Promethium compounds

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 Pm³⁺ 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 Pm³⁺ is [Xe] 4f⁴, 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/cm³.^[7] The oxalate, Pm₂(C₂O₄)₃·10 H₂O, has the lowest solubility of all lanthanide oxalates.^[8]

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

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/cm3
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 Pm³⁺ 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.