Ciancaleoni Gianluca, Belpassi Leonardo, Marchetti Fabio
Dipartimento di Chimica e Chimica Industriale, Università degli Studi di Pisa , via Giuseppe Moruzzi 13, Pisa 56124, Italy.
Istituto di Scienze e Tecnologie Molecolari del CNR (CNR-ISTM), c/o Dipartimento di Chimica, Biologia e Biotecnologie, Università degli Studi di Perugia , via Elce di Sotto 8, Perugia I-06123, Italy.
Inorg Chem. 2017 Sep 18;56(18):11266-11274. doi: 10.1021/acs.inorgchem.7b01635. Epub 2017 Aug 31.
In the last years, some N-heterocyclic carbene (NHC) complexes of high-valent d transition-metal halides have been structurally characterized, showing a significant short distance between the carbene carbon and the cis-halide ligands (Cl). Some authors attributed this arrangement to a halide → C unusual "back-donation", whereas, according to others, the M-carbene bond is purely σ. More, in general, the ability of d metal centers to provide back-donation to suitable ligands is still debated, and detailed bond analyses for this class of systems are missing in the literature. In this contribution, we analyze in detail the Nb-L bond within neutral, cationic, and anionic derivatives of NbCl, with L = NHC, CO, CNH, and CN. In [NbCl(NHC)] complexes, with NHC being either a model carbene (1,3-dimethylimidazol-2-ylidene, IMe) or a realistic one [1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, IPr], we demonstrate that the metal center is really capable of back-donation to the carbene ligand by a charge flux that involves the chloride in the trans position and, directly, the metal. In this case, a direct interaction between Cl and C can be excluded, while if different π-acceptor ligands, such as CO or CNH, are used (instead of NHC), the direct Cl → L interligand interaction becomes predominant.
在过去几年中，一些高价态d过渡金属卤化物的N-杂环卡宾（NHC）配合物已得到结构表征，显示出卡宾碳与顺式卤化物配体（Cl）之间存在显著的短距离。一些作者将这种排列归因于卤化物→C的不寻常“反向配位”，而另一些人则认为M-卡宾键纯粹是σ键。更一般地说，d金属中心向合适配体提供反向配位的能力仍存在争议，并且文献中缺少对这类体系的详细键分析。在本论文中，我们详细分析了NbCl的中性、阳离子和阴离子衍生物中Nb-L键，其中L = NHC、CO、CNH和CN。在[NbCl(NHC)]配合物中，NHC可以是模型卡宾（1,3-二甲基咪唑-2-亚基，IMe）或实际的卡宾[1,3-双（2,6-二异丙基苯基）咪唑-2-亚基，IPr]，我们证明金属中心确实能够通过涉及反位氯和直接涉及金属的电荷流对卡宾配体进行反向配位。在这种情况下，可以排除Cl与C之间的直接相互作用，而如果使用不同的π-受体配体，如CO或CNH（而不是NHC），则Cl→L配体间直接相互作用占主导地位。
