Iron complexes, especially bis(imino)pyridyliron dichlorides, had been extensively explored for their catalytic behaviour in ethylene oligomerization and polymerization. The fatal natures of late-transition metal pro-catalysts generally show that the catalytic activities are decreased for producing oligomers when the reaction temperatures are elevated. Iron complexes have recently been approved as useful pro-catalysts because such catalytic systems could be controlled for either oligomerization or polymerization, and catalytic activities remained high at the elevated temperature.