Group 4 transition metal sandwich complexes: Still fresh after almost 60 years

Metal sandwich compounds, more commonly known as metallocenes, are some of the most enduring molecules in organometallic chemistry. Beginning with Wilkinson's quest for titanocene, [(?⁵-C₅H ₅)₂Ti], the synthesis and exploration of the electronic structure and reactivity of group 4 metal sandwich complexes have been an ongoing story for almost six decades. This review recounts the historical origins of group 4 metallocenes and highlights synthetic efforts to observe and stabilize formally d² metal sandwich compounds. Specific attention is devoted to recent developments in the field, including dinitrogen coordination by various substituted titanocene derivatives as well as our group's interest in bis(indenyl)zirconium and -hafnium compounds.