Comprehensive structural study of lanthanide(III) chloride hydrates: [<i>RE</i>Cl₃·<i>x</i>H₂O (<i>RE</i> = La–Nd, Sm–Lu; <i>x</i> = 6, 7)]

A comprehensive crystallographic study is presented of the complete series of rare-earth(III) chloride hydrates. Early lanthanides form dimeric [(H₂O)₇ <i>RE</i>(μ-Cl)₂ <i>RE</i>(H₂O)₇]⁴⁺ binuclear complexes in which each <i>RE</i> atom (<i>RE</i> = La³⁺, Ce³⁺) is coordinated by seven H₂O mol-ecules and two bridging inner-sphere chloride ions. Di-μ-chlorido-bis-[hepta-aqua-lanthanide(III)] tetra-chloride, [(H₂O)₇ <i>RE</i>(μ-Cl)₂ <i>RE</i>(H₂O)₇]Cl₄, crystallizes in the triclinic space group <i>P</i>1. Heavier lanthanides exhibit monomeric [<i>RE</i>Cl₂(H₂O)₆]⁺ units where each <i>RE</i> atom (<i>RE</i> = Pr³⁺, Nd³⁺, Sm³⁺-Lu³⁺) is coordinated by six H₂O mol-ecules and two inner-sphere chloride ions. Hexa-aqua-dichlorido-lanthanide(III) chlorides, [<i>RE</i>Cl₂(H₂O)₆]Cl, adopt the monoclinic space group <i>P</i>2/<i>c.</i> In both structures, the cationic <i>inner-sphere</i> complex is counter-charged by the corresponding number of <i>outer-sphere</i> Cl^- anions, in which the metal ion and outer-sphere chloride ion lie on crystallographic twofold axes. Crystal structures for all compounds were determined in high quality with refined H-atom positions and were collected at the same temperature (100 K), providing a uniform structural dataset and addressing discrepancies in previous reports. The crystal structure of [HoCl₂(H₂O)₆]Cl is reported for the first time.