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)(4)(2+) + nCH(3)C(=O)N(-CH3)O- <--(-->) Be(H2O)(4 - 2n)(CH3C(=O)N(-CH3)O)(n)((2-n) +) + 2nH(2)O. The stability constants of the mono and bis complexes, determined by the combined pH-spectral titration method at 25 +/- 0.1 degrees C in aqueous medium of 0.1 M ionic strength, were found to be 10(6.93(5)) and 10(10.76(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 (H(3)dfb) 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-3(OH)(3)(3+) species. The stability constants for the two mononuclear complexes, related to the following equations: Be(H2O)(4)(2+) + Hdfb(2-) + 2H(+) <--(-->) Be(H2O)(2)(H(3)dfb)(2+) + 2H(2)O, and Be(H2O)(4)(2+) + Hdfb(2-) + H+ <--(-->) Be(H(2)dfb)(+) + 4H(2)O (Hdfb(2-) is an anion formed by deprotonation of all three hydroxamate protons), were determined as 10(27.63(4)) and 10(22.165(5)), respectively. The formation of the trinuclear complex, described by Eq. (3) Be(H2O)(4)(2+) + Hdfb(2-) <--(-->) Be-3(OH)(3)(Hdfb)(+) + 9H(2)O + 3H(+), is characterized by the stability constant value of 10(5.85(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'. (C) 1999 Elsevier Science Inc. All rights reserved.