Sandwich compound-based planar chiral N-heterocyclic carbene (NHC) ligands having a bulky η5-cyclopentadienyl-cobalt-η4-tetraphenylcyclobutadiene core were developed, and their donor strengths were evaluated by determining the Tolman electronic parameter (TEP) of the Rh dicarbonyl complex. The TEP was high at 2048 cm-1, as expected for an NHC ligand. The utility of the carbenes as chiral ligands was examined in the Rh-catalyzed asymmetric ring-opening reaction of oxabenzonorbornadienes with benzylamine, and the corresponding products were obtained with moderate enantioselectivities (up to 66% ee).