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RESEARCH PRODUCT

VIOLURATO COMPLEXES OF Cr(III). SYNTHESIS AND CHARACTERIZATION. PROTONATION-DEPROTONATION EQUILIBRIA OF THE COORDINATED LIGANDS. EVIDENCE OF THE COORDINATION OF VIOLURIC ACID AS A NEUTRAL LIGAND

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Abstract The formation of violurato complexes of Cr(III) has been studied in aqueous solution. Two compounds, [Cr(H2V)3].5H2O and Na[Cr(H2V)2(OH)2].4H2O have been synthesized and characterized. [Cr(H2V)3] behaves in aqueous solution as a triprotic acid but it can also add a proton yielding [Cr(H2V)2(H3V)]+. The acidity constants of this species have been determined: pKa1 =3.32, pKa2 =4.25, pKa3 =4.83 and pKa4 =6.99 (25°C, 0.1 M NaC1O4). [Cr(H2V)2(OH)2]− undergoes a deprotonation of the two H2V− ligands and a protonation of the hydroxo ligands. The acidity constants of [Cr(H2V)2(H2O)2]+ have been also determined: pKa1 =3.5, pKa2 =4.6, pKa3 =7.1 and pKa4 =9.2 (25°C, 0.1 M NaC1O4). The coordination of a violuric acid molecule as a neutral ligand is reported for the first time.