cyclobutadiene

chemical compound

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June 25, 2026

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Cyclobutadiene (C₄H₄) is a cyclic hydrocarbon that has resisted attempts at chemical synthesis due to its high reactivity. While it has four carbon atoms and alternating single and double bonds, it does not possess the special stability required to be classified as aromatic. Unlike benzene, which is aromatic, cyclobutadiene has a rectangular structure with alternating single and double bonds, preventing electron delocalization necessary for aromaticity. It is known to exist in stable complexes with metal atoms, where coordination stabilizes the otherwise unstable molecule.12

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hydrocarbon
Nonbenzenoid aromatic compounds
The earliest targets were cyclobutadiene (C₄H₄) and cyclooctatetraene (C₈H₈).
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The earliest targets were cyclobutadiene (C₄H₄) and cyclooctatetraene (C₈H₈).
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organometallic compound
Cyclic polyene ligands
The cyclobutadiene ligand is a four-electron donor. It is unstable as the free (i.e., uncombined) hydrocarbon, but it is known to exist in stable complexes, including Ru(C₄H₄)(CO)₃.
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The cyclobutadiene ligand is a four-electron donor. It is unstable as the free (i.e., uncombined) hydrocarbon, but it is known to exist in stable complexes, including Ru(C₄H₄)(CO)₃.
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