An ab initio classical trajectory method, the dynamic reaction coordinate (DRC) method based on ab initio electronic structure calculations, is applied to a study of the chemical reaction SiH4 + H- --> SiH5-. Both side attack (C-2v symmetry) and front attack (C-3v symmetry) of H- on SiH4 are examined. To analyze the nature of the intramolecular vibrational energy transfer, the DRC and its corresponding momentum are mapped onto normal modes of both reactant and product systems. These analyses show that Berry pseudorotation occurs repeatedly in the SiH5- produced by the side attack, whereas the S(N)2 reaction H- + SiH4 --> SiH4 + H- often occurs upon front attack depending on the initial relative velocity.