13C and 207Pb NMR Chemical Shifts of Dirhodio- and Dilithioplumbole Complexes: A Quantum Chemical Assessment.

Density functional theory (DFT) and zeroth-order regular approximation DFT calculations were performed to investigate the electronic structures and 13C and 207Pb nuclear magnetic resonance (NMR) chemical shifts of metal-coordinated plumboles, namely, monorhodioplumbole ([Rh-plumbole]-), dirhodioplumbole (Rh2-plumbole), and dilithioplumbole (Li2-plumbole), which have a five-membered ring containing lead. The molecular orbital correlation diagram and extended transition state-natural orbitals for chemical valence analysis of the [Rh-plumbole]- and Rh2-plumbole complexes showed that the plumbole is primarily a π-donor, with π-donation being dominant in the Rh2-plumbole complex. The present calculations show that the Pb-Cα internuclear distances are longer in the Rh2-plumbole complex than in [Rh-plumbole]- because of the combined effect of strong π-donation and weak π-back-donation in the Rh2-plumbole complex. The calculated 207Pb and 13Cα NMR chemical shifts agree with the experimental trends reasonably well. The influences of the relativistic effect, role of the functional, effect of the solvent, and dependence of the exact exchange admixture on the calculated 207Pb and 13Cα NMR chemical shifts were investigated. The NMR chemical shift trend of the 207Pb atom in the complexes originates from the paramagnetic and spin-orbit contributions. NMR component analysis revealed that the upfield shift of the 13Cα atoms of the [Rh-plumbole]- and Rh2-plumbole complexes compared to that of the Li2-plumbole complex is mainly due to the decrease in the paramagnetic term.