The addition reaction of SiX2 (X = H, CH3, t-Bu, F, and Cl) to acetylene is considered using ab initio calculations. Second-order perturbation theory (MP2) and coupled cluster CCSD(T) calculations confirm there is no barrier for SiH2, Si(CH3)2, and Si(t-Bu)2 additions, while electronegative substituents such as F and Cl have nonzero barriers. It is concluded that electronegativity is more important than steric effects in determining barrier heights for these reactions. The CCSD(T)/6-31+G* barriers for SiF2 and SiCl2 additions are computed to be about 24 and 8 kcal/mol, respectively.