Strained Metallocenophanes and Related Organometallic Rings Containing π-Hydrocarbon Ligands and Transition-Metal Centers

The structures, bonding, and ring‐opening reactions of strained cyclic carbon‐based molecules form a key component of standard textbooks. In contrast, the study of strained organometallic molecules containing transition metals is a much more recent development. A wealth of recent research has revealed fascinating nuances in terms of structure, bonding, and reactivity. Building on initial work on strained ferrocenophanes, a broad range of strained organometallic rings composed of a variety of different metals, π‐hydrocarbon ligands, and bridging elements has now been developed. Such strained species can potentially undergo ring‐opening reactions to functionalize surfaces and ring‐opening polymerization to form easily processed metallopolymers with properties determined by the presence of the metal and spacer. This Review summarizes the current state of knowledge on the preparation, structural characterization, electronic structure, and reactivity of strained organometallic rings with π‐hydrocarbon ligands and d‐block metals. Tension you can cut without a knife: Over the past 40 years, metallocenophanes and related species have emerged as the first broad class of strained organometallic rings to be systematically developed and studied (for examples, see scheme; Mes=2,4,6‐trimethylphenyl). This Review focuses on the synthesis, structures, and reactivities of these interesting species based on π‐hydrocarbon ligands and transition‐metal centers.