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, molybdenum salts may be toxic.

Molybdenum NMR

Molybdenum (Mo) has two NMR active nuclei that have a very wide chemical shift range. ⁹⁵Mo and ⁹⁷Mo, are both quadrupolar although ⁹⁵Mo yields very narrow lines in small complexes. ⁹⁵Mo is preferred as it has the higher sensitivity and yields much narrower signals than ⁹⁷Mo (fig. 1). Molybdenum NMR is used for the study of molybdenum complexes and, using its relaxation rate, its binding to larger molecules.

Fig. 1. Comparison of the molybdenum isotopes under comparable conditions for Na₂MoO₄ (1 M) in D₂O

Both the isotopes have the same chemical shifts (fig. 2).

Fig. 2. Chemical shift ranges for molybdenum NMR

⁹⁵Molybdenum NMR

(⁹⁵Mo) ⁹⁵Molybdenum is a spin 5/2 nucleus and is therefore quadrupolar. As a result, the signal width increases with asymmetry of the environment but its very small quadrupole moment means that very sharp signals are observed (fig. 3) in small complexes containing one molybdenum atom. Larger complexes (2-4 molybdenums) yield lines that are hundreds to thousands Hertz wide. ⁹⁵Mo is more sensitive and yields much sharper signals than ⁹⁷Mo so ⁹⁵Mo is the nucleus of choice for molybdenum NMR unless studying isotopic enrichment.

Fig. 3. ⁹⁵Mo-NMR spectrum of Na₂MoO₄ (1 M) in D₂O

Because molybdenum has such a wide chemical shift range and ⁹⁵Mo gives narrow signals, the slightest effect can be resolved as in the spectrum in fig. 4 where replacing ³²S with ³⁴S gives extra signals.

Fig. 4. ⁹⁵Mo-NMR spectrum of of (Et₄N)₂MoS₄ in D₂O

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Properties of ⁹⁵Mo

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⁹⁷Molybdenum NMR

(⁹⁷Mo) ⁹⁷Molybdenum is a spin 5/2 nucleus and is therefore quadrupolar. As a result, the signal width increases with asymmetry of the environment. ⁹⁷Mo is less sensitive and yields much wider lines (fig. 5) than ⁹⁵Mo so is not the molybdenum nucleus of choice.

Fig. 5. ⁹⁷Mo-NMR spectrum of Na₂MoO₄ (1 M) in D₂O

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Properties of ⁹⁷mo

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References

1. O. Lutz, A. Nolle and P. Kroneck, "Use of ⁹⁵Mo NMR for identification of molybdenum (VI) chalcogenide anions in aqueous solution", Z. Naturforsch. A, 32, 505-506 (1977).
2. A. F. Masters, R. T. C. Brownlee, M. J. O'Connor, A. G. Wedd and J. D. Cotton, "Molybdenum-95 nuclear magnetic resonance. Applications to substituted carbonyls", J. Organometal. Chem., 195, C17-C20 (1980).
3. P. Kronech, O. Lutz and A. Nolle, "NMR investigations of ¹⁷O, ³³S, ⁹⁵Mo, and ⁹⁷Mo in thiomolybdates", Z. Naturforsch. A, 35, 226-229 (1980).
4. S. Dysart, I. Georgii and B. E. Mann, "Molybdenum-95 NMR spectra of some molybdenum carbonyl and related compounds", J. Organometal. Chem., 213, C10-C12 (1981).
5. G. M. Gray and R. J. Gray, "Synthesis and a multinuclear spectroscopic study of some Mo(CO)₂(PPh₂XR) (X = O, NH; R = 1-4 carbon alkyls) complexes. Steric effects on ³¹P and ⁹⁵Mo chemical shifts", Organometal., 2, 1026-1031 (1983).
6. S. F. Gheller, T. W. Hambley. R. T. C. Brownlee, M. J. O'Connor , M. R. Snow and A. G. Wedd, "Applications of molybdenum-95 NMR spectroscopy. 7. Studies of metal-metal bonded systems including aqueous molybdenum(IV) and molybdenum(V). Crystal and molecular structure of Na₂[Mo₃O₄((O₂CCH₂)₂NCH₃)₃].7H₂O", J. Am. Chem. Soc., 105, 1527-1532 (1983).
7. J. W. Faller and B. C. Whitmore, "Analysis of conformation isomers of molybdenum allyl complexes using ⁹⁵Mo NMR spectroscopy", Organometal., 5, 752-755 (1986).
8. J. C. Green, R. A. Grieves and J. Mason, "Molybdenum-95 and carbon-13 nuclear magnetic shielding and bonding relationships in sandwich complexes", J. Chem. Soc. Dalton Trans., 1313-1316 (1986).
9. E. C. Alyea, A. Malek and J. Malito, "⁹⁵Mo NMR studies of some carbonylate anions", Polyhedron, 5, 403-406 (1986).
10. C. G. Young, M. Minelli, J. H. Enemark, W. Hussain, C. J. Jones and J. A. McCleverty, "A molybdenum-95 and nitrogen-14 nuclear magnetic resonance study of six-co-ordinate hydrotris(3,5-dimethyl-1-pyrazoyl)borate complexes containing a sixteen-electron {Mo(NO)}₄ core", J. Chem. Soc. Dalton Trans., 619-621 (1987).
11. E. C. Alyea and A. Somogyvari, "Molybdenum-95 nuclear magnetic resonance studies on disubstituted molybdenum(0) carbonyls", Can. J. Chem., 66, 397-400 (1988).
12. H, Schumann, J. H. Enemark, M. J. Labarre, M. Bruck and P. Wexler, "⁹⁵Mo NMR investigation on cationic [C₅H₅Mo(CO)₂L₂]BF₄ complexes (L = group 15 donor ligands)", Polyhedron, 10, 665-671 (1991).
13. T. C. Wong, Q. Huang, N. Zhu and X. Wu, "⁹⁵Mo NMR studies of five heterometallic trinuclear incomplete cubane-like clusters", Polyhedron, 16, 2987-2990 (1997).

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