Validation of Relativistic DFT Approaches to the Calculation of NMR Chemical Shifts in Square-Planar Pt²⁺ and Au³⁺ Complexes

Recently implemented hybrid density functional methods of calculating nuclear magnetic shielding using the two-component zeroth-order regular approximation approach (J. Phys. Chem. A 2009, 113, 11495) have been employed for a series of compounds containing heavy transition-metal atoms. These include Pt²⁺, Pd²⁺, and Au³⁺ organometallics and metal complexes with azines, some of which exhibit interesting biological and catalytic activities. In this study we investigate the effects of geometry, exchange–correlation functional, solvent, and scalar relativistic and spin–orbit corrections on the nuclear magnetic shieldingmainly for ¹³C and ¹⁵N atoms connected to a heavy-atom center. Our calculations demonstrate that the B3LYP method using effective core potentials and a cc-pwCVTZ-PP/6-31G** basis set augmented with the polarizable continuum model of the dimethylsulfoxide solvent provides geometries for the complexes in question which are compatible with the experimental NMR results in terms of both the trends and the absolute values of the ¹³C shifts. The important role of the exact exchange admixture parameter for hybrid functionals based on B3LYP and PBE0 is investigated systematically for selected Pt²⁺ and Au³⁺ complexes. The ¹³C and ¹⁵N NMR chemical shifts are found to be best reproduced by using a B3LYP or PBE0 approach with 30% and 40–50% exact exchange admixtures for the Pt²⁺ and Au³⁺ complexes, respectively. The spin–orbit contributions to the ¹⁵N NMR chemical shifts reflect metal–ligand bonding that is much more ionic for the Au³⁺ than for the Pt²⁺ complex. Finally, an optimized density functional method is applied to a series of transition-metal complexes to estimate the scope and the limitations of the current approach.