Japan 60, 1961 (1987).
(14) L.Qiwang, H. Yuwai, L. Fengzhe, and H. Jinshun, Acta
Phys.-Chim. Sin. 2, 68 (1986); A. V. Lesikar, J. Chem.
Phys. 40, 2746 (1964); L. Korecz, H. Abou, G. Ortaggi,
M. Graziani, U. Belluco, and K. Burger, Inorg. Chim.
Acta 9, 209 (1974).
(15) See, for example, H. Schottenberger, K. Wurst, and R. H.

HerberHerber, J.Organomet. Chem. 625, 200 (2001).
(16) C.-Y. Song and J. G. Mullen, Nucl. Instr. Methods 140, 81(1977).
(17) J. D. Dunitz and J. Bernstein, Acc. Chem. Res. 28, 193(1995), and references therein(18) A. I. Kitaigorodskii, Ad. Struct. Res. Diffr. Methods 3,
173 (1970).
(19) A. Kubo, R. Ikeda, and D. Nakamura, Z. Naturf. A 43, 78(1988).
(20) J. Polin, thesis, Universität Innsbruck, 1992, p 8ff; H.
Schottenberger, private communic.
(21) K. L. Rinehart Jr., K. L. Motz, and S. Moon, J. Amer.
Chem. Soc. 79, 2749 (1975); Y. Sasaki and C. U. Pittman
Jr., J. Org. Chem. 38, 3723 (1973); M. Omori, M. Kurono,
and S. Yajima, Bull. Chem. Soc. Japan, 48, 1291 (1975);
J. Davis and D. H. Vaughan, J. Chem. Educ. 72, 266(1995).