Relativistic DFT calculations of the NMR properties and reactivity of transition metal methane σ-complexes: insights on C-H bond activation.

Relativistic ZORA DFT methods have been employed to predict the NMR properties of methane and methyl hydride complexes of rhodium and iridium. Two of these compounds, the rhodium methane and the iridium methyl hydride complexes, have been recently characterized by NMR spectroscopy. Calculations reveal that relativistic effects are largely responsible of the high shielding observed for the proton and carbon resonances of the methane moiety. The key steps for the reaction mechanism of C-H cleavage catalyzed by both compounds have been investigated at the relativistic level. Although the structure of the intermediates and TSs for the Rh and Ir complexes is rather similar, subtle differences in the energetics are responsible of the different catalytic activity of the two complexes.