Transition-and inner-transition metal hydride complexes are crucial reagents in a great variety of stoichiometric and catalytic transformations, including CÀH bond activation. [1] As hydrogen atoms near heavy-metal centers are difficult to locate by X-ray diffraction, often their prime characterization is by 1 H NMR spectroscopy, sometimes augmented by IR spectroscopy. A significant part of the utility of 1 H NMR spectroscopy in this field arises from the fact that the chemical shifts of metal-bound protons are characteristic and occupy extreme positions in the proton shift range, even for diamagnetic compounds. For instance, complexes with d 6 or d 8 metal configuration exhibit shifts below d = 0 ppm, in record cases down to below d = À50 ppm for iridium hydride complexes. [2,3] While this phenomenon was explained as early as the 1960s by Buckingham and Stephens as being due to offcenter paramagnetic ring currents [4] (see Ref.[5] for the earliest DFT results), we have recently shown that the largest low-frequency shifts of this kind are, to an appreciable part, caused by relativistic spin-orbit (SO) effects. [2] These heavyatom induced SO effects are mediated through the Fermi contact mechanism to which proton shifts are particularly susceptible, owing to the large hydrogen 1 s-orbital contributions to bonding (the transfer of SO-induced spin polarization to the NMR nucleus is decisive in this situation). [6] In contrast, d 10 metal hydride complexes of mercury or gold exhibit large high-frequency shifts up to d =+ 17 ppm, again predominantly because of SO coupling. [2] Some d 0 metal hydrides have been studied by 1 H NMR spectroscopy as well. Similarly to d 10 systems they often also exhibit shifts in the very highfrequency range (Scheme 1). [7] As shown in Scheme 1, SO effects again play a very important role in these NMR shift values, increasingly so moving down a group in the periodic table (cf. 1 H NMR shifts within the [H 2 MCp* 2 ] series, M = Ti, Zr, Hf, Cp* = h-C 5 Me 5 ). Probably the largest known shift value of such a d 0 complex is the d =+ 18.9 ppm of the tantalum complex in Scheme 1.We note in passing, that deshielding SO shifts are generally related to high-lying occupied orbitals with ssymmetry relative to the bond between the SO center and NMR atom (e.g. for the abovementioned d 10 and d 0 metal hydride complexes), whereas p-type occupied orbitals provide shielding SO contributions (e.g. in the d 6 and d 8 hydride complexes or for heavy halogen substituents). [8] In view of these observations, we wondered about the magnitude of hydride shifts when the d 0 transition-metal center is replaced by an actinide ion to form the corresponding f 0 species, as SO effects should be particularly large in this case. A literature survey provided only a few diamagnetic hydride complexes: the thorium systems 1-3 shown in Scheme 2 [9] and a few borohydride complexes of Th IV and UO 2 2+ . [10] The hydride shifts in 1-3 are in the high-frequency range, comparable to the abovementioned Ta complex (only some prot...

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.

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.

Previous relativistic quantum-chemical predictions of unusually large H andC NMR chemical shifts for ligand atoms directly bonded to a diamagnetic uranium(vi) center (P. Hrobárik, V. Hrobáriková, A. H. Greif and M. Kaupp, Angew. Chem., Int. Ed., 2012, 51, 10884) have been revisited by two- and four-component relativistic density functional methods. In particular, the effect of the exchange-correlation response kernel, which had been missing in the previously used two-component version of the Amsterdam Density Functional program, has been examined. Kernel contributions are large for cases with large spin-orbit (SO) contributions to the NMR shifts and may amount to up to ∼30% of the total shifts, which means more than a 50 ppm difference for the metal-bonded carbon shifts in some extreme cases. Previous calculations with a PBE-40HF functional had provided overall reasonable predictions, due to cancellation of errors between the missing kernel contributions and the enhanced exact-exchange (EXX) admixture of 40%. In the presence of an exchange-correlation kernel, functionals with lower EXX admixtures give already good agreement with experiments, and the PBE0 functional provides reasonable predictive quality. Most importantly, the revised approach still predicts unprecedented giant H NMR shifts between +30 ppm and more than +200 ppm for uranium(vi) hydride species. We also predict uranium-bonded C NMR shifts for some synthetically known organometallic U(vi) complexes, for which no corresponding signals have been detected to date. In several cases, the experimental lack of these signals may be attributed to unexpected spectral regions in which some of theC NMR shifts can appear, sometimes beyond the usual measurement area. An extremely large uranium-bonded C shift above 550 ppm, near the upper end of the diamagneticC shift range, is predicted for a known pincer carbene complex. Bonding analyses allow in particular the magnitude of the SO shifts, and of their dependence on the functional, on the ligand position in the complex, and on the overall electronic structure to be better appreciated, and improved confidence ranges for predicted shifts have been obtained.

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