The Serendipitous Discovery of Rollover Cyclometalation: From Unrecognized Observation to Transformative Chemistry

Rollover cyclometalation is a distinctive variant of classical cyclometalation where a chelated heterocyclic ligand undergoes internal rotation, releasing one donor atom from coordination and subsequently activating a remote C─H bond with high regioselectivity. The resulting rollover complexes bear an uncoordinated donor atom that confers unique reactivity unavailable to conventional cyclometalated species. The discovery of rollover cyclometalation represents a remarkable example of serendipity in organometallic chemistry, a reaction observed multiple times over decades before being recognized as a distinct and valuable transformation. First appearing in the literature in 1975 and formally named in 1985, rollover cyclometalation involves an unconventional ligand rearrangement to form a new metallacycle. This review traces the serendipitous discovery narrative, emphasizing how early researchers observed the reaction without recognizing its novelty, and examines how subsequent mechanistic studies established rollover cyclometalation as a distinct reaction class. The review explores the reaction’s broad scope beyond 2,2′‐bipyridine, its novel features including reversible retro‐rollover processes, mechanistic insights in gas‐ and condensed‐phase, and growing applications in catalysis, materials science, and novel metallodrugs. This discovery exemplifies Thomas Kuhn’s observation that scientific (r)evolutions often begin with anomalies that challenge existing frameworks, demonstrating how preparedness and systematic investigation can transform unexpected observations into transformative knowledge.