The spectroscopic effects of electron-phonon (e-ph) coupling in mixed-valence halogen-bridged chain complexes are investigated through a parallel infrared and Raman study of three compounds with decreasing metal-metal distance along the chain. The e-ph interaction is analyzed in terms of the Herzberg-Teller coupling scheme. We take into account the quadratic term and define a precise reference state. The force field relevant to this state is constructed, whereas the electronic structure is analyzed in terms of a simple phenomenological model, singling out a trimeric unit along the chain. In this way we are able to account for all the available optical data of the three compounds, and to estimate the relevant microscopic parameters, such as the e-ph coupling constants and the charge-density-wave amplitude. The latter correlates linearly with the reciprocal metal-metal distance.