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Mechanisms of Ligand Hyperfine Coupling in Transition-Metal Complexes: σ and π Transmission Pathways

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Year of publication 2024
Type Article in Periodical
Magazine / Source Inorganic Chemistry
MU Faculty or unit

Faculty of Science

Web DOI: 10.1021/acs.inorgchem.3c04425
Keywords EPR;pNMR;hyperfine interaction;Fermi contact;pi-conjugation;sigma-hyperconjugation
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Description Theoretical interpretation of hyperfine interactions was pioneered in the 1950s-1960s by the seminal works of McConnell, Karplus, and others for organic radicals and by Watson and Freeman for transition-metal (TM) complexes. In this work, we investigate a series of octahedral Ru(III) complexes with aromatic ligands to understand the mechanism of transmission of the spin density from the d-orbital of the metal to the s-orbitals of the ligand atoms. Spin densities and spin populations underlying ligand hyperfine couplings are analyzed in terms of pi-conjugative or sigma-hyperconjugative delocalization vs. spin polarization based on symmetry considerations and restricted open-shell vs. unrestricted wave function analysis. The transmission of spin density is shown to be most efficient in the case of symmetry-allowed pi-conjugative delocalization, but when the pi-conjugation is partially or fully symmetry-forbidden, it can be surpassed by the sigma-hyperconjugative delocalization. Despite a lower spin population of the ligand in sigma-hyperconjugative transmission, the hyperfine couplings can be larger because of the direct involvement of the ligand s-orbitals in this delocalization pathway. We demonstrate a quantitative correlation between the hyperfine couplings of aromatic ligand atoms and the characteristics of the metal-ligand bond modulated by the trans substituent, a hyperfine trans effect.
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