This article describes the preparation, characterization and reactions of the diene complexes of group 2-5 metals, whose structure and reactivity are highly depending on the kind of metal as follows. For group 2 metals, the structures_ of calcium and strontium diene complexes are between η
4-diene and metallacyclo-3-pentene, while the structure of magnesium-diene complex is metallacyclo-3-pentene. For lanthanide elements, metallic lanthanide reacted with cyclooctatetraene and an oxidant to give [LnX(η
8-C
8H
8) (thf)
m]
n, and the reaction of lanthanum with 1, 4-diphenyl-1, 3-butadiene and iodine afforded an inverse sandwich complex, [LaI
2(thf)
3{μ-(η
4:η
4-1, 4-diphenyl-1, 3-butadiene)} LaI
2 (thf)
3]. The s-trans diene coordination mode as well as s-cis diene coordination mode were found for metallocene complexes of zirconium and hafnium. Diene complexes of niobium and tantalum MX
2(η
5-C
5R
5)(η
4-diene) in combination with methylaluminoxane are the catalyst precursors of the living polymerization of ethylene to give polyethylene with very narrow polydispersities (M
w/M
n, as low as 1.05).
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