The structural parameter of spin states of C ₁-symmetric four-coordinate cobalt complexes: D ₄

Herein, we reveal the correlation between structure and spin-states for C ₁-symmetric four-coordinate cobalt complexes. A series of C ₁-symmetric four-coordinate cobalt complexes with various steric, electronic, anions and chelation modes were synthesized and characterized. X-ray diffraction studies revealed a structural distortion from square-planar to distorted tetrahedral pyramidal geometry. Combining paramagnetic nuclear magnetic resonance (pNMR), magnetic measurement (SQUID), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT), the electronic structure with a cobalt(ii) (S _Co = 1/2 to 3/2) ion antiferromagnetically coupled to a radical anion ligand (S _Ligand = 1/2) was established. A new structural parameter, D ₄, was introduced as an improved parameter for quantitatively assessing the spin state in four-coordinate complexes.