4 H2O + 4 KOH + K2Cr2O7 + 3 Na2SO3 → 3 Na2SO4 + 2 K3[Cr(OH)6]
This is a complexation reaction: K3[Cr(OH)6] is the formed complex.
This is an oxidation-reduction (redox) reaction:
3 SIV - 6 e- → 3 SVI (oxidation)
2 CrVI + 6 e- → 2 CrIII (reduction)
Na2SO3 is a reducing agent, K2Cr2O7 is an oxidizing agent.