We present a theoretical study of the geometric structure of Co(corrin)(L)(R) + (R = CH 3 , i-C 3 H 7 and adenosyl and L = NH 3 , pyridine and 5,6-dimethylbenzimidazole) as model systems for the vitamin B 1 2 coenzyme. We have optimized the geometries of nine corrin derivatives with axial ligands of varying degrees of steric bulkiness. In addition, we have included reoptimized geometries for nine dimethylglyoxime derivatives. Our goal was to compare and contrast calculated geometries for corrin models to those found for the corresponding dimethylglyoxime derivatives, which are often used as a model of the corrin group in experimental systems. We found that the dimethylglyoxime model is appropriate for axial Co-C distances, but not for axial Co-N distances.