Magneto‐Structural Correlation of Five‐Coordinate Trigonal Bipyramidal High Spin Cobalt(II) Complexes

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.