5 years ago

Insights into trans-Ligand and Spin-Orbit Effects on Electronic Structure and Ligand NMR Shifts in Transition-Metal Complexes

Insights into trans-Ligand and Spin-Orbit Effects on Electronic Structure and Ligand NMR Shifts in Transition-Metal Complexes
Martin Kaupp, Peter Hrobárik, Anja H. Greif
Surprisingly general effects of trans ligands L on the ligand NMR shifts in third-row transition-metal complexes have been found by quasi-relativistic computations, encompassing 5d10, 5d8, and to some extent even 5d6 situations. Closer analysis, with emphasis on 1H shieldings in a series of linear HAuILq complexes, reveals a dominance of spin-orbit (SO) effects, which can change sign from appreciably shielding for weak trans ligands to appreciably deshielding for ligands with strong trans influence. This may be traced back to increasing destabilization of a σ-type MO at scalar relativistic level, which translates into very different σ-/π-mixing if SO coupling is included. For the strongest trans ligands, the σ-MO may move above the highest occupied π-type MOs, thereby dramatically reducing strongly shielding contributions from predominantly π-type spinors. The effects of SO-mixing are in turn related to angular momentum admixture from atomic spinors at the metal center. These SO-induced trends hold for other nuclei and may also be used to qualitatively predict shifts in unknown complexes. σ-Antibonding is the key! Surprisingly general trans ligand effects on NMR shifts in transition metal complexes are revealed by relativistic quantum-chemical analyses of ligand shifts, notably 1H shifts, in 5d10, 5d8, and 5d6 complexes. They are traced back to changes in σ-/π-spinor mixing by spin-orbit coupling due to a destabilization of a σ-type MO by strong trans ligands (see graphic). The insights allow qualitative and quantitative predictions of a wide range of ligand effects.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/chem.201700844

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