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)(C o)(4)](-) (M = Ti, Zr, Hf) and in related species has been investigate d by using a combination of ab initio effective-core potentials (ECPs) and density-functional theory (DFT). The ECP/SOS-DFPT(IGLO) calculati ons indicate a considerable reduction in the anisotropy of the C-13(CO ) chemical shift tensors compared to terminal carbonyl ligands in ''no rmal'' complexes, This is due to large paramagnetic contributions from metal d AO type (d(z2), d(xy)) orbitals to the parallel component, si gma(33), of the shielding tensors of the carbonyl carbon atoms, The ne utral 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 successive ly 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 anisot ropy, but have carbonyl C-13 shift tensors comparable to regular octah edral carbonyl complexes. The special situation for the four-legged co mplexes is related to the presence of high-lying occupied metal d orbi tals, and particularly to the favorable spatial arrangement of these d orbitals with respect to the carbonyl ligands, Bent-sandwich d(2) com plexes 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 str uctures of the complexes are also discussed.