We have prepared several pyrazolyl palladium and nickel complexes ([(L1)PdCl2] (1), [(L2) PdCl2] (2), [(L3) PdCl2] (3), [(L1) NiBr2] (4), [(L2) NiBr2] (5) and [(L3) NiBr2] (6)) by reacting 3,5-dimethyl-1H-pyrazole (L1), 3,5-di-tert-butyl-1H-pyrazole (L2) and 5-ferrocenyl-1H-pyrazole(L3) with [PdCl2(NCMe)2] or [NiBr2(DME)] to afford mononuclear palladium and nickel complexes, respectively. These complexes were then investigated as pre-catalysts in the hydrogenation of 2,4-hexadienoic acid (sorbic acid). The active catalysts from these complexes demonstrate significant activities under mild experimental conditions. Additionally, the active catalysts show that the hydrogenation of sorbic acid proceeds in a sequential manner, where the less hindered C=C bond (4-hexenoic acid) is preferentially reduced over the more hindered C=C bond (2-hexenoic acid)
