Promethium compounds

Promethium(III) nitrate, a promethium compound[1]

Promethium compounds are compounds containing the element promethium, which normally take the +3 oxidation state. Promethium belongs to the cerium group of lanthanides and is chemically very similar to the neighboring elements.[2] Because of its instability, chemical studies of promethium are incomplete. Even though a few compounds have been synthesized, they are not fully studied; in general, they tend to be pink or red in color.[3][4] Treatment of acidic solutions containing Pm^3+ ions with ammonia results in a gelatinous light-brown sediment of hydroxide, Pm(OH)₃, which is insoluble in water.[5] When dissolved in hydrochloric acid, a water-soluble yellow salt, PmCl₃, is produced;[5] similarly, when dissolved in nitric acid, a nitrate results, Pm(NO₃)₃. The latter is also well-soluble; when dried, it forms pink crystals, similar to Nd(NO₃)₃.[5] The electron configuration for Pm^3+ is [Xe] 4f^4, and the color of the ion is pink. The ground state term symbol is ^5I₄.[6] The sulfate is slightly soluble, like the other cerium group sulfates. Cell parameters have been calculated for its octahydrate; they lead to conclusion that the density of Pm₂(SO₄)₃·8 H₂O is 2.86 g/cm^3.[7] The oxalate, Pm₂(C₂O₄)₃·10 H₂O, has the lowest solubility of all lanthanide oxalates.[8]

Unlike the nitrate, the oxide is similar to the corresponding samarium salt and not the neodymium salt. As-synthesized, e.g. by heating the oxalate, it is a white or lavender-colored powder with disordered structure.[5] This powder crystallizes in a cubic lattice upon heating to 600 °C. Further annealing at 800 °C and then at 1750 °C irreversibly transforms it to monoclinic and hexagonal phases, respectively, and the last two phases can be interconverted by adjusting the annealing time and temperature.[9]

Formula	symmetry	space group	No	Pearson symbol	a (pm)	b (pm)	c (pm)	Z	density, g/cm^3
α-Pm	dhcp[10][11]	P63/mmc	194	hP4	365	365	1165	4	7.26
β-Pm	bcc[11]	Fm3m	225	cF4	410	410	410	4	6.99
Pm2O3	cubic[9]	Ia3	206	cI80	1099	1099	1099	16	6.77
Pm2O3	monoclinic[9]	C2/m	12	mS30	1422	365	891	6	7.40
Pm2O3	hexagonal[9]	P3m1	164	hP5	380.2	380.2	595.4	1	7.53

Promethium forms only one stable oxidation state, +3, in the form of ions; this is in line with other lanthanides. According to its position in the periodic table, the element cannot be expected to form stable +4 or +2 oxidation states; treating chemical compounds containing Pm^3+ ions with strong oxidizing or reducing agents showed that the ion is not easily oxidized or reduced.[2]

Promethium halides[12]

Formula	color	coordination number	symmetry	space group	No	Pearson symbol	m.p. (°C)
PmF3	Purple-pink	11	hexagonal	P3c1	165	hP24	1338
PmCl3	Lavender	9	hexagonal	P63/mc	176	hP8	655
PmBr3	Red	8	orthorhombic	Cmcm	63	oS16	624
α-PmI3	Red	8	orthorhombic	Cmcm	63	oS16	α→β
β-PmI3	Red	6	rhombohedral	R3	148	hR24	695

Bibliography

• Lavruk︠h︡ina, Avgusta Konstantinovna; Pozdni︠a︡kov, Aleksandr Aleksandrovich (1970). Analytical chemistry of technetium, promethium, astatine and francium. S2CID 94160197.

References

1. Driscoll, Darren M.; White, Frankie D.; Pramanik, Subhamay; Einkauf, Jeffrey D.; Ravel, Bruce; Bykov, Dmytro; Roy, Santanu; Mayes, Richard T.; Delmau, Lætitia H.; Cary, Samantha K.; Dyke, Thomas; Miller, April; Silveira, Matt; VanCleve, Shelley M.; Davern, Sandra M. (May 2024). "Observation of a promethium complex in solution". Nature. 629 (8013): 819–823. Bibcode:2024Natur.629..819D. doi:10.1038/s41586-024-07267-6. ISSN 1476-4687. PMC 11111410. PMID 38778232.
2. Lavruk︠h︡ina & Pozdni︠a︡kov 1970, p. 120.
3. Emsley, J. (2011). Nature's Building Blocks: An A-Z Guide to the Elements. OUP Oxford. p. 429. ISBN 978-0-19-257046-8.
4. promethium. Encyclopædia Britannica Online
5. Lavruk︠h︡ina & Pozdni︠a︡kov 1970, p. 121.
6. Aspinall, H. C. (2001). Chemistry of the f-block elements. Gordon & Breach. p. 34, Table 2.1. ISBN 978-9056993337.
7. Lavruk︠h︡ina & Pozdni︠a︡kov 1970, p. 122.
8. Lavruk︠h︡ina & Pozdni︠a︡kov 1970, p. 123.
9. Chikalla, T. D.; McNeilly, C. E.; Roberts, F. P. (1972). "Polymorphic Modifications of Pm2O3". Journal of the American Ceramic Society. 55 (8): 428. doi:10.1111/j.1151-2916.1972.tb11329.x.
10. Pallmer, P. G.; Chikalla, T. D. (1971). "The crystal structure of promethium". Journal of the Less Common Metals. 24 (3): 233. doi:10.1016/0022-5088(71)90101-9.
11. Gschneidner Jr., K.A. (2005). "Physical Properties of the rare earth metals" (PDF). In Lide, D. R. (ed.). CRC Handbook of Chemistry and Physics (86th ed.). Boca Raton, FL: CRC Press. ISBN 978-0-8493-0486-6. Archived from the original (PDF) on 2012-09-18. Retrieved 2012-06-20.
12. Cotton, Simon (2006). Lanthanide And Actinide Chemistry. John Wiley & Sons. p. 117. ISBN 978-0-470-01006-8.