Density functional theory calculations for the cyclopropanation reactions of several mono zinc carbenoids and their corresponding gem-dizinc carbenoids with ethylene are reported. The mono zinc carbenoids react with ethylene via an asynchronous attack on one CH2 group of ethylene with a relatively high barrier to reaction in the 20-25 kcal/mol range similar to other Simmons-Smith type carbenoids previously studied. In contrast, the gem-dizinc carbenoids react with ethylene via a synchronous attack on both CH2 groups of ethylene and substantially lower barriers to reaction (about 15 kcal/mol) compared to their corresponding mono zinc carbenoid. Both mono zinc and gem-dizinc carbenoid reactions can be accelerated by the addition of Znl(2) groups as a Lewis acid, and this lowers the barrier by another 1.0-5.1 kcal/mol and 0.0-5.5 kcal/mol, respectively, for addition of one Znl(2) group. Our results indicate that gem-dizinc carbenoids react with C=C bonds with significantly lower barriers to reaction and in a noticeably different manner than Simmons-Smith type mono zinc carbenoids. The three gem-dizinc carbenoids have a substantially larger positive charge distribution than those in the mono zinc carbenoids and, hence, a stronger electrophilic character for the gem-dizinc carbenoids.