# Mean Amplitudes of Vibration of theIF8− Anion

Authors: Enrique J. Baran
Journal: Research Letters in Inorganic Chemistry
 (2008)

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## Abstract

The mean amplitudes of vibration of the interestingIF8− anion (D4d-symmetry), containing iodine (VII), were calculated from known spectroscopic and structural data in the temperature range between 0 and 1000 K. The results are discussed in comparison with those of related species.


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## Body

## 1. Introduction

Mean amplitudes of vibration are very useful and
valuable parameters for the analysis of molecular structures and their
vibrational behavior. In a similar way to vibrational
frequencies  and force constants, they can be very characteristic values for both
bonded and nonbonded atoms [1, 2].During
years we have calculated mean amplitudes of vibration for a large series of
molecules and ions containing halogen-halogen or halogen-oxygen bonds (for a
recent review cf. [3]) and in this paper we present the results of our
calculations for the interesting IF8− anion, which
vibrational-spectroscopic behavior was only very recently definitely clarified
[4].As it is
well known, the structure of this anion is a practically perfect Archimedean
square antiprism [5], constituting the unique example of an interhalogen species
presenting this geometry, which structural peculiarities are similar to those of the few other
known examples of homoleptic species of this type, namely, ReF82−,
ReF8−,  WF82−,
UF82−, and XeF82− [6, 7].


## 2. Calculations

The mean amplitudes of vibration were calculated with
the method of the 		“characteristic vibrations” of Müller et al. [8]   (cf. also
[2, 9]). The necessary vibrational-spectroscopic data were taken from the recent
paper of Dixon et al. [4] and
the geometrical parameters from the paper of Mahjoub and Seppelt [5].


## 3. Results and Discussion

The obtained results, in the temperature range
between 0 and 1000 K, are shown in Table1. Regarding the nonbonded F⋯F
pairs, F⋯F (in plane) refers to the pairs within one hemisphere of
the anion, whereas F⋯F (betw. planes) refers to neighboring pairs
belonging to different hemispheres.Table 1
Mean amplitudes of vibration (inÅ) for the IF8− anion in the temperature range between 0 and 1000 K.
T (K)uI–FuF⋯F (in plane)uF⋯F (betw.
planes)00.04260.0620.0721000.04260.0620.0722000.04330.0640.079298.160.04550.0690.0893000.04560.0690.0894000.04870.0750.0995000.05210.0820.1096000.05560.0880.1187000.05900.0940.1278000.06240.1000.1359000.06560.1050.14310000.06880.1100.150The
analysis of the so far available data of mean amplitude values for I–F bonds
has shown that the extreme values lie between 0.0377Å  (for IF6+)
and 0.0602 Å  (for IF52−)  [3, 10], in agreement with the
fact that in the first case iodine presents the oxidation state +7 and a positive charge whereas in
the other one the iodine is in the oxidation state +3 and not only presents two
negative charges but also an important congestion effect on the molecular
plane, in which the fluorine atoms are practically in contact  [3, 10, 11]. Besides,
these two species present also the greatest differences in bond lengths found
in I/F species (1.75 Å  for IF6+,
2.095 Å  for IF52−) [3, 12]. Furthermore,
the specially high mean amplitude value of IF52− is in
good agreement with the very low force constant calculated for the I–F bonds in
this anion (1.53 mdyn/Å  [11]).The values
of the mean amplitudes of vibration calculated for the I–F bonds ofIF8− fall clearly into the mentioned range as it can also be seen from the
comparative data presented in Table 2. This comparison shows that the values
for IF8− are appreciably higher than those found for IF6+ showing again the effect of the geometry and of the negative charge over bond
weakening [3]. Besides, these amplitude values are only somewhat higher than
those calculated for the equatorial IF7 bonds.Table 2
Mean amplitudes of vibration (inÅ) of some iodine
(VII) species at three different temperatures ((eq): equatorial I–F bonds;
(ax): axial I–F bonds).
Species0 K298.16 K600 KReferenceIF6+0.03670.03770.0433[13]IF7 (eq)0.04080.04300.0516[14]IF7 (ax)0.03780.03900.0454[14]IOF6− (eq)0.04180.04440.0537[15]IOF6− (ax)0.03960.04140.0491[15]IO2F52−0.04550.04990.0625[16]IO2F4−0.04210.04480.0544[17]IF8−0.04260.04550.0556this workOn the other hand, the values calculated forIF8− lie relatively close to those of the equatorial
I–F-bonds of IOF6−. In comparison with the interhalogen
bonds of the other two fluorooxoanions containing iodine (VII), IF8− presents lower mean amplitudes of vibration (i.e., stronger I–F bonds) than IO2F52− but weaker I–F bonds than IO2F4−, in the full
temperature range.Concerning
the amplitude values of the nonbonded pairs, those of the same hemispheres are
always lower and show a smaller temperature dependence than those between
F-atoms belonging to the different hemispheres.


## Conclusions

Mean amplitudes of vibration of theIF8− anion clearly lie in the expected range determined for I–F bonds. These values
point to relatively weak bonds, when compared with iodine fluorine bonds present
in other simple iodine (VII) species, such as IF7 or IF6+, in
agreement with the higher coordination number and with the presence of a negative charge in the case of the IF8− anion.



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*Source: 147831-2008-12-28.xml*