The synthesis, crystal structure determination, magnetic properties and bonding interaction analysis of a novel 3d transition-metal complex, [CrBr₂(NCCH₃)₄](Br₃), are reported. Single-crystal X-ray diffraction results show that [CrBr₂(NCCH₃)₄](Br₃) crystallizes in space group C2/m (No. 12) with a symmetric tribromide anion and the powder X-ray diffraction results show the high purity of the material specimen. X-ray photoelectron studies with a combination of magnetic measurements demonstrate that Cr adopts the 3+ oxidation state. Based on the Curie–Weiss analysis of magnetic susceptibility data, the Néel temperature is found to be around 2.2 K and the effective moment (μ_eff) of Cr³⁺ in [CrBr₂(NCCH₃)₄](Br₃) is ∼3.8 µ_B, which agrees with the theoretical value for Cr³⁺. The direct current magnetic susceptibility of the molecule shows a broad maximum at ∼2.3 K, which is consistent with the theoretical Néel temperature. The maximum temperature, however, shows no clear frequency dependence. Combined with the observed upturn in heat capacity below 2.3 K and the corresponding field dependence, it is speculated that the low-temperature magnetic feature of a broad transition in [CrBr₂(NCCH₃)₄](Br₃) could originate from a crossover from high spin to low spin for the split d orbital level low-lying states rather than a short-range ordering solely; this is also supported by the molecular orbital diagram obtained from theoretical calculations.

中文翻译：

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反铁磁分子的晶体生长和物理性质：反式二溴四（乙腈）三溴化铬（III），[CrBr2（NCCH3）4]（Br3）

报道了一种新型 3 d过渡金属配合物 [CrBr ₂ (NCCH ₃ ) ₄ ](Br ₃ )的合成、晶体结构测定、磁性和键相互作用分析。单晶X射线衍射结果表明[CrBr ₂ (NCCH ₃ ) ₄ ](Br ₃ )在空间群C 2/ m 中结晶(No. 12) 具有对称的三溴阴离子，粉末 X 射线衍射结果表明材料样品纯度高。结合磁性测量的 X 射线光电子研究表明，Cr 采用 3+ 氧化态。基于磁化率数据的居里-魏斯分析，发现 Néel 温度约为 2.2 K，并且[CrBr ₂ (NCCH ₃ ) ₄ ](Br ₃ )中 Cr ³⁺的有效矩 (μ _eff ) 为 ~ 3.8 µ _B，与 Cr ³⁺的理论值一致. 该分子的直流磁化率在~2.3 K 处显示出一个广泛的最大值，这与理论 Néel 温度一致。然而，最高温度没有显示出明显的频率依赖性。结合观察到的低于 2.3 K 的热容上升和相应的场依赖性，推测 [CrBr ₂ (NCCH ₃ ) ₄ ](Br ₃ ) 中广泛转变的低温磁性特征可能源于交叉对于分裂的d轨道水平低位状态，从高自旋到低自旋，而不是单独的短程排序；从理论计算得到的分子轨道图也支持这一点。