GROUP-14 METHYLENE METHANE-TYPE COMPLEXES - A THEORETICAL-STUDY

Neutral complexes between Group 14 singlet methylene- and methane-type fragments, H-3X-H ... XH-2, have been investigated by ab initio calculations. Beyond carbon, such singly bridged complexes are found to be significantly bound, as has recently been reported for silicon. At our best level of treatment (MP4 energies on SCF-DZP geometries), the binding energies relative to the set XH-4 + XH-2 range from 5 to 10 kcal mol-1 from silicon to lead. Starting from these complexes, the pathways for methylene insertion into methane are examined for carbon and all its analogues. The rearrangement H-3X-H-XH-2 --> H-3X --> XH-3 becomes less exothermic with heavier elements. The corresponding energy differences range from 49 to 9 kcal mol-1 from silicon to lead. This is on a par with increasing energy barriers, which are calculated to be near zero for carbon and silicon, and increases up to 20 kcal mol-1 for lead. Other aspects of the X2H-6 potential surfaces are discussed, in particular a low-energy saddle point is found for hydrogen exchange between SiH4 and SiH2.