Isolation of the simplest 4π three‐membered heterocycles (1H‐azirine, oxirene, thiirene, and selenirene) remains a big challenge due to their π‐antiaromaticity and significant ring strain. Here we demonstrate that the incorporation of a transition‐metal fragment could stabilize the antiaromatic selenirene and pentalene frameworks simultaneously by density functional theory (DFT) calculations. Experimental verification leads to the Se‐containing metallapolycycles, osmapentaloselenirenes, with remarkable thermal stability. The osmaselenirene unit in the metallapolycycle is determined to be the first example of σ‐aromaticity dominating in an unsaturated Se‐containing ring. Our results not only highlight a remarkable stabilization by the transition‐metal but also widen the scope of σ‐aromaticity in unsaturated rings, which is traditionally reserved for the domain of π‐aromaticity.

https://onlinelibrary.wiley.com/doi/abs/10.1002/chem.201703870