From Ligand Fields to Molecular Orbitals: Probing the Local Valence Electronic Structure of Ni²⁺ in Aqueous Solution with Resonant Inelastic X-ray Scattering

Abstract

Bonding of the Ni 2+ (aq) complex is investigated with an unprecedented combination of resonant inelastic X-ray scattering (RIXS) measurements and ab initio calculations at the Ni L absorption edge. The spectra directly reflect the relative energies of the ligand-field and charge-transfer valence-excited states. They give element-specific access with atomic resolution to the ground-state electronic structure of the complex and allow quantification of ligand-field strength and 3d−3d electron correlation interactions in the Ni 2+ (aq) complex. The experimentally determined ligand-field strength is 10Dq = 1.1 eV. This and the Racah parameters characterizing 3d−3d Coulomb interactions B = 0.13 eV and C = 0.42 eV as readily derived from the measured energies match very well with the results from UV−vis spectroscopy. Our results demonstrate how L-edge RIXS can be used to complement existing spectroscopic tools for the investigation of bonding in 3d transition-metal coordination compounds in solution. The ab initio RASPT2 calculation is successfully used to simulate the L-edge RIXS spectra. ■ INTRODUCTION3d transition metals in aqueous solution are interesting from a fundamental point of view as their hexaaqua complexes [M(H 2 O) 6 ] n+ represent the archetype of Werner complexes with coordinative bonding in octahedral symmetry. 1 Considering hydrogen bonding between the various coordination spheres of water molecules, they could even be regarded as chelate complexes, 2 and new insight into their electronic structure could help understanding the interplay of bonding, structure, and dynamics in these complexes. This in turn forms the basis for understanding their thermodynamic properties and reactivity, having implications for modeling and understanding phenomena ranging from water exchange reactions 3,4 to photochemical processes 5 and atmospheric liquid chemistry of 3d transition-metal ions in solution. 6 While the structure of 3d transition metal (TM) ions and their complexes in aqueous solution can be addressed experimentally with numerous scattering and spectroscopic methods, 7 bonding can be assessed with a limited set of spectroscopic tools among which, historically, UV−vis spectroscopy has played a dominant role. Complementing such experimental results and due to the strongly polarized character of the coordinate bond, ligand-field theory 8,9 can be successfully applied to hexaaqua complexes. In particular, the dependence of the ligand-field (LF) state energies on ligand-field strength and 3d−3d Coulomb interactions for all 3d n complexes were first calculated by Tanabe and Sugano in their seminal work from 1954. 10,11 The low-energy electronic excited state spectrum of a typical TM coordination complex can thus be described by, with increasing photon energy, the LF excitations, followed by the charge-transfer (CT) and ligand-centered excitations. At present, the excitations to LF states of octahedral TM complexes are very well understood at the semiempirical level of ligand-field theory. Today, ab ini...

Viewing the Valence Electronic Structure of Ferric and Ferrous Hexacyanide in Solution from the Fe and Cyanide Perspectives

“…Note however that metal L2,3-edge RIXS probes the ground state 10Dq, similarly to UV/vis absorption. 33,53,54 The ferricyanide RIXS spectra taken at the aM and aL resonances ( Figure 5A) show, in addition to the elastic scattering peak, a main RIXS peak denoted 1aM,L at an energy transfer of 4 eV with a weaker peak denoted 2aM,L at ~7 eV. For excitation at the aM resonance at 705.6 eV the elastic peak is intense due to six (nearly) degenerate t2g…”

K- and L-edge X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) determination of differential orbital covalency (DOC) of transition metal sites

“…In the soft X-ray (0.1–2 keV) regime, RIXS has been performed on, for example, the C, N, and O K-edges in small molecules [85], metal oxides [86], and coordination compounds [87]. 2p3d RIXS (2p→3d excitation, 3d→2p emission) is also commonly used to study d-d interactions, ligand field, and charge transfer effects in Ti [88], Mn [89], Fe [87], and Ni [89, 90] compounds. In the hard X-ray (>2 keV) regime, 2p5d RIXS can be used to probe the valence orbitals of 5 th row transition metals such as platinum [91].…”

Ionic Solutions Probed by Resonant Inelastic X-ray Scattering

“…Moreover, photons offer bulk information as the penetration length is much higher compared to electrons. Overall X-ray spectroscopy is an ideal tool to study the local electronic structure [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23].…”