Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, USA.
Inorg Chem. 2011 Oct 17;50(20):9766-72. doi: 10.1021/ic201237d. Epub 2011 Aug 22.
Ligand noninnocence occurs for complexes composed of redox-active ligands and metals, with frontier orbitals of similar energy. Usually methods of analysis can be used to define the charge distribution, and cases where the metal oxidation state and ligand charge are unclear are unusual. Ligands derived from o-benzoquinones can bond with metals as radical semiquinonates (SQ(•-)) or as catecholates (Cat(2-)). Spectroscopic, magnetic, and structural properties can be used to assess the metal and ligand charges. With the redox activity at both the metal and ligands, reversible multicomponent redox series can be observed using electrochemical methods. Steps in the series may occur at either the ligand or metal, and ligand substituent effects can be used to tune the range of ligand-based redox steps. Complexes that appear as intermediates in a ligand-based redox series may contain both SQ and Cat ligands "bridged" by the metal as mixed-valence complexes. Properties reflect the strength of metal-mediated interligand electronic coupling in the same way that ligand-bridged bimetallics conform to the Robin and Day classification scheme. In this review, we will focus specifically on complexes of first-row transition-metal ions coordinated with three ligands derived from tetrachloro-1,2-benzoquinone (Cl(4)BQ). The redox activity of this ligand overlaps with the potentials of common metal oxidation states, providing examples of metal- and ligand-based redox activity, in some cases, within a single redox series. The strength of the interligand electronic coupling is important in defining the separation between ligand-based couples of a redox series. The complex of ferric iron will be described as an example where coupling is weak, and the steps associated with the Fe(III)(Cl(4)SQ)(3)/Fe(III)(Cl(4)Cat)(3) redox series are observed over a narrow range in electrochemical potential.
配体非定域性发生在由氧化还原活性配体和金属组成的配合物中，这些配体具有相似能量的前线轨道。通常可以使用分析方法来定义电荷分布，而金属氧化态和配体电荷不清楚的情况并不常见。来自邻苯醌的配体可以与金属键合形成自由基半醌酸盐 (SQ(•-)) 或儿茶酸盐 (Cat(2-))。光谱、磁性和结构性质可用于评估金属和配体的电荷。由于金属和配体都具有氧化还原活性，可以使用电化学方法观察到可逆的多组分氧化还原系列。该系列中的步骤可以发生在配体或金属上，并且配体取代基效应可以用于调节基于配体的氧化还原步骤的范围。在基于配体的氧化还原系列中作为中间体出现的配合物可能包含通过金属“桥接”的 SQ 和 Cat 配体作为混合价配合物。这些性质反映了金属介导的配体间电子耦合的强度，与配体桥联双金属配合物一样符合 Robin 和 Day 分类方案。在这篇综述中，我们将特别关注与四氯-1,2-苯醌 (Cl(4)BQ) 衍生的三种配体配位的第一行过渡金属离子的配合物。该配体的氧化还原活性与常见金属氧化态的电势重叠，在某些情况下，在单个氧化还原系列中提供了基于金属和配体的氧化还原活性的例子。配体间电子耦合的强度对于定义氧化还原系列中基于配体的偶对之间的分离很重要。将描述三价铁配合物作为耦合较弱的例子，并且与 Fe(III)(Cl(4)SQ)(3)/Fe(III)(Cl(4)Cat)(3) 氧化还原系列相关的步骤在电化学电势的窄范围内观察到。
