Continual advancements in the development of synchrotron radiation sources have resulted in X-ray based spectroscopic techniques capable of probing the electronic and structural properties of numerous systems. This review gives an overview of the application of metal K-edge and L-edge X-ray absorption spectroscopy (XAS), as well as K resonant inelastic X-ray scattering (RIXS), to the study of electronic structure in transition metal sites with emphasis on experimentally quantifying 3d orbital covalency. The specific sensitivities of K-edge XAS, L-edge XAS, and RIXS are discussed emphasizing the complementary nature of the methods. L-edge XAS and RIXS are sensitive to mixing between 3d orbitals and ligand valence orbitals, and to the differential orbital covalency (DOC), that is, the difference in the covalencies for different symmetry sets of the d orbitals. Both L-edge XAS and RIXS are highly sensitive to and enable separation of and donor bonding and back bonding contributions to bonding. Applying ligand field multiplet simulations, including charge transfer via valence bond configuration interactions, DOC can be obtained for direct comparison with density functional theory calculations and to understand chemical trends. The application of RIXS as a probe of frontier molecular orbitals in a heme enzyme demonstrates the potential of this method for the study of metal sites in highly covalent coordination sites in bioinorganic chemistry.

中文翻译：

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K边和L边X射线吸收光谱（XAS）和共振非弹性X射线散射（RIXS）测定过渡金属位点的微分轨道共价（DOC）。

同步加速器辐射源的开发不断进步，导致了基于X射线的光谱技术能够探测众多系统的电子和结构特性。这篇综述概述了金属K边缘和L边缘X射线吸收光谱（XAS）以及K共振非弹性X射线散射（RIXS）在过渡金属位点电子结构研究中的应用。重点是通过实验量化3d轨道速度。讨论了K-edge XAS，L-edge XAS和RIXS的特定敏感性，强调了方法的互补性。L-edge XAS和RIXS对3d轨道和配体价轨道之间的混合以及微分轨道共价（DOC）敏感，即 d轨道不同对称集的共价差。L-edge XAS和RIXS都对粘合高度敏感，能够分离供体键合和背键合。应用配体场多重态模拟，包括通过价键构型相互作用进行电荷转移，可以得到DOC，以与密度泛函理论计算进行直接比较并了解化学趋势。RIXS作为血红素酶前沿分子轨道探针的应用证明了该方法在研究生物无机化学中高共价配位位点中的金属位点方面的潜力。应用配体场多重态模拟，包括通过价键构型相互作用进行电荷转移，可以得到DOC，以与密度泛函理论计算进行直接比较并了解化学趋势。RIXS在血红素酶中作为前沿分子轨道的探针的应用证明了该方法在研究生物无机化学中高共价配位位点中的金属位点方面的潜力。应用配体场多重态模拟，包括通过价键构型相互作用进行电荷转移，可以得到DOC，以与密度泛函理论计算进行直接比较并了解化学趋势。RIXS作为血红素酶前沿分子轨道探针的应用证明了该方法在研究生物无机化学中高共价配位位点中的金属位点方面的潜力。