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Complexation of beryllium(II) with hydroxamic acids in aqueous medium; Spectrophotometric determination of stability constants

Beryllium(II) in aqueous solution forms mono and bis N-methylacetohydroxamate complexes depicted by the equation: Be(H2O)42+ + nCH3C(=O)N(-CH3)O- V Be(H2O)4-2n(CH3C(=O)N(-CH3)O)n(2-n)+ + 2nH2O. The stability constants of the mono and bis complexes, determined by the combined pH-spectral titration method at 25 ą 0.1 oC in aqueous medium of 0.1 M ionic strength, were found to be 106.93(5) and 1010.76(8), respectively. Results of the pH-spectral titration indicate that, under similar experimental conditions, beryllium(II) does not form analogous complexes with acetohydroxamic acid in aqueous solution. However, beryllium(II) forms three different complexes with desferrioxamine B (H3dfb) in aqueous solution. Two of these were found to be mononuclear, with the desferrioxamine B bidentately and tetradentately coordinated to the metal ion, whereas the third complex was found to be trinuclear with the ligand hexadentately coordinated to the Be(OH)33+ species. The stability constants for the two mononuclear complexes, related to the following equations: Be(H2O)42+ + Hdfb2- + 2H+ V Be(H2O)2(H3dfb)2+ + 2H2O, and Be(H2O)42+ + Hdfb2- + H+ V Be(H2dfb)+ + 4H2O (Hdfb2- is an anion formed by deprotonation of all three hydroxamate protons), were determined as 1027.63(4) and 1022.165(5), respectively. The formation of the trinuclear complex, described by equation 3Be(H2O)42+ + Hdfb2- V Be3(OH)3(Hdfb)+ + 9H2O + 3H+, is characterized by the stability constant value of 105.85(4). The obtained results reveal that the electronic properties of the N-substituent play an important role in determining the stability of beryllium-hydroxamate complexes, possibly by influencing the size of the hydroxamate coordinating "bite".

complexation; beryllium(II); hydroxamic acids; spectrophotometry; stability constants

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