The kinetics and mechanism of the complexation of La3+ and Cu2+ ions with desferrioxamine B (H(4)dfb(+)) and N-methylacetohydroxamic acid (NMHA) in aqueous medium were studied by stopped-flow and H-1 NMR methods. The equilibrium constants for reactions (Mn+ f HA reversible arrow MA((n-1)+) + H+) of NMHA with Cu2+ and La3+ were determined by the combined pll-spectral titration method at 25 +/- 0.1 degrees C as (1.58 +/- 0.02) x 10(-1) and (3.5 +/- 0.2) x 10(-4), respectively. In aqueous solution of 2 M ionic strength, the kinetic parameters for complexation of La3+ and Cu2+ with NMHA were determined as k(25 degrees C) = 3.0 +/- 0.3 s(-1), Delta H double dagger = 76 +/- 3 kJ mol(-1), Delta S double dagger = 19 +/- 7 J K-1 mol(-1), Delta V double dagger = +5.3 +/- 0.5 cm(3)/mol and k(25 degrees C) = 3.4 +/- 0.2 s(-1), Delta H double dagger = 69 +/-1 kJ mol(-1), Delta S double dagger = -3 +/- 3 J K-1 mol(-1), Delta V double dagger = +5.0 +/- 0.5 cm(3)/mol, whereas with H(4)dfb(+) they were determined as k(25 degrees C) = 2.9 +/-0.3 s(-1), Delta H double dagger = 76 +/- 1 kJ mol(-1), Delta S double dagger = 34 +/- 4 J K-1 mol(-1), Delta V double dagger = +5.2 +/- 0.5 cm(3)/mol; and k(25 degrees C) = 3.0 +/-0.4 s(-1), Delta H double dagger = 72 +/- 1 kJ mol(-1), Delta S double dagger = 5 +/- 2 J K-1 mol(-1), Delta V double dagger = +3.4 +/- 0.2 cm(3)/mol, respectively. The rotation about the C-N hydroxamate bonds in NMHA and H4dfb+ is characterized by k(25 degrees C) = 11 +/- 2 s(-1), Delta H double dagger = 76 +/- 5 kJ mol(-1), Delta S double dagger = 31 +/- 16 J K-1 mol(-1), Delta(r)V((cis reversible arrow trans)) = +1.5 +/- 0.8 cm(3)/mol, k(trans-->cis)(25 degrees C) = 2.8 +/- 0.5 s(-1), Delta V double dagger(trans-->cis) = +12 +/- 4 cm(3)/mol and by k(25 degrees C) = 9 +/- 1 s(-1), Delta H double dagger = 69 +/-6 kJ mol(-1), Delta S double dagger = 6 +/- 18 J K-1 mol(-1), Delta(r)V((cis reversible arrow trans)) = +0.6 +/- 0.3 cm(3)/mol, k(trans-->cis)(25 degrees C) = 2.6 +/- 0.3 s(-1), Delta V double dagger(trans-->cis) = +5 +/- 2 cm(3)/mol, respectively. The results suggest that the slow rotation around the hydroxamate C-N bond is the rate-determining step for the complexation reactions.