Crystal structures of the penta- and hexa-hydrate of thulium nitrate.

Wilhelm Klein

doi:10.1107/s2056989020015388

Abstract

Tm(NO3)3·5H2O and Tm(NO3)3·6H2O, or more precisely [Tm(NO3)3(H2O)4]·H2O and [Tm(NO3)3(H2O)4]·2H2O, respectively, have been obtained from a concentrated solution of Tm2O3 in HNO3. The crystal structures of the two hydrates show strong similarities as both crystallize in space group P with all atoms at general positions and contain neutral, mol-ecular [Tm(NO3)3(H2O)4] complexes, i.e. ten-coordinated TmIII cations with three nitrate anions as bidentate ligands and four coordinating water mol-ecules, and one or two additional crystal water mol-ecules, respectively. All building units are connected by medium-strong to weak O-H⋯O hydrogen bonds. Tm(NO3)3·6H2O represents the maximally hydrated thulium nitrate as well as the heaviest rare earth nitrate hexa-hydrate known to date.

Highlights

The nitrates of the rare earth metals have long been used to separate and purify these elements
When thulium was discovered (Cleve, 1897), fusion of nitrates was already used to separate the element from the erbiumcontaining earth, and a hydrate of thulium nitrate in substance was already described more than 100 years ago with four equivalents of water of crystallization and of highly hygroscopic nature (James, 1911)
Considering the structural information for the maximally hydrated rare earth nitrates, a general tendency of a decreasing amount of water with increasing atomic number is obvious: for the lighter homologues La–Nd and Sm–Tb, the hexahydrates are found as maximally hydrated compounds for the nitrates (La: Eriksson et al, 1980; Ce: Milinski et al, 1980; Pr: Decadt et al, 2012; Nd: Rogers et al, 1983; Sm: Kawashima et al, 2000; Eu: Stumpf & Bolte, 2001; Gd: Taha et al, 2012; Tb: Moret et al, 1990) while for the heavier elements Dy–Er and Yb, only pentahydrates have been reported (Ho: Rincke et al, 2017; other: Junk et al, 1999)

Summary

Chemical context

The nitrates of the rare earth metals have long been used to separate and purify these elements. Considering the structural information for the maximally hydrated rare earth nitrates, a general tendency of a decreasing amount of water with increasing atomic number is obvious: for the lighter homologues La–Nd and Sm–Tb, the hexahydrates are found as maximally hydrated compounds for the nitrates (La: Eriksson et al, 1980; Ce: Milinski et al, 1980; Pr: Decadt et al, 2012; Nd: Rogers et al, 1983; Sm: Kawashima et al, 2000; Eu: Stumpf & Bolte, 2001; Gd: Taha et al, 2012; Tb: Moret et al, 1990) while for the heavier elements Dy–Er and Yb, only pentahydrates have been reported (Ho: Rincke et al, 2017; other: Junk et al, 1999). While the pentahydrate of Tm(NO3) fills the gap within the known compounds containing Er and Yb, the hexahydrate represents the highest hydrated nitrate including Tm and shifts the border of known stable compounds notably to heavier rare earth elements

Structural commentary

Database survey

Refinement