Journal of Nuclear and Radiochemical Sciences, Vol. 7, No.1, pp. 13-15, 2006

Fe Mössbauer Parameters of Two Crystal Polymorphs of Fc+AsF6 - and the Sign of theQuadrupole Splitting in the Ferrocenium Ion
57

Rolfe H. Herber,*, a Israel Nowik,a Dario Braga,b and Marco PolitobabRacah Institute of Physics, the Hebrew University, 91904 Jerusalem, Israel
Department of Chemistry “G.Ciamician” University of Bologna, 40126 Bologna, Italy

Received: April 26, 2006; In Final Form: May 30, 2006
Temperature-dependent 57Fe Mössbauer effect spectroscopy has been used to characterize the hyperfine anddynamical properties of the metal atom in two crystallographic forms of ferrocenium hexafluoroarsenate. Thetemperature dependence of the recoil-free fraction and the spin-lattice relaxation rates have been determined overthe interval 89<T<300 K and shows a small difference between the two crystallographic forms. The sign of thequadrupole splitting parameter in the Fc+ cation has been elucidated in a transverse magnetic field experiment atroom temperature.

Salts of the ferrocenium ion, Fc + , were among the firstorganometallic compounds characterized by Mössbauer effect(ME) spectroscopy,1,2 and its hyperfine parameters have playeda central role in arriving at the correct description of the bonding interaction between the metal atom and its ligands. It is
well known that this ion can exhibit polymorphism in the solidstate. In a recent study, Braga et al.3,4 have described the successful isolation of both monoclinic (M) and trigonal (T)Fc+AsF6- using the technique of heteromolecular seeding. The
two polymorphs have distinctly different crystallographic properties. The T form exists in space group P3121 with three formula units per unit cell, whereas the M form exists in spacegroup P21/c with Z=4. Both of these crystallographic modifications are blue. The only other crystallographic study
reported in the literature is that of Fc + SbF6 -, a deep red compound reported by Hendrickson et al.5
In the present study, temperature dependent 57Fe ME spectroscopy has been used to determine the hyperfine parametersof the M and T form of Fc +AsF6 - and elucidate the metal atom
dynamics in the two forms. In addition, a room temperaturetransverse magnetic field has been applied to Fc+AsF6- to further
elucidate the sign of the quadrupole hyperfine interaction inthe cation.
2. ExperimentalSamples. Microcrystalline powder samples of the M and T
form of the subject compounds were prepared as previously
reported 3,4 by the Bologna group, mixed with BN to assure
random crystallite orientation, and transferred to plastic sample
holders. These, in turn, were mounted in a suitable cryostat
and examined in transmission geometry as a function of temperature as described earlier.
ME Spectroscopy. The ME transmission spectroscopy wascarried out using a 57Co in Rh source at room temperature aspreviously detailed.6 Temperature monitoring and control overthe period of data accumulation (typically 7 to 20 hours)
showed that the reported temperatures were constant to better
than ±0.2 K. Spectrometer calibration was effected using a 20
mg/cm 2 α -Fe foil at room temperature, and all isomer shifts*Corresponding author. E-mail: HERBER@VMS.HUJI.AC.IL
Fax:972 2 6586347

are reported with respect to the centroid of such spectra.
Magnetic Field Experiments. Transmission geometry
experiments in a transverse magnetic field of 2.13 T were performed on a finely powdered sample of Fc + AsF 6 - using aNewport Instruments Model 369 electromagnet slightly above
room temperature (due to resistive magnet heating), with a current of 6 –7 ampers at 55 volts. The magnet gap was ~ 8 mm
and the powdered sample was confined in a brass sample
holder having an area of 6×10 mm2.
3. Results and Discussion
Variable temperature experiments. a representative ME
spectrum of the M form of Fc +AsF6 - at room temperature (~296 K) is shown in the upper trace of Figure 1 and is typical ofspectra showing spin-lattice relaxation, as reported earlier.7
The ME spectra of Fc+PF6- have qualitatively the same appearance, and the data analysis was carried out on the two forms of
the hexafluoroarsenates, assuming a simple spin-lattice relaxation resonance. The temperature dependence of the isomer

RELATIVE TRANSMISSION

1. Introduction

VEROCITY / mm s-1

Figure 1. 57Fe Mössbauer spectra of the monoclinic form of Fc +AsF6at room temperature. The upper trace is in zero external field, thelower trace in a 2.13 T transverse magnetic field. The isomer shift
scale is with respect to the centroid of a room temperature α -Feabsorber spectrum in identical geometry.

