Probing the spin state and a putative Ru-Ru bond in a dinuclear Ru(II) complex with X-ray quantum crystallography

Determining the spin state of the metal complexes is central to understanding the electronic structure and the magnetic properties of materials. Inferring spin states from conventional crystallography is often ambiguous because it is largely limited to detecting geometric variations associated with different spin states. In this study, X-ray quantum crystallography (QCr) is employed to determine the spin state of a dinuclear ruthenium (II) complex by comparatively analyzing restricted and unrestricted wavefunctions fitted to high-resolution X-ray diffraction data. QCr models obtained from Hirshfeld Atom Refinement (HAR) and Xray constrained wavefunction (XCW) fitting at different spin multiplicities indicate that the ruthenium centers adopt a low-spin state consistent with the electronic configuration of Ru(II), which is further corroborated by NMR spectroscopy. Additionally, electron density and bond-order analyses of the putative Ru–Ru bond based on the X-ray-derived wavefunction reveal the absence of bonding features between the Ru centers, in contrast to the single bond expected from conventional electron-counting methods.