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Abstract Here, we combined magnetometry, multi‐frequency electronic paramagnetic resonance, and wave function based ab initio calculations to investigate magnetic properties of two high spin Co(II) complexes Co(BDP R P) (BDP R P=2,6‐bis((2‐(S)‐di(4‐R)phenylhydroxylmethyl‐1‐pyrrolidi‐nyl)methyl)pyridine, R=H for 8 ; R= t Bu for 9 ). Complexes 8 and 9 featuring effective D 3h symmetry were found to possess D =24.0 and 32.0 cm −1 , respectively, in their S =3/2 ground states of . Ligand field analyses revealed that the low‐lying d‐d excited states make either positive or vanishing contributions to D . Hence, total positive D values were measured for 8 and 9 , as well as related D 3h high spin Co(II) complexes. In contrast, negative D values are usually observed for C 3v congeners. In‐depth analyses suggested that lowering symmetry from D 3h to C 3v induces orbital mixing between and and admixes excited state into the ground state. Both factors turn the total D value progressively negative with the increasing distance (δ) of the Co(II) center out of the equatorial plane. Therefore, δ determines the sign and magnitude of final D values of five‐coordinate trigonal bipyramidal S =3/2 Co(II) complexes as measured for a series of such species with varying δ.
Zhang et al. (Tue,) studied this question.