The carbon-13 and oxygen-17 nuclear magnetic resonance spectroscopic shielding behavior, as well as the oxygen-27 nuclear quadrupole coupling constants (NQCC), in the four metal-CO systems Fe(CO)5, Fe2(CO)9, Ni2(ε5-C5H5)2(CO)2, and Rh6(CO)16 have been investigated both experimentally and by density functional calculations. Characteristics of the spectroscopic observables and bonding for the most common types of metal-carbonyl coordination, μ1- , μ2-, and μ3-CO, may thus be compared in detail. There is generally very good agreement between the theoretical predictions and the experimental measurements, including the 17O shift predictions for Fe2(CO)9 and Rh6(CO)16 made previously. Interestingly, the bridging oxygen shift tensor in Fe2(CO)9 has its most deshielded component parallel to the C-O axis. This is highly unusual for carbonyl ligands, but is the normal behavior seen in organic carbonyl groups. To explain this and other observations, the computed shielding tensors and electric field gradients have been broken down into contributions from various localized, delocalized, or mixed sets of molecular orbitals. In addition to the common IGLO procedure, these analyses also include 'partial IGLO' and IGLO-Pipek-Mezey methods. The results give new insights into both the magnitudes and orientations of the shielding and nuclear quadrupole coupling tensors. The potential for the combined use of solid-state NMR and quantum chemical methods in various areas of transition metal chemistry is discussed.
ASJC Scopus subject areas
- Colloid and Surface Chemistry