Large Magnetic Anisotropy in Linear CoII Complexes – Ab Initio Investigation of the Roles of Ligand Field, Structural Distortion, and Conformational Dynamics

Abstract

Linear or near‐linear bicoordinate mononuclear CoII complexes are studied as potential single‐molecule magnets owing to the strong spin–orbit coupling constant of CoII and its low coordination number, which results in the retention of the unquenched metal‐orbital angular momentum. The spin–orbit‐coupled unquenched orbital angular momentum of the metal center produces strong magnetic anisotropy through ligand‐field interactions. The role of the ligand environment, structural distortions, and conformational changes on the magnetic anisotropy of CoII complexes are investigated through ab initio electronic‐structure calculations. The zero‐field splitting parameters, g tensors, and transition‐magnetic‐moment matrix elements among the Kramers pairs are evaluated to obtain the effective anisotropy barriers (Ueff) and the mechanism for the relaxation of magnetization in a series of CoII complexes, for which the estimated values of Ueff range between 394 and 974 cm–1 and include the largest effective anisotropy barriers reported for transition‐metal complexes. The calculations reveal that the ligand‐field strength, structural distortion, and conformational changes not only affect the magnetic anisotropy barrier but also significantly alter the mechanism of relaxation of the magnetic moments.