We have used femtosecond impulsive excitation to time-resolve the vibrational motions following the excitation of the optical intervalence charge transfer transition in a quasi-one-dimensional molecular solid, the mixed-valence metal-halide linear chain complex [Pt(en)(2)][Pt(en)(2)Br-2].(PF6)(4) (en = ethylenediamine, C2H8N2) Wavelength-resolved measurements reveal an induced absorption signal assigned to the formation of the self-trapped exciton that is strongly modulated by vibrational wavepacket oscillations originating on both the ground and excited electronic states. The ground-state response, which includes the fundamental chain axis symmetric stretch frequency and its harmonics, is discussed in terms of resonantly enhanced impulsive stimulated Raman excitation of a strongly coupled electron-phonon system.