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Abstract Displacement of 2-methvlpropene from [Fp(Me2C = CH2)]BF4 (Fp = (η5-C5H5)Fe(CO)2) by Z-1,2-dimethoxyethylene (Z-DME) gives [Fp(Z-DME)]BF4. Displacement by E-1,2-dimethoxyethylene (E-DME), in contrast, does not give the expected complex of E-DME, but rather the complex of the Z-alkene. A mechanism involving scission of the C = C bond of the coordinated alkene to give a bis-carbene intermediate is proposed.