SOLID-STATE NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPIC AND QUANTUM-CHEMICAL INVESTIGATION OF C-13 AND O-17 CHEMICAL-SHIFT TENSORS, O-17 NUCLEAR-QUADRUPOLE COUPLING TENSORS, AND BONDING IN TRANSITION-METAL CARBONYL-COMPLEXES AND CLUSTERS

The carbon-13 and oxygen-17 nuclear magnetic resonance spectroscopic s hielding behavior, as well as the oxygen-17 nuclear quadrupole couplin g constants (NQCC), in the four metal-CO systems Fe(CO)(5), Fe-2(CO)(9 ), Ni-2(eta(5)-C5H5)(2)(CO)(2), and Rh-6(CO)(16) have been investigate d both experimentally and by density functional calculations. Characte ristics of the spectroscopic observables and bonding for the most comm on types of metal-carbonyl coordination, mu(1)-, mu(2)-, and mu(3)-CO, may thus be compared in detail. There is generally very good agreemen t between the theoretical predictions and the experimental measurement s, including the O-17 shift predictions for Fe-2(CO)(9) and Rh-6(CO)(1 6) made previously. Interestingly, the bridging oxygen shift tensor in Fe-2(CO)(9) has its most deshielded component parallel to the C-O axi s. This is highly unusual for carbonyl ligands, but is the normal beha vior seen in organic carbonyl groups. To explain this and other observ ations, the computed shielding tensors and electric field gradients ha ve been broken down into contributions from various localized, delocal ized, or mixed sets of molecular orbitals. In addition to the common I GLO procedure, these analyses also include ''partial IGLO'' and IGLO-P ipek-Mezey methods. The results give new insights into both the magnit udes and orientations of the shielding and nuclear quadrupole coupling tensors. The potential for the combined use of solid-state NMR and qu antum chemical methods in various areas of transition metal chemistry is discussed.