KINETICS AND MECHANISM OF THE REACTION BETWEEN CHROMIUM(II) AND 1,2-BIS(2-PYRIDYL)ETHYLENE IN ACIDIC AQUEOUS-SOLUTIONS

The reduction of 1,2-bis(2-pyridyl)ethylene by chromium(II) perchlorate in aqueous perchloric acid to the corresponding dipyridylethane has been shown to take place through a sigma-bonded organochromium intermediate. The acidolysis of this complex takes place in two stages, the first of which has an observed rate constant k(obs) = k0 + k1[H+]. The corresponding activation parameters are DELTAH-0 double-dagger = 92.0 =/- 1.6 kJ mol-1, DELTAS0 double-dagger = -15.1 +/- 4.9 J K-1 mol-1, DELTAH-1 double-dagger = 142 +/- 3 kJ mol-1, and DELTAS1 double-dagger = 142 +/- 7 J K-1 mol-1. The experimental results are discussed in terms of the nature of the lowest unoccupied molecular orbital of the substrate, which is 'ethylenic'.