Shee James, Loipersberger Matthias, Hait Diptarka, Lee Joonho, Head-Gordon Martin
Kenneth S. Pitzer Center for Theoretical Chemistry, Department of Chemistry, University of California, Berkeley, California 94720, USA.
Department of Chemistry, Columbia University, New York, New York 10027, USA.
J Chem Phys. 2021 May 21;154(19):194109. doi: 10.1063/5.0047386.
In this work, we provide a nuanced view of electron correlation in the context of transition metal complexes, reconciling computational characterization via spin and spatial symmetry breaking in single-reference methods with qualitative concepts from ligand-field and molecular orbital theories. These insights provide the tools to reliably diagnose the multi-reference character, and our analysis reveals that while strong (i.e., static) correlation can be found in linear molecules (e.g., diatomics) and weakly bound and antiferromagnetically coupled (monometal-noninnocent ligand or multi-metal) complexes, it is rarely found in the ground-states of mono-transition-metal complexes. This leads to a picture of static correlation that is no more complex for transition metals than it is, e.g., for organic biradicaloids. In contrast, the ability of organometallic species to form more complex interactions, involving both ligand-to-metal σ-donation and metal-to-ligand π-backdonation, places a larger burden on a theory's treatment of dynamic correlation. We hypothesize that chemical bonds in which inter-electron pair correlation is non-negligible cannot be adequately described by theories using MP2 correlation energies and indeed find large errors vs experiment for carbonyl-dissociation energies from double-hybrid density functionals. A theory's description of dynamic correlation (and to a less important extent, delocalization error), which affects relative spin-state energetics and thus spin symmetry breaking, is found to govern the efficacy of its use to diagnose static correlation.
在这项工作中，我们在过渡金属配合物的背景下提供了对电子关联的细致入微的观点，将单参考方法中通过自旋和空间对称性破缺的计算表征与配体场理论和分子轨道理论中的定性概念相协调。这些见解提供了可靠诊断多参考特征的工具，我们的分析表明，虽然在线性分子（如双原子分子）以及弱键合和反铁磁耦合（单金属 - 非无辜配体或多金属）配合物中可以发现强（即静态）关联，但在单过渡金属配合物的基态中很少发现。这导致了一种对于过渡金属而言，静态关联并不比例如有机双自由基类更复杂的情况。相比之下，有机金属物种形成更复杂相互作用的能力，包括配体到金属的σ - 给予和金属到配体的π - 反馈，给理论处理动态关联带来了更大的负担。我们假设，电子对之间关联不可忽略的化学键不能用使用MP2关联能的理论充分描述，并且确实发现双杂化密度泛函计算的羰基解离能与实验值存在很大误差。发现理论对动态关联（以及在较小程度上对离域误差）的描述，它影响相对自旋态能量，从而影响自旋对称性破缺，决定了其用于诊断静态关联的有效性。
