  We study with the Raman scattering technique two types of phase transitions
in the pyroxene compounds NaMSi$_2$O$_6$ (with M=Ti, V, and Cr). In the quasi
one-dimensional S=1/2 system NaTiSi$_2$O$_6$ we observe anomalous
high-temperature phonon broadening and large changes of the phonon energies and
line-widths across the phase transition at 210 K. The phonon anomalies
originate from an orbital order-disorder phase transition and these results
--combined with theoretical considerations-- indicate that the high temperature
dynamical Jahn-Teller phase of NaTiSi$_2$O$_6$ exhibits a spontaneous breaking
of translational symmetry into a dimerized, Jahn-Teller distorted, orbital
ordered state under the formation of spin valence bonds. In S=1 NaVSi$_2$O$_6$
orbital degrees of freedom are strongly suppressed and the magnetic excitations
are well described within a Heisenberg model, indicating that at T$_N$=19K this
system orders antiferromagnetically.
