The reaction of bis (acetonitrile) bis (diethyl fumarate) cobalt (0) with dibromomethane gave diethyl trans-1, 2-cyclopropanedicarboxylate in a 67% yield based on the cobalt complex. Dimethyl cis-1, 2-cyclopropanedicarboxylate was obtained in a 52% yield from the reaction of dimethyl maleate with dibromomethane in the presence of acetonitrilebis (dimethyl maleate) nickel (0) accompanied by the formation of the trans-isomer in a 14% yield. The ...