The hybrid density functional B3LYP method has been used to study the mechanism of disilane formation from silylenes. The main findings can be summarized as follows: (a) Lewis acid-base complexes between silylenes and halocarbons only play a limited role in silylene insertions, and therefore the acid-base complex mechanism proposed by West et al. does not apply to disilane formation. (b) Regardless of whether transient or stable, aft silylenes follow the energetically favorable general reaction pathway: (1) Y2Si:+ClCR3 --> TS1 --> Y2ClSi-CR3. (II) Y2Si:+Y2ClSi-CR3 --> TS2 --> Y2ClSi-SiY2CR3. (C) 2003 Elsevier Science B.V. All rights reserved.