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Methylenecyclopropane (1a) reacts with bis(trimethylphosphane)titanocene (8) and (η2-1-butene)(trimethylphosphane)zirconocene (12a) via olefin complexes; e.g. 9, in a [2+2] cycloaddition to afford the metallacyclopentane derivates 10, 13 and 14. In contrast, a [3+2] cycloaddition reaction is observed with bis(η5-lndenyl)(η2-1-butene)(trimethylphosphane)zirconium (12b) to furnish the 1-zircona-4-methylenecyclohexane 15. Similarly, reactions of the complexes 8 and 12a with benzylidenecyclopropane (1c) give the olefin complexes 9c and 20, while a direct reaction between the indenyl complex 12b with 1c occurs to furnish the 1-zircona-2-benzylidenecyclobutane complex 21b which was characterised by X-ray crystallography. [Copyright &y& Elsevier]