The overall formation
constant, β3, of tris[2,3-bis(2- pyridyl)pyrazine]iron(II) was
determined spectrophotometrically at 25, 40, 60 and 80�C as 8.9 x 107,
2.9 x 107, 1.0 x 107 and 1.8 x 106 (mol l-1)-3
respectively at pH > 5.5. The pKa of the ligand is 3.1 at 25�C
and decreases to about 2.0 at 80�C. The variation of β3 with
temperature is attributed to differences in the temperature dependence of the
rate constants of the complex formation process and of dissociation. The
overall energy of activation for complex formation is -870 J mol-1.
The complex dissociates according to first order kinetics with rate constants
of 4.3 x 10-3 s-1 and 6.8 x 10-2 s-1
at 25 and 35�C respectively, and the activation energy of the dissociation
reaction is about 3000 J mol-1. The molar absorptivity of the
complex, 11000 1. mol-1 cm-1 at the absorption maximum
(λmax 535 nm), is independent of temperature from 25 to 80�C.
Complex formation constant of ferric ion with Gly, Pro-Hyp and Gly-Pro-Hyp
