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(BDPRP) (BDPRP=2,6-bis((2-(S)-di(4-R)phenylhydroxylmethyl-1-pyrrolidi-nyl)methyl)pyridine, R=H for 8; R=tBu for 9). Complexes 8 and 9 featuring effective D3h symmetry were found to possess D=24.0 and 32.0 cm-1, respectively, in their S=3/2 ground states of 1e''dxz/yz41e'dxy/x2-y221a1'dz21 ${{\left(1{{\rm e}}<^>{{\rm { {<^>\prime}}}{\rm { {<^>\prime}}}}\right({d}_{xz/yz}\left)\right)}<^>{4}{\left(1{{\rm e}}<^>{{\rm { {<^>\prime}}}}\right({d}_{{xy/{x}<^>{2}-y}<^>{2}}\left)\right)}<^>{2}{\left(1{{\rm a}}_{1}<^>{{\rm { {<^>\prime}}}}\right({d}_{{z}<^>{2}}\left)\right)}<^>{1}}$ . 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 D3h high spin Co(II) complexes. In contrast, negative D values are usually observed for C3v congeners. In-depth analyses suggested that lowering symmetry from D3h to C3v induces orbital mixing between 1edxz/yz ${1{\rm e}\left({d}_{xz/yz}\right)}$ and 2edxy/x2-y2 ${2{\rm e}\left({d}_{{xy/{x}<^>{2}-y}<^>{2}}\right)}$ and admixes excited state 4A21e -> 2e ${{}<^>{4}{{\rm A}}_{2}\left(1e\to 2e\right)}$ into the ground state. Both factors turn the total D value progressively negative with the increasing distance (delta) of the Co(II) center out of the equatorial plane. Therefore, delta 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 delta. The combination of magnetometry and multi-frequency electronic paramagnetic resonance spectroscopy reveal large positive axial zero-field splitings, D's, for 8 and 9 (24 cm-1 and 32 cm-1), and in-depth ligand field analyses on top of wave function based ab initio calculations demonstrated that the Co(II) out-of-equatorial plane distance, delta, determines the sign and magnitude of D values of five-coordinate trigonal bipyramidal high spin Co(II) complexes. image

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