The importance of the role of the ion-lattice coupling in determining the energy levels of $latex Cr^{3+}$ ions in congruent $latex LiNbO _ {3}$ crystals doped with 2.9% of ZnO is demonstrated in this paper. The Racah parameters: $latex B = 533 cm^{-1 }$, $latex C = 3244 cm^{-1}$ and the crystal-field intensity $latex Dq = 1527 cm^{-1}$ were determined and the Tanabe-Sugano diagram was constructed. The characteristics of the absorption and the emission spectra of $latex Cr^{3+}$ ions in these crystals can be explained in terms of the configurational-coordinate diagram in the harmonic approximation, with good agreement obtained. The Huang-Rhys parameter $latex S = 6.86$ and the lattice phonon energy $latex \hbar \omega = 343 cm ^{-1}$ are also reported in this work.