© 2006 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 06/30/2006

HerberJ. Nucl. Radiochem. Sci., Vol. 7, No. 1, 2006
-MonoclTrig

ISOMER SHIFT / mm s-1

shifts (IS) for the two Fc AsF6 forms is summarized graphically in Figure 2 and shows that this parameter is the same forboth forms within experimental error. The temperature dependencies of the recoil-free fraction, which scales directly withthe temperature dependency of the area under the resonancecurve for an optically “thin” absorber, are shown in Figure 3.
Both data sets show curvature over the accessible temperature
range, but the data suggest a slightly “softer” lattice (as deduced
from -d ln A/dT of the iron atom) for the T form as comparedto the M form. However, not wishing to over-interpret the ME
data, it is worth noting that the Fe–ring carbon bond distances
at 293 K are identical (within experimental error) for the Mform (2.080 ± 0.018 Å) and the T form (2.079 ± 0.008 Å) as
extracted from the X-ray crystallographic data.
The collapse of the well-resolved quadrupole splitting in theparent ferrocene [QS at room temperature = 2.41(2) mm s-1]
when the neutral compound is subjected to a one-electron oxidation has been reported extensively in the literature.8,9 The
QS hyperfine interaction in the Fc+AsF6- cation is clearly negative [QS = −0.133(12) mm s-1], as deduced from the χ 2 values
obtained in the fitting procedure. The broad resonance line
envelope observed for the Fc+ cation is due to spin lattice relaxation, and a relative relaxation rate for the M and T forms of
the hexafluoroarsenate salt can be calculated using spin 1/2
relaxation theory10 to fit the spectra. Such relaxation rates are
only relative since the magnitude of the magnetic hyperfine
field due to the unpaired electron is not known. The data, summarized in Figure 4 suggest both a temperature-dependent
spin-lattice relaxation rate, as observed earlier, as well as aslightly faster rate for the T form than for the M form, although
the error limits, especially at the higher temperature, make thisconclusion somewhat tenuous.
ME spectra of Fc +BF4 - (no crystallographic form specified)were reported in an early study by Zahn et al.1 who reported anIS of 0.25± 0.04 and a QS of 0.37± 0.04 mm s-1 at room temperature. Despite the employment of a 57Co in stainless steel(Sandvik 2R2) source, these authors were able to resolve thebroad resonance line envelope into two components, with theline at lower velocity having the greater intensity. In the present study, the zero-field ME spectrum of Fc+BF4- was analyzed
using a relaxation algorithm, employing the Nowik-Wickman
Mössbauer spectra relaxation formalism 11 and resulted in avalue of IS=0.400 ± 0.006 and a QS of −0.133± 0.012 mm s-1 atroom temperature, in only modest agreement with the earlier
results. The sign of the QS is consistent with those reported
earlier in related ferrocenium cations.12
The lower trace of Figure 1 shows the ME absorption spectrum of the M form of Fc +AsF6 - in a transverse magnetic fieldusing the data analysis algorithm which has recently beendeveloped in this laboratory.13 The QS is negative in the cation,
as has been observed for similar ferrocenoid complexes,13 andthe effective magnetic field is larger than the nominal appliedfield (determined from similar studies with diamagnetic complexes) by Heff −Hext = 3.6 kOe. The induced hyperfine field is
produced by the electron vacancy in the 3d electron shell as has
been reported in other nominally Fe(III) complexes.14 This
induced hyperfine field is given by Heff −Hext = (µ BHext /kBT) Hhf,
which yields H hf = + 720 kOe. Such a positive large field isgenerally interpreted15 in terms of orbital contributions to thehyperfine field acting on the nucleus.

TEMPERATURE / K

Figure 2. The temperature dependence of the isomer shift for the Tform (filled data points) and the M form (open data points) of
Fc +AsF6-.

Trig
MonoclIn [A(T)/A(90)]+

TEMPERATURE / K

Figure 3. The temperature dependence of the recoil-free fraction(normalized to the 90 K data points) for the T form (filled datapoints) and M form (open data points) of Fc +AsF6-.

MonoclTrigRLX

14TEMPERATURE / KFigure 4. Temperature dependence of the spin-lattice relaxation
rates for the two polymorphic forms of Fc +AsF6-. The data are normalized to the 90 K data points as discussed in the text.

4. Summary and Conclusions
Two crystallographically distinct forms of ferrocenium
hexafluoroarsenate have been studied by temperature-dependent 57 Fe ME spectroscopy to elucidate the influence of thecrystal form on the hyperfine and dynamical parameters of theiron atom. Within experimental error, the IS parameter is thesame for both forms over the temperature interval 87<T<295

K, but the temperature dependence of the recoil-free fraction
and the spin-lattice relaxation rate appear to differ between thetwo forms. The sign of the quadrupole splitting in the monoclinic form is negative, in contrast to its positive sign in theneutral (diamagnetic) precursor.

57

Fe Mössbauer Parameters of Two Crystal Polymorphs

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Wertheim and R. H. Herber, J. Chem. Phys. 9, 2106 (1963)
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J. Nucl. Radiochem. Sci., Vol. 7, No. 1, 2006

15(9) N. N. Greenwood and T. C. Gibb, Mössbauer
Spectroscopy, Chapman Hall Ltd. London, 1971, p 233
and references therein.
(10) H. H. Wickman and G. K. Wertheim. Chemical Applicationsof Mössbauer Spectroscopy, Eds. V. I. Gol’danskii and R. H.
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