Reaction of [Mo₂(CO)₆(η-C₅Me₅)₂] with methylenecyclopropane and AgBF₄ in CH₂Cl₂ affords the cationic trimethylenemethane complex [Mo{η⁴-C(CH₂)₃}(CO)₂(η-C₅Me₅)][BF₄]. Methylenecyclopropane and 2,2-dimethylmethylenecyclopropane react with [Mo(NCMe)₂(CO)₂(η-C₅H₅)][BF₄] to give [Mo{η⁴-C(CH₂)₃}(CO)₂(η-C₅H₅)][BF₄] and [Mo{η⁴-C(CH₂)₂CMe₂}(CO)₂(η-C₅H₅)][BF₄] respectively. A single-crystal X-ray diffraction study of [Mo{η⁴-C(CH₂)₃}(CO)₂(η-C₅Me₅)][BF₄] confirmed that ring opening of methylenecyclopropane had occurred. The complex crystallises in the centrosymmetric orthorhombic space group Pbca with a= 12.822(2), b= 12.311 (3), c= 22.660(4)Å, and Z= 8 ion pairs. The structure has been solved by conventional methods and refined by full-matrix least squares to R= 0.0674 for 2 747 observed reflections at 268 ± 1 K. In the cation the trimethylenemethane ligand adopts an orientation that is syn with respect to the OC–Mo–CO angle. It is pyramidal with the CH₂ groups bent towards the molybdenum atom by an average of 12.4°. There is intramolecular congestion involving the C₅Me₅ and C(CH₂)₃ ligands that may contribute towards the observed asymmetric bonding of the former to the metal atom. Extended Hückel molecular-orbital calculations suggest, that the observed syn stereochemistry is electronically preferred, and that the barrier to rotation of the η⁴-C(CH₂)₃ ligand relative to a Mo(CO)₂(η-C₅H₅)⁺ fragment is high. The stereochemistry of the ring-opening reaction is disrotatory-out as exemplified by the conversion of cis-and trans-2,3-dimethylmethylenecyclopropane into syn,syn-dimethyl and syn,anti-dimethyl-trimethylene-methane complexes. The reaction of these cationic η⁴-trimethylenemethane complexes with the nucleophiles BH₄^–, OH^–, CuMe₂^–, and SPh^– affords η³-allylic complexes derived from attack on the peripheral carbons.