The dynamical behavior of the coordinated H-2 Molecule of the PtH(H-2)[P(t-Bu)(3)](2)](+) complex is examined by the ONIOM-molecular dynamics (MD) method that we recently developed. The ONIOM-MD simulations reveal that the dynamical environmental effects of the t-Bu substituents of the phosphine ligands, which increase the magnitude of the energy fluctuations of the active part, significantly promote both rotation and dissociation of the coordinated H-2 molecule. The Matsubara-RRK (M-RRK) theory proposed in this study and Matsubara's equation verify that the dynamical environmental effects are crucial factors to determine the chemical reactivity.