Mechanism of Spin–Orbit Effects on the Ligand NMR Chemical Shift in Transition-Metal Complexes: Linking NMR to EPR

Abstract

Relativistic effects play an essential role in understanding the nuclear magnetic resonance (NMR) chemical shifts in heavy-atom compounds. Particularly interesting from the chemical point of view are the relativistic effects due to heavy atom (HA) on the NMR chemical shifts of the nearby light atoms (LA), referred to as the HALA effects. The effect of Spin-Orbit (SO) interaction originating from HA on the nuclear magnetic shielding at a neighboring LA, σ(SO), is explored here in detail for a series of d(6) complexes of iridium. Unlike the previous findings, the trends in σ(SO) observed in this study can be fully explained neither in terms of the s-character of the HA-LA bonding nor by trends in the energy differences between occupied and virtual molecular orbitals (MOs). Rather, the σ(SO) contribution to the total NMR shielding is found to be modulated by the d-orbital participation of the heavy atom (Ir) in the occupied and virtual spin-orbit active MOs, i.e., those which contribute significantly to the σ(SO). The correlation between the d-character of σ(SO)-active MOs and the size of the corresponding SO contribution to the nuclear magnetic shielding constant at LA is so tight that the magnitude of σ(SO) can be predicted in a given class of compounds on the basis of d-orbital character of relevant MO with relative error smaller than 15%. This correspondence is supported by an analogy between the perturbation theory expressions for the spin-orbit induced NMR σ-tensor and those for the EPR g-tensor as well as the A-tensor of the ligand. This correlation is demonstrated on a series of d(5) complexes of iridium. Thus, known qualitative relationships between electronic structure and EPR parameters can be newly applied to reproduce, predict, and understand the SO-induced contributions to NMR shielding constants of light atoms in heavy-atom compounds.

Relativistic Effects in NMR of New Prospective Water-Soluble Ligands SeP(CH2OH)3 and H[Se2P(CH2OH)2]

“…It appears that the biggest changes are observed for three terms corresponding to P-C bonds, P-Se bonds, and Se lone pairs (Table 4). P-Se bond in SeP(CH 2 OH) 3 has bigger contribution of phosphorus s-orbital than we would expect for sp 3 hybridization (29.02 % of the phosphorus 55.27 % of the bonding orbital), while Se part of the bonding orbital is 89.40 % of p-character, this agrees well with the mechanism of the so-called HALA (Heavy Atom to Light Atom) relativistic effects [50,51], where Se acts as heavy atom. On the other hand, the role of P-C bonds that have 22-24 % of s-orbital in the phosphorous parts of corresponding P-C bonding orbitals is unexpectedly big.…”

[FeFe]‐hydrogenase H‐cluster mimics mediated by mixed (S, Se) and (S, Te) bridging moieties: Insight into molecular structures and electrochemical characteristics

“…Indeed, carbon atoms directly bound to heavy nuclei, for example, Te, are known to experiences a drastic high field shift. This effect is called heavy atom effect . In addition, the 77 Se{ 1 H} NMR spectrum of complex 1 shows one signal at 273.2 ppm for the Se atom while the 125 Te{ 1 H} NMR spectrum of complex 2 displays one signal at 421.6 ppm for the Te atom.…”

A Relativistic Quantum-Chemical Analysis of the trans Influence on 1H NMR Hydride Shifts in Square-Planar Platinum(II) Complexes

“…9 The predicted anisotropies in the latter systems have very recently been confirmed by solid-state 1 H/ 2 H NMR experiments. 52 MO analyses of the SO contributions may be obtained by subtraction of MO contributions obtained in scalar-relativistic calculations from the corresponding MO (spinor) contributions of two-component ZORA-SO calculations. Table 5 shows the dominant contributions to the σ SO tensor, which stem in most cases from coupling between the occupied in-plane and out-of-plane d π -type and virtual σ*(Pt−H) MOs (note that σ and π-type symmetry is given here with respect to the Pt−H bond).…”