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

Abstract

Empirical correlations between characteristic (1)H NMR shifts in Pt(II) hydrides with trans ligand influence series, Pt-H distances, and (195)Pt shifts are analyzed at various levels of including relativistic effects into density-functional calculations. A close examination of the trans ligand effects on hydride NMR shifts is shown to be dominated by spin-orbit shielding σ(SO). A rather complete understanding of the trends has been obtained by detailed molecular orbital (MO)-by-MO and localized MO analyses of the paramagnetic and spin-orbit (SO) contributions to the chemical shifts, noting that it is the perpendicular shift-tensor components that determine the trend of the (1)H hydride shifts. In contrast to previous assumptions, the change of the Pt-H distance in given complexes does not allow correlations between hydride shifts and metal-hydrogen bond length to be understood. Instead, variations in the polarization of metal 5d orbitals by the trans ligand affects the SO (and partly paramagnetic) shift contributions, as well as the Pt-H distances and the covalency of the metal-hydrogen bond (quantified, e.g., by natural atomic charges and delocalization indices from quantum theory atoms-in-molecules), resulting in a reasonable correlation of these structural/electronic quantities with hydride σ(SO) shieldings. Our analysis also shows that specific σ(p)- and σ(SO)-active MOs are not equally important across the entire series. This explains some outliers in the correlation for limited ranges of trans-influence ligands. Additionally, SO effects from heavy-halide ligands may further complicate trends, indicating some limitations of the simple one-parameter correlations. Strikingly, σ-donating/π-accepting ligands with a very strong trans influence are shown to invert the sign of the usually shielding σ(SO) contribution to the (1)H shifts, by a substantial reduction of the metal 5d orbital involvement in Pt-H bonding, and by involvement of metal 6p-type orbitals in the magnetic couplings, in violation of the Buckingham-Stephens "off-center ring-current" picture.

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

“…As for the more restricted ligand set discussed above, we see that the overall trend is dominated by σ SO , but even σ p changes sign as a function of the ligand (Figure ). The latter point contrasts with our earlier observations for Pt II hydride complexes . We note in passing that the paramagnetic and SO shieldings are dominated by the shielding tensor components perpendicular to the H‐Au‐L bonding axes, whereas the parallel component does not change as much for the different ligands (cf.…”

“…Figure for neutral ligands; different correlation lines are obtained for neutral and anionic complexes; Figure S2 in the Supporting Information). However, similarly as found in our previous study of Pt II hydride complexes, 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 (see below).…”

“…Compared to the complexes discussed so far, the σ‐type MOs exhibit a larger 6p z contribution, which increases with the stronger trans ligand (already slightly discernible in Table ). Together with the effects of energetic destabilization (see discussion above for HAuL), the enhanced M(6p z ) character helps to reduce shielding or even create deshielding SO contributions, as noted previously . The percentage 6p z character of the d 6 and d 8 complexes increases from about 10 % (Cl) to above 15 % (BH 2 ).…”

“…Note that a similar destabilization of a 2σ‐type MO with stronger trans ligands was also found for anionic [HAuL] − complexes (Figure S3 in the Supporting Information). Indeed, a closely related σ(Μ‐Η)‐type MO was seen to be important in our previous analyses of d 8 square‐planar Pt II complexes, pointing to a general origin of the observed trends …”

“…Earlier studies suggested that SO shieldings on directly neighboring atoms induced by occupied MOs with σ symmetry with respect to the bond between the heavy neighboring atom and that nucleus are generally negative, whereas those induced by π orbitals are positive . Our recent analysis of trans ligand effects on 1 H shifts in a series of square‐planar Pt II complexes, and a comparison between 13 C and 14 N shifts in square‐planar Au III and Pt II complexes by Vicha et al., gave a somewhat more varied picture, whereby seemingly similar bonding arrangements may give rise to either shielding or deshielding SO effects depending on the trans ligand or the central metal. Adding the also frequently important paramagnetic contributions (σ p ) to the shifts further complicates the situation and makes general predictions without explicit quantitative calculations more difficult.…”