Kinetics and mechanism of the reaction between chromium(II) and 1,2-bis(2-pyridyl)ethylene in acidic aqueous solutions

Abstract

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 σ-bonded organochromium intermediate. The acidolysis of this complex takes place in two stages, the first of which has an observed rate constant k_obc=k_o+k₁[H⁺]. The corresponding activation parameters are ΔH_o^‡= 92.0 ± 1.6 kJ mol^–1, ΔS_o^‡=–15.1 ± 4.9 J K^–1 mol^–1, ΔH₁^‡= 142 ± 3 kJ mol^–1, and ΔS₁^‡= 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’.