Structural Systematics of Rare Earth Complexes. I. Structural Characterization of Lanthanoid(III) Picrate Hydrates: Monoclinic (P2₁/C) (Quasi-)Dodecahydrates of the Related La → Pr and Nd → Tb Families

Abstract

Single-crystal room-temperature X-ray studies on hydrated specimens (deposited from aqueous solutions) of the sequence of trivalent lanthanoid picrates La → Tb have shown, with certain qualifications/reservations discussed herein and in Parts III and IV following, their existence as dodecahydrates in two closely related but subtly different monoclinic P2₁/c structural types. Nd → Tb generally conform to a 'low-β' type: a 7.767(1) → 7.603(3), b 27.525(9) → 27.440(10), c 18.198(11) → 18.182(8) Ǻ, β 105.11(3) → 105.18(4)°, V 3712(3) → 3674(3) Ǻ₃, Z = 4 formula units, while La → Pr are a 'higher-β' type: a 7.933(1) → 7.9435(9), b 26.856(6) → 26.74(1), c 18.244(6) → 18.215(2) Ǻ, β 107.96(2) → 108.238(8)°, V 3697(2) → 3675(2) Ǻ³, Z = 4 formula units; the latter form is frequently found partially desolvated toward the undecahydrate without loss of crystal integrity, as water is lost from tunnels in the lattice, with the actual hydrate stoichiometry dependent on the history of the specimen. In both sets of compounds, similar complex species [ Ln ( pic )₂(OH₂)₆]+ are observed, with one picrate uncoordinated as counter ion, and six lattice water molecules. The lanthanoid ion is >8-coordinate, one of the coordinated picrate moieties being quasi-bidentate , with an ortho-nitro group oxygen atom having a long contact to the metal atom in both series.