NMR Chemical‐Shift Anomaly and Bonding in Piano‐Stool Carbonyl and Related Complexes–an Ab Initio ECP/DFT Study

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Abstract
The origin of the unusually large carbonyl 13C shifts and of unusual periodic trends in four‐legged piano‐stool complexes [M(n5‐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 13C(CO) chemical shift tensors compared to terminal carbonyl ligands in “normal” complexes. This is due to large paramagnetic contributions from metal d AO type (dz2, dxy) orbitals to the parallel component, σ33, of the shielding tensors of the carbonyl carbon atoms. The neutral d4 Group 5 and 6 complexes [M(n5‐C5H5)(CO)4] (MV, Nb, Ta) and [M(n5‐C5H5)(CO)3CH3] (MCr, Mo, W) exhibit successively smaller but still significant paramagnetic d‐orbital contributions to σ33, consistent with the observed less dramatic deshielding. The three‐legged d6 piano‐stool complexes [M(n5‐C5H5)(CO)3] (MMn, Tc, Re) do not exhibit these reductions of the shielding anisotropy, but have carbonyl 13C 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 d2 complexes like [Zr(n5‐C5H5)2(CO)2] exhibit comparable deshielding contributions from an occupied metal d orbital. For similar reasons, the 17O 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 (n5‐C5H5)‐ligand atoms and the structures of the complexes are also discussed.

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