Safety note: Some of the materials mentioned here are very dangerous. Ask a qualified chemist for advice before handling them. Qualified chemists should check the relevant safety literature before handling or giving advice about unfamiliar substances. NMR solvents are toxic and most are flammable. Specifically, osmium salts are toxic and osmium tetroxide causes burns: wear eye protection and protective gloves.

Osmium NMR

Osmium (Os) has two NMR active nuclei ¹⁸⁷Os and ¹⁸⁹Os. ¹⁸⁹Os is a quadrupolar low sensitivity nucleus that yields a very broad signal for the highly symmetrical OsO₄ molecule and cannot be observed for other molecules using a high resolution NMR spectrometer. ¹⁸⁷Os is a spin ½ nucleus and yields sharp signals over a very wide chemical shift range. However, ¹⁸⁷Os has an extremely low sensitivity so is usually observed in complexes by indirect detection via proton coupling. Osmium NMR is used for studying osmium complexes.

Each type of osmium has its characteristic chemical shift range (fig. 1).

Fig. 1. Chemical shift ranges for osmium NMR

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¹⁸⁷Osmium NMR

¹⁸⁷Os yields sharp signals however, it is much less sensitive and than ¹⁸⁹Os. Therefore ¹⁸⁷Os is so insensitive that it is usually studied by indirect detection via proton coupling. It is still the nucleus of choice because only OsO₄ can be studied using ¹⁸⁹Os NMR.

One-bond ¹H-¹⁸⁷Os couplings are between 25 and 85 Hz, and two-bond are about 5 Hz. One-bond ¹³C-¹⁸⁷Os couplings are about 100 Hz and ³¹P-¹⁸⁷Os couplings are about 300 Hz.

We have not succeeded in observing ¹⁸⁷Os NMR in our laboratory due to its low sensitivity although its observation is possible and we are willing to make the extra effort to do so if the need arises. We do not have the equipment observe ¹⁸⁷Os by indirect detection.

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Properties of ¹⁸⁷Os

(Click here for explanation)

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¹⁸⁹Osmium NMR

¹⁸⁹Os is a quadrupolar nucleus that yields very broad signals for the most symmetrical of compounds but is much more sensitive and than ¹⁸⁷Os. However, ¹⁸⁹Os signals are too broad cannot be observed on a high resolution NMR spectrometer with the exception of OsO₄. Therefore, with the exception of OsO₄, ¹⁸⁹Os is not the osmium nucleus of choice for NMR. Quadrupolar signals increase in line-width with increasing magnetic field. In our experience, OsO₄ yields a broad signal on a 200 MHz spectrometer (fig. 2) and is unobservable on a 400 MHz spectrometer.

Fig. 2. ¹⁸⁹Os NMR for OSO₄ (1 M) in CCl₄

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Properties of ¹⁸⁹Os

(Click here for explanation)

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References

1. B. E. Mann, C. Masters and B. L. Shaw, "¹J(osmium-187-hydrogen) coupling in the proton nuclear magnetic resonance spectra of some osmium hydrides" J. Chem. Soc. D: Chem. Comm., 1041 (1970).
2. J. A. Cabeza, B. E. Mann, C. Brevard and P. M. Maitlis, "Dinuclear p-cymeneosmium hydride complexes: the measurement of osmium-187 chemical shifts using ¹H-¹⁸⁷Os two-dimensional NMR spectroscopy", J. Chem. Soc., Chem. Comm., 65-67 (1985).
3. R. Benn, E. Joussen, H. Lehmkuhl, F. Lopez Ortiz and A. Rufinska, "Determination of d(¹⁸⁷Os), J(Os,X), and T₁(¹⁸⁷Os) in osmium complexes via indirect 2D NMR spectroscopy" J. Am. Chem. Soc., 111, 8754-8756 (1989).

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