X-ray diffraction and inelastic X-ray scattering studies have been performed across the spin (~100 K) and semiconductor-metal (~500 K) transitions in $LaCoO_{3}$ single crystals. The quadratic increase with temperature of the oxygen displacement parameters parallel and perpendicular to the Co-O bond has been correlated with softening of the TO2 and hardening of the $TO_{1}$ phonon branches along the [0 ξ ξ] high symmetry direction. The latter effect can be associated with the weakening of the Co-O bond strength derived from the increase of Co-O bond length and angle as expected upon increasing the high spin state population of the system with temperature.