Morris Robert H
Department of Chemistry, University of Toronto, 80 Saint George St., Toronto, Ontario, Canada, M5S3H6.
Chem Soc Rev. 2024 Mar 18;53(6):2808-2827. doi: 10.1039/d3cs00979c.
The success and power of homogeneous catalysis derives in large part from the wide choice of transition metal ions and their ligands. This tutorial review introduces examples where the reactivity of a ligand is completely reversed (umpolung) from Lewis basic/nucleophilic to acidic/electrophilic or vice versa on changing the metal and co-ligands. Understanding this phenomenon will assist in the rational design of catalysts and the understanding of metalloenzyme mechanisms. Labelling a metal and ligand with Seebach donor and acceptor labels helps to identify whether a reaction involving the intermolecular attack on the ligand is displaying native reactivity or reactivity umpolung. This has been done for complexes of nitriles, carbonyls, isonitriles, dinitrogen, Fischer carbenes, alkenes, alkynes, hydrides, methyls, methylidenes and alkylidenes, silylenes, oxides, imides/nitrenes, alkylidynes, methylidynes, and nitrides. The electronic influence of the metal and co-ligands is discussed in terms of the energy of (HOMO) d electrons. The energy can be related to the pLACa (LAC is ligand acidity constant) of the theoretical hydride complexes [H-[M]-L] formed by the protonation of pair of valence electrons on the metal in the [M-L] complex. Preliminary findings indicate that a negative pLACa indicates that nucleophilic attack by a carbanion or amine on the ligand will likely occur while a positive pLACa indicates that electrophilic attack by strong acids on the ligand will usually occur when the ligand is nitrile, carbonyl, isonitrile, alkene and η-arene.
均相催化的成功与强大在很大程度上源于过渡金属离子及其配体的广泛选择。本教程综述介绍了一些例子，即在改变金属和共配体时，配体的反应性会从路易斯碱性/亲核性完全反转（极性反转）为酸性/亲电性，反之亦然。理解这一现象将有助于合理设计催化剂以及理解金属酶的作用机制。用西巴赫给体和受体标签标记金属和配体，有助于确定涉及对配体进行分子间攻击的反应是显示天然反应性还是极性反转反应性。对腈、羰基、异腈、二氮、费歇尔卡宾、烯烃、炔烃、氢化物、甲基、亚甲基和亚烷基、硅烯、氧化物、酰亚胺/氮烯、亚烷基炔、亚甲基炔和氮化物的配合物都进行了这样的研究。从（最高占据分子轨道）d电子的能量角度讨论了金属和共配体的电子影响。该能量可以与通过[M-L]配合物中金属上一对价电子的质子化形成的理论氢化物配合物[H-[M]-L]的pLACa（LAC是配体酸度常数）相关。初步研究结果表明，负的pLACa表明碳负离子或胺对配体的亲核攻击可能会发生，而正的pLACa表明当配体为腈、羰基、异腈、烯烃和η-芳烃时，强酸对配体的亲电攻击通常会发生。
