Low-/High-Spin Cobaltaboratranes Stabilized by Cis-/Trans-Isomeric Bisphosphines

Electronic spin isomerism in low-valent cobalt complexes can potentially be achieved through the rational design of ligand fields, but such instances have rarely been reported up to now. Herein, we crystallized two cobaltaboratrane complexes featuring Co^I → B dative covalent bonds by leveraging the rigidity of the C═C bond from cis-/trans-isomeric bisphosphine ligands. The Co^I cores in CoB-cis/CoB-trans are stabilized in six- and five-coordinate octahedral and trigonal bipyramidal geometries, respectively, resulting in low- and high-spin ground states, as demonstrated by magnetic measurements. Ab initio ligand field analysis revealed that both the ligand field and the Co^I → B dative bond play crucial roles in the disparities in the energies of the d orbitals and their spin configurations.