Synthesis and reactivity of functionalized cycloheptatrienyl–cyclopentadienyl sandwich complexes

This feature article provides an overview of the synthesis and reactivity of functionalized cycloheptatrienyl-cyclopentadienyl transition metal sandwich complexes of the type [(eta(7)-C(7)H(7))M(eta(5)-C(5)H(5))] (M = group 4, 5 or 6 metal), which can be used as building blocks for the preparation of metallopolymers and polymetallic complexes. Emphasis is placed on 16-electron group 4 complexes (M = Ti, Zr, Hf) and their reactivity towards sigma-donor/pi-acceptor ligands, which indicates that these complexes bear a close resemblance to Lewis acidic M(+IV) complexes. Based on theoretical calculations, this behavior can be mainly attributed to the strong and appreciably covalent metal-cycloheptatrienyl interaction with the cycloheptatrienyl ring acting more as a -3 ligand than as a +1 ligand in these mixed ring complexes.