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8157716 
Journal Article 
Early Stages of the Hydrolysis of Chromium(III) in Aqueous Solution. 1. Characterization of a Tetrameric Species 
Stänzi, H; Marty, W 
1983 
Yes 
Inorganic Chemistry
ISSN: 0020-1669
EISSN: 1520-510X 
22 
15 
2145-2150 
English 
The first few members of the series of hydrolytic polymers of Cr(III), formed upon addition of hydroxide ion (<1 equiv) to the chromium(III) aqua ion, were separated on Sephadex SP C-25. Elution with sodium Perchlorate (0.5-4 M) gave fractions containing Cr3+(aq), and, respectively. Two higher polymers, presumably pentamer and hexamer, were eluted with 4 M LiC1O4. The pKa values of the first oligomers were determined (I = 1 M (NaC1O4) at 25 °C) from pH titrations: 4.29, 6.1 (monomer); 3.68, 6.04 (dimer); 4.35, 5.63, 6.0 (trimer); 2.55, 5.08 (tetramer). The visible spectra of the monomer, dimer, and trimer agree with those given in the literature, and has λmax = 426 ran (e = 30.3) and 580 nm (e = 15.6). The spectrum of the hexamer is very similar to that of the trimer. The tetramer is most conveniently synthesized by adding hydroxide ion to solutions of the pure dimer, but in acidic solution it cleaves rapidly and almost exclusively into monomer and trimer. The order of robustness (in acidic solution) and stability is dimer < trimer >> tetramer. The above observations agree best with a trimer consisting of a trigonal array of the three chromium ions sharing a common hydroxide bridge (2). The properties of the tetramer, suggest that the fourth chromium center may be bound through OH bridges to two of the chromium ions of a trimer unit (3a). On deprotonation, intramolecular condensation is proposed to give (3b) with a central tetracoordinated oxo ligand. Reaction of 51Cr-labeled monomer with excess deprotonated trimer afforded labeled tetramer, which was isolated and cleaved with acid to give equal amounts of label in the monomer and trimer. The tetramer thus contains two equivalent metal centers, consistent with the proposed structures of the tetramer and trimer. © 1983, American Chemical Society. All rights reserved.