Comparative vapour phase FTIR spectra and vibrational assignment of manganese pentacarbonyls derivatives of the type XMn(CO)5: (where X=Br, Cl, I, H, D, CH3, CD3, CF3)

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Abstract

The infrared spectra of pure Mn(CO)5X in the region 4000–400 cm−1 has been obtained in the vapour phase. The observed spectrum has been analyzed to distinguish the fundamental frequencies, the rotational–vibrational and structure, and overtone and combination frequencies. The assignment of the observed vapour phase frequencies to the fundamental modes of vibration has been made on C4V symmetry. The weak peaks due to XMn(CO)4(13CO) molecules have been measured and assigned for all molecules. This study provides a comprehensive comparison of these compounds, with all of these data the assignment of frequencies is reviewed and a set of quite unambiguous assignments made. The significant finding in this regards are that, it is not necessary to assume lower than C4V symmetry for XMn(CO)5 as has been done in previous consideration of some infrared spectrum of these compounds.

Introduction

The vibrational spectra of metal carbonyl and their derivatives have been extensively studied particularly in the CO stretching region. There have also been studied of lower frequency vibrations, such as the metal–carbon stretching [ν(MC)] and metal–carbon–oxygen bending [δ(MCO)] vibration in some metal carbonyl halides and their derivatives [1], [2].

Subsequent work on the series of XMn(CO)5 compounds dealt with the application of the IR spectroscopic method of assigning and analyzing the CO stretching modes to a variety of octahedral molecules derived from the carbonyls of the group VIA and group VIIA metals and to M2(CO)10 (M = Mn, Re) molecules [3]. Other authors [4] have used this method in varying degree, and often in association with the kind of intensity argument suggested by Orgel [5] in order to analyse an assign the CO stretching spectra of numerous M(CO)xL6−x molecules, where L6−x represent a collection of donor molecules or univalent groups which are not necessarily all the same. It is clear from preceding work there has been no unified comparative study in the region 4000–400 cm−1, for the compounds XMn(CO)5 (where = Br, Cl, I, H, D, CH3, CD3, CF3). It is still subject to certain possible ambiguities that the molecular symmetry is lower than C4v although no vibrational assignment were made and more detailed study its reliability would be of value. In order to resolve this disparity we have to perform a comparative series of infrared spectroscopic investigations of gaseous samples of manganese carbonyl derivatives of the type XMn(CO)5 were known to be pure, following the number of metal–carbon stretching and metal–carbon–oxygen bending vibration active in the 5 μm region. Accurate value for the band maxima in the 4000–400 cm−1 region based upon C4v symmetry have been obtained for these complex.

Section snippets

BrMn(CO)5, ClMn(CO)5 and IMn(CO)5

The manganese pentacarbonyl halides, Mn(CO)5 X, (where X = Cl, Br, I) are considered as pseudo-octahedral compounds having C4V symmetry. The irreducible representation for the normal modes of vibrations for this symmetry areΓvibC4v=7A1+A2+4B1+2B2+8E

A1 and E modes are both infrared and Raman inactive, B1 and B2 are only Raman active, and the A2 mode is both Raman and infrared inactive. For a total of eight symmetric, six unsymmetric, and eight doubly degenerate modes.

To establish description for

Conclusion

There was no major difference between the gas phase spectra in the current work and previous studies on solution and solid-state apart from frequency shift. It is difficult to make an unequivocable assignment without observing every possible band. There is no boubt that substantial mode mixing occurs in the fundamental modes in 5 μm region. This the Mnsingle bondC stretching motion and the Mnsingle bondCsingle bondO bending motion have a pronounced tendency to mix and one can expect some mixing of the axial and planar

Experimental

Derivatives XMn(CO)5 (X = Br, Cl, I, H, D, CH3, CD3, CF3) were prepared according to the methods described in literature [26], [27], [28], [29]. Purity was checked using low resolution Fourier transform mid-infrared spectroscopy and 1H NMR spectroscopy, the spectra should no evidence of any impurities.

Gaseous sample were examined in a 10 cm long cell equipped with ZnSe windows. The sample pressure was generally of the order of 1–5 Torr except when the intense carbonyl peaks were examined for which

Uncited reference

[23].

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