Computational study and molecular orbital analysis of NMR shielding, spin-spin coupling, and electric field gradients of azido platinum complexes

¹⁹⁵Pt, ¹⁴N, and ¹⁵N NMR data for five azido (N₃^-) complexes are studied using relativistic density functional theory (DFT). Good agreement with experiment is obtained for Pt and N chemical shifts as well as Pt-N J-coupling constants. Calculated ¹⁴N electric field gradients (EFGs) reflect experimentally observed trends for the line broadening of azido ¹⁴N NMR signals. A localized molecular orbital analysis of the nitrogen EFGs and chemical shifts is performed to explain some interesting trends seen experimentally and in the first-principles calculations: (i) ¹⁴N NMR signals for the Pt-coordinating (N_α) nuclei in the azido ligands are much broader than for the central (N_β) or terminal (N _γ) atoms. The N_β signals are particularly narrow; (ii) compared to N_γ, the N_α nuclei are particularly strongly shielded; (iii) N_β nuclei have much larger chemical shifts than N_α and N_γ; and (iv) The Pt-N_αJ-coupling constants are small in magnitude when considering the formal sp hybridization of N_α. It is found that for N_α a significant shielding reduction due to formation of the dative N_α-Pt bond is counterbalanced by an increased shielding from spin-orbit (SO) coupling originating at Pt. Upon coordination, the strongly delocalized π system of free azide localizes somewhat on N _β and N_γ. This effect, along with rehybridization at N_α upon bond formation with Pt, is shown to cause a deshielding of N_γ relative to N_α and a strong increase of the EFG at N_α. The large 2p character of the azide σ bonds is responsible for the particularly high N _β chemical shifts. The nitrogen s-character of the Pt-N _α bond is low, which is the reason for the small J-coupling. Similar bonding situations are likely to be found in azide complexes with other transition metals.