NMR chemical-shift anomaly and bonding in piano-stool carbonyl and related complexes - An ab initio ECP/DFT study

The origin of the unusually large carbonyl C-13 shifts and of unusual periodic trends in four-legged piano-stool complexes [M(eta(5)-C5H5)(Co)(4)](-) (M = Ti, Zr, Hf) and in related species has been investigated by using a combination of ab initio effective-core potentials (ECPs) and density-functional theory (DFT). The ECP/SOS-DFPT(IGLO) calculations indicate a considerable reduction in the anisotropy of the C-13(CO) chemical shift tensors compared to terminal carbonyl ligands in ''normal'' complexes, This is due to large paramagnetic contributions from metal d AO type (d(z2), d(xy)) orbitals to the parallel component, sigma(33), of the shielding tensors of the carbonyl carbon atoms, The neutral d(4) Group 5 and 6 complexes [M(eta(5)-C5H5)(CO)(4)] (M = V, Nb, Ta) and [M(eta(5)-C5H5)(CO)(3)CH3] (M = Cr, Mo, W) exhibit successively smaller but still significant paramagnetic d-orbital contributions to sigma(33). consistent with the observed less dramatic deshielding. The three-legged d(6) piano-stool complexes [M(eta(5)-C5H5)(CO)(3)] (M = Mn, Tc, Re) do not exhibit these reductions of the shielding anisotropy, but have carbonyl C-13 shift tensors comparable to regular octahedral carbonyl complexes. The special situation for the four-legged complexes is related to the presence of high-lying occupied metal d orbitals, and particularly to the favorable spatial arrangement of these d orbitals with respect to the carbonyl ligands, Bent-sandwich d(2) complexes like [Zr(eta(5)-C5H5)(2)(CO)(2)] exhibit comparable deshielding contributions from an occupied metal d orbital, For similar reasons, the O-17 resonances for these piano-stool and bent-sandwich complexes are also predicted to be at unusually high frequencies, with low shift anisotropy. NMR shifts for the (eta(5)-C5H5)-ligand atoms and the structures of the complexes are also discussed.