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

Abstract

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 the metal-ligand (M-L) bond. A correlation between the (back)-donation character of the M-L bond in d 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 the 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 σ-bonding metal orbital with the NMR spectator atom L. The deshielding paramagnetic contribution is linked to the σ-type M-L bonding orbitals, which are notably affected by the trans ligand. The SO deshielding role of σ-type orbitals is enhanced in d Hg(II) complexes with the Hg 6p atomic orbital involved in the M-L bonding. In contrast, in d Pt(II) complexes, occupied π-type orbitals play a dominant role in the SO-altered magnetic couplings due to the accessibility of vacant antibonding σ-type MOs in formally open 5d-shell (d). This results in a significant SO shielding at the light atom. The energy- and composition-modulation of σ- vs π-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).

Trans‐philicity (trans‐influence/trans‐effect) ladders for square planar platinum(II) complexes constructed by 35Cl NMR probe

“…Magnetic shielding tensors have been computed with the gauge‐including atomic orbitals DFT method, as implemented in the Gaussian09 series of programs. It is well established that relativistic and spin orbit effects besides influencing the NMR signal of the heavy atom (HA) itself (HAHA effect), the NMR chemical shifts of the light atoms (LA) bound to the HA center are affected as well (HALA effect) and therefore strongly contribute to computed isotropic magnetic shielding constants in Pt(II) coordination compounds . In this context, 35 Cl shielding constants were calculated by inclusion of scalar relativistic effects and switching on the DKH approximation as implemented in the Gaussian 09 program suite.…”

“…and the catalytic activity of Pd(II)‐ and Pt(II)‐based catalysts . Although there has been much experimental and theoretical work done on the overall trans ‐effect and the trans ‐influence, it has often proved difficult to produce a systematic set of data; hence, the proliferation of different series concerning the magnitude of the effect for various trans ‐ligands. The trans ‐effect of a ligand in square planar d 8 complexes is evident in its overall effect on the rate of ligand substitutions, while the trans ‐influence is generally evident in changes in the metal–ligand bond length, IR frequencies, and NMR chemical shifts.…”

“…Quasi‐relativistic calculations on third‐row transition metal complexes encompassing [Au(L)H] − (5 d 10 ), [Pt(PMe 3 ) 2 (L)H] and [Pt(CO) 2 (L)H] (5 d 8 ) and [Os(R 2 PCH 2 CH 2 PR 2 )(L)H] (5 d 6 ) complexes (L = NO 3 − , Cl − , Me − , and BH 2 − ) revealed a strikingly general trans ligand effect on 1 H shifts of hydride ligands which are also operative for other metal‐bound NMR probes . Marek and coworkers presented a detailed analysis of the paramagnetic NMR shielding constants of the light atom NMR probe bound to the heavy transition metal centers in Au(I), Au(III), Hg(II), and Pt(II) complexes employing relativistic two‐component SO‐ZORA and four‐component Dirac−Kohn−Sham (DKS) formalism based on the Dirac‐Coulomb Hamiltonian for the calculation of the NMR shielding constants. A chemical link between the structural altered paramagnetic contributions to the light atom NMR shielding constants and the energy and composition characteristics of M − L bonding was provided.…”

Relativistic heavy atom effect on 13C NMR chemical shifts initiated by adjacent multiple chalcogens

“…Heavy atom on light atom (HALA) effect is among the most important relativistic phenomena influencing the NMR parameters of the light atoms, which are known to be very sensitive to the presence of heavy surrounding . The nonrelativistic quantum chemical calculations of 13 C NMR chemical shifts have already become some sort of well‐established protocols, augmented by special details of the particular problem, which are capable to provide a qualitative description of the 13 C NMR spectra of compounds containing only the light elements.…”

“…The HALA effects induced by neighboring heavy atoms (HAs) on 13 C chemical shifts have been studied for a long time, though, the most part of works deals with the HALA effect induced by the heavy halogens or by neighboring heavy transition metals . At the present moment, with the exception for several casual communications, there is no systematic study of the HALA effect on carbon chemical shifts induced by heavy chalcogens.…”