An unusual carbobnyl chemical shift in a carbonylhexairidium cluster: a combined solid-state NMR and DFT approach

The 13C NMR spectrum of TMBA2[Ir6(CO)15] [TMBA =(CH3)3N(CH2C6H5)] shows, at low temperatures, an unprecedented 2-bridging carbonyl low-frequency shift, with the resonances of the terminal 1-carbonyl ligands placed at higher frequencies. The chemical shift tensors and the shielding anisotropies of the carbonyl ligands, obtained from solid-state NMR analysis, allow us to determine the nature of the M-CO interaction. The results have been compared with the 13C MAS data of Ir6(CO)16 where 3-CO ligands are present. Further evidence for the assignment and for the peculiar chemical shift value of bridging carbonyl ligands in TMBA2[Ir6(CO)15] has been obtained by the DFT calculation of the NMR parameters. The scalar and spin-orbit (SO) relativistic two-component zero-order regular approximation (ZORA) methods were employed in the geometry optimization and NMR chemical shift calculations, respectively. The large SO contribution ( = 26.6 ppm) to the 2-bridging CO ligand 13C chemical shifts accounts for the position of the experimentally observed resonance