The structure–property correlation of [Co(terpy)₂]²⁺, which shows a spin crossover at 270 K, has been computationally investigated based on its variable temperature crystal structures. Among the employed DFT functionals, only the re-parametrized hybrid B3LYP* functional could describe the correct spin transition temperature. Explicit consideration of metal–ligand sigma bonding with dynamic electron correlation is found to be necessary for an accurate determination of the SCO temperature with multi-reference calculations. The metal–ligand axial bond distances are found to be the most significant internal coordinates in deciding SCO. A small structural change along the axial distance causes a change in the t_2g orbital splitting pattern and a reorientation of the magnetization axes at the SCO temperature. The complex shows an unusual triaxial magnetic anisotropy, with an easy axis of magnetization developing at higher temperatures. The strong coupling of low-frequency wagging motion of the two terpyridine ligands with the spin states of the complex provides an effective pathway for the relaxation of magnetization, resulting in a small magnetic anisotropy barrier.

中文翻译：

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[Co(terpy)2]2+ 变温晶体构象中磁各向异性和热自旋跃迁的从头算


[Co(terpy) ₂ ] ²⁺ 的结构-性质相关性在 270 K 时显示出自旋交叉，已根据其变温晶体结构进行了计算研究。在所采用的 DFT 泛函中，只有重新参数化的混合 B3LYP* 泛函可以描述正确的自旋转变温度。研究发现，通过多参考计算准确确定 SCO 温度，必须明确考虑金属-配体西格玛键合与动态电子相关性。发现金属-配体轴向键距是决定 SCO 的最重要的内坐标。沿轴向距离的微小结构变化会导致 t _2g 轨道分裂模式的变化以及 SCO 温度下磁化轴的重新定向。该复合体表现出不寻常的三轴磁各向异性，在较高温度下会形成易磁化轴。两个三联吡啶配体的低频摇摆运动与配合物的自旋态的强耦合为磁化弛豫提供了有效的途径，从而产生了小的磁各向异性势垒。