Effects of equatorial ligand bulk on the orientation of axial ligands in methyl organocobalt B12 models assessed by X-ray and NMR methods

In the new complexes, [CH3Co((DO)(DOBF2)pn)(L)]+ with L = pyridine and 1,5,6-trimethylbenzimidazole ((DO)(DOBF2)pn = BF2-substituted (DO)(DOH)pn), NMR and X-ray methods show L is oriented over the double bonds. We advance the concept (applicable to a broad series of complexes) that steric interactions between L and the equatorial ligands are alleviated by a combination of Co-Nax bond elongation and opening of the Neq-Co-Nax angles.