The preparation and structural characterization of dirhodium(II) tetrakis[ N,N-dimethyl-2-pyrrolidone-5( S)-carb- oxamide], Rh 2(5 S-DMAP) 4, a new sterically-demanding catalyst for enantioselective metal carbene transformations, is described. The pyrrolidone ligands are arrayed around the dirhodium(II) core with two oxygen and two nitrogen donor atoms, each oriented cis, bound to each octahedral rhodium. The crystal structure of this compound has been determined to be that of Rh 2(5 S-DMAP) 4(CH 3CN) 2·CH 3CN·6H 2O: space group P2 12 12 1 with cell constants a= 12.685(4), b=15.050(3), c=24.035(4) Å; V=4588.5(1.9) Å 3, Z=4, R=0.0316, RhRh distance =2 4538(5) Å. Decreased activity for diazodecomposition catalyzed by Rh 2(5 S-DMAP) 4 is observed, and enantiocontrol for cyclopropanation and carbon-hydrogen insertion is lower than expected by analogy to the corresponding di- rhodium(II) tetrakis[methyl 2-pyrrolidone-5( S)-carboxylate], Rh 2(5 S-MEPY) 4 Electronic stabilization of the in- termediate metal carbene is absent in reactions catalyzed by Rh 2(5 S-DMAP) 4.