The theoretical barriers and reaction energetics have been determined, using ab initio calculations at the MP4/6-3IG(d) level, with 3-2IG* geometries, for the insertions of methylene and silylene into the X-Y bonds of ethane, methylsilane, disilane, and cyclopropane and into the X-H bonds of disilane and ethane. A correlation was observed between barrier heights and lengths of substrate bonds. The largest barriers were observed for methylene and silylene inserting into the C-C bonds of ethane. The barriers became successively lower for the C-Si and Si-Si bonds. Steric interactions thus appear to be a major factor in determining barrier heights. The observed barriers for silylene and methylene inserting into the strained c-c bonds of cyclopropane were approximately 40 kcalfmol less than those for the analogous unstrained compounds.