Structure of the hydrated, hydrolysed and solvated zirconium(iv) and hafnium(iv) ions in water and aprotic oxygen donor solvents. A crystallographic, EXAFS spectroscopic and large angle X-ray scattering study

Abstract

The tetrameric hydrolysis products of zirconium(IV) and hafnium(IV), the zirconyl(IV) and hafnyl(IV) ions, [M(4)(OH)(8)(OH(2))(16)(8+)], often labelled MO(2+).5H(2)O, are in principle the only zirconium(IV) and hafnium(IV) species present in aqueous solution without stabilising ligands and pH larger than zero. These complexes are furthermore kinetically very stable and do not become protonated even after refluxing in concentrated acid for at least a week. The structures of these complexes have been determined in both solid state and aqueous solution by means of crystallography, EXAFS and large angle X-ray scattering (LAXS). Each metal ion in the [M(4)(OH)(8)(OH(2))(16)](8+) complex binds four hydroxide ions in double hydroxo bridges, and four water molecules terminally. The M-O bond distance to the hydroxide ions are markedly shorter, ca. 0.12 A, than to the water molecules. The hydrated zirconium(IV) and hafnium(IV) ions only exist in extremely acidic aqueous solution due to their very strong tendency to hydrolyse. The structure of the hydrated zirconium(IV) and hafnium(IV) ions has been determined in concentrated aqueous perchloric acid by means of EXAFS, with both ions being eight-coordinated, most probably in square antiprismatic fashion, with mean Zr-O and Hf-O bond distances of 2.187(3) and 2.160(12) A, respectively. The dimethyl sulfoxide solvated zirconium(IV) and hafnium(IV) ions are square antiprismatic in both solid state and solution, with mean Zr-O and Hf-O bond distances of 2.193(1) and 2.181(6) A, respectively, in the solid state. Hafnium(IV) chloride does not dissociate in N,N'-dimethylpropyleneurea, dmpu, a solvent with good solvating properties but with a somewhat lower permittivity (epsilon= 36.1) than dimethyl sulfoxide (epsilon= 46.4), and an octahedral HfCl(4)(dmpu)(2) complex is formed.

Hydrolysis Constant and Coordination Geometry of Zirconium(IV)

“…Also, an antiprismatic structure is assumed for mononuclear hydrolysis species, which has been suggested for the Zr(IV) ion in aqueous solution by means of EXAFS. 7) For the formation of polynuclear species, a polymerization process similar to the case of tetravalent actinide ions was assumed by considering the formation of polymers bridged by hydroxide ions. For simplicity, the formation of oxygen bridges is not taken into account in the present study.…”

Periodates of Tetravalent Titanium, Zirconium, Hafnium and Thorium: Synthesis, Characterisation and EXAFS Study

“…In large part, this is a result of the complex and only partially understood solution chemistry of the group 4 M IV ions in which oxo-or hydroxo-bridge oligomeric hydrolysis products are common features. [16,17] Often this results in rather intractable chemistry. Our general approach was to carry out a variety of exploratory syntheses, examine the products formed by energy dispersive X-ray analysis (EDX) to establish the elements present and their ratios, and use IR spectroscopy to examine the form ([IO 4 ] -or [H 5-n IO 6 ] n-) of any periodate present.…”

Rare earths, zirconium and hafnium distribution in coastal areas: The example of Sabella spallanzanii (Gmelin, 1791)

“…This coherence is absent during aqueous processes due to the different grades of Zr and Hf hydrolysis and dissolved complexation in natural waters (Aja et al, 1995;Veyland et al, 1998;Byrne, 2002;Hagfeldt et al, 2004;Messner et al, 2011a;2011b;Inguaggiato et al, 2015Inguaggiato et al, , 2016Censi et al, 2017). Aqueous environments usually make Hf more surface-reactive than Zr, which changes the associated Zr/Hf with respect to the range of values recognised in rocks and minerals and provides a useful signature of solid-liquid processes.…”