Osmocenium(IV) dimer (2B, [CpOsIV(C5H4)](2)(BF4)(2)) was obtained by the oxidation reaction of biosmocene with p-benzoquinone and BF3. The result of single-crystal X-ray structural analysis showed that the C-C single bond of the fulvalene ligand was cleaved during the reaction. It was found that the reaction proceeded via a stable intermediate 2A.