Publication details

Linking the Character of the Metal-Ligand Bond to the Ligand NMR Shielding in Transition-Metal Complexes: NMR Contributions from Spin-Orbit Coupling



Year of publication 2017
Type Article in Periodical
Magazine / Source Journal of Chemical Theory and Computation
MU Faculty or unit

Central European Institute of Technology

Web DOI: 10.1021/acs.jctc.7b00444
Field Physical chemistry and theoretical chemistry
Keywords NMR chemical shift; spin-orbit coupling; relativistic DFT; gold; platinum; mercury;
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Description Relativistic effects significantly affect various spectroscopic properties of compounds containing heavy elements. Particularly in Nuclear Magnetic Resonance (NMR) spectroscopy, the heavy atoms strongly influence the NMR shielding constants of neighboring light atoms. In this account we analyze paramagnetic contributions to NMR shielding constants and their modulation by relativistic spin-orbit effects in a series of transition-metal complexes of Pt(II), Au(I), Au(III), and Hg(II). We show how the paramagnetic NMR shielding and spin-orbit effects relate to the character of metal-ligand (M-L) bond. A correlation between the (back)-donation character of the M-L bond in d10 Au(I) complexes and the propagation of the spin-orbit (SO) effects from M to L through the M-L bond influencing the ligand NMR shielding via Fermi-contact mechanism is found and rationalized by using third-order perturbation theory. The SO effects on the ligand NMR shielding are demonstrated to be driven by both the electronic structure of M and the nature of the trans ligand, sharing the sigma-bonding metal orbital with the NMR spectator atom L. The deshielding paramagnetic contribution is linked to the sigma-type M-L bonding orbitals, which are notably affected by the trans ligand. The SO deshielding role of sigma-type orbitals is enhanced in d10 Hg(II) complexes with Hg 6p atomic orbital involved in the M-L bonding. In contrast, in d8 Pt(II) complexes, occupied pi-type orbitals play a dominant role in the SO-altered magnetic couplings due to the accessibility of vacant antibonding sigma-type MOs in formally open 5d-shell (d8). This results in a significant SO shielding at the light atom. The energy- and composition-modulation of sigma- vs pi-type orbitals by spin-orbit coupling is rationalized and supported by visualizing the SO-induced changes in the electron density around the metal and light atoms (spin-orbit electron deformation density, SO-EDD).
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