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

The C-13 NMR spectrum of TMBA(2)[Ir-6(CO)151 [TMBA = (CH3)(3)N(CH2C6H5)] shows, at low temperatures, an unprecedented mu(2)-bridging carbonyl low-frequency shift, with the resonances of the terminal mu(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 C-13 MAS data of lr6(CO)16 where mu(3)-CO ligands are present. Further evidence for the assignment and for the peculiar chernical shift value of bridging carbonyl ligands in TMBA(2)[Ir-6(CO)15] has been obtained by the DFT calculation of the NNIR 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 (delta = 26.6 ppm) to the mu(2)-bridging CO ligand C-13 chemical shifts accounts for the position of the experimentally observed resonance. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).