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

Abstract

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 5d , 5d , and to some extent even 5d situations. Closer analysis, with emphasis on H shieldings in a series of linear HAu L 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.

13C NMR Shifts as an Indicator of U–C Bond Covalency in Uranium(VI) Acetylide Complexes: An Experimental and Computational Study

“…10 We note in passing that this finding is consistent with previous theoretical studies on HALA (heavy-atom-effects on the lightatom) "spin-orbit" shifts in transition-metal and actinide complexes. 34,36,[50][51][52][53][54][55][56] Similarly, owing to the large hydrogen 1s-orbital contribution to the An-H bonding, as-yet elusive uranium(VI)…”

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

“…Kaupp and co‐workers showed that the trans ligand influence is related with the Au‐H bond lengths in HAuL complexes and the observed correlations between hydride shifts and Au‐H distances arise indirectly from the effect of the trans ligand on the polarization of the metal d (and p ) orbitals, which in turn affects both the structures and the hydride shifts. Accurate linear correlations between σ SO ( 15 N) and the Au−N distances in a series of Au(I) complexes were also reported .…”

“…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.…”