Hydrogenations catalyzed by Wilkinson's catalyst involve stereospecific syn hydrometallation of the multiple bond followed by stereospecific reductive elimination. Hence the hydrogenation of olefins or alkynes result in syn addition products.
E.g. Hydrogenation of Maleic acid or Fumaric acid with D2 in presence of Wilkinson's catalyst is diastereoselective. Hydrogenation of Maleic acid with D2 give meso compound exclusively.
stereoselctivity of wilkinson hydrogenation: maleic acid