Analysis of C-13 and O-17 chemical shift tensors and an ELF view of bonding in Fe-2(CO)(9) and Rh-6(CO)(16)

Carbon and oxygen chemical shift tensors for bridging and terminal carbonyl ligands in Fe-2(CO)(9) and Rh-6(CO)(16) were calculated by sum-over-states density-functional perturbation theory (SOS-DFPT). Agreement with experimental C-13 shift tensors is excellent, and O-17 shift tensors are predicted. The reduced anisotropy values of the shift tensors for the bridging compared to terminal carbonyl ligands are due to large deshielding contributions to delta(33) from non-bonding or bridge-bonding orbitals. Comparison to recent computational results for a series of unusual piano-stool and bent-sandwich group-4 complexes is made. The electronic structures of the clusters are discussed by using plots of electron localization functions (ELF). Bonding electrons within the Rh-6 cluster are mainly localized on the unbridged octahedral faces. This leads to a heterocubane-like arrangement of ELF maxima above all octahedral faces (four bridging CO ligands, four M-M 3-center-bonding maxima), in analogy to previous results for B6H62-.