Terminal imido complexes containing metal–nitrogen multiple bonds have been widely used in organometallic chemistry and homogeneous catalysis. The role of terminal imido ligands spans from reactive sites to spectator motifs, largely depending on the nature of the metal center and its specific coordination sphere. Aiming at identifying reactivity descriptors for M–N multiple bonds, we herein explore solid-state ¹⁵N NMR spectroscopy (ssNMR) on early transition metal terminal imido complexes augmented by computational studies and show that the asymmetry parameter, κ (skew, 1 ≥ κ ≥ −1), readily available from experiments or calculations, is diagnostic for the reactivity of M–N multiple bonds in imido complexes. While inert imido ligands exhibit skew values (κ) close to 1, highly reactive imido moieties display significantly lower skew values (κ ≪ 1) as found in metallocene or bis-imido complexes. Natural chemical shielding analysis shows that skew values away from 1 are associated with an asymmetric development of π-orbitals around the M–N multiple bond of the imido moiety, with a larger double-bond character for reactive imido. Notably, this descriptor does not directly relate to the M–N–C bond angle, illustrating the shortcoming of evaluating bonding and hybridization from geometrical parameters alone. Overall, this descriptor enables to obtain direct experimental evidence for the π-loading effect seen in bis(imido) and related complexes, thus explaining their bonding/reactivity.

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15N NMR 特征编码的 d0 过渡金属亚氨基配合物的键合和反应性

含有金属-氮多重键的末端酰亚胺配合物已广泛应用于有机金属化学和均相催化领域。末端亚氨基配体的作用涵盖从反应位点到观察者基序，很大程度上取决于金属中心的性质及其特定的配位球。为了识别 M-N 多重键的反应性描述符，我们在本文中探索了通过计算研究增强的早期过渡金属末端亚氨基配合物的固态¹⁵ N NMR 光谱 (ssNMR)，并表明不对称参数 κ (skew, 1 ≥ κ ≥ -1)，很容易从实验或计算中获得，可诊断亚氨基复合物中 M-N 多重键的反应性。虽然惰性亚氨基配体的偏斜值 (κ) 接近 1，但高反应性酰亚胺基团却显示出明显较低的偏斜值 (κ < 1)，如茂金属或双亚氨基配合物中所发现的那样。天然化学屏蔽分析表明，偏离 1 的偏斜值与亚氨基部分 M-N 多重键周围 π 轨道的不对称发展有关，反应性亚氨基具有较大的双键特征。值得注意的是，该描述符与 M-N-C 键角没有直接关系，说明了仅从几何参数评估键合和杂化的缺点。总体而言，该描述符能够获得双（酰亚胺）和相关复合物中的 π 负载效应的直接实验证据，从而解释它们的键合/反应性。