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 13 C shifts and of unusual periodic trends in four‐legged piano‐stool complexes [M( n 5 ‐C 5 H 5 )(CO) 4 ] − (MTi, 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 13 C(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 z 2 , d xy ) orbitals to the parallel component, σ 33 , of the shielding tensors of the carbonyl carbon atoms. The neutral d 4 Group 5 and 6 complexes [M( n 5 ‐C 5 H 5 )(CO) 4 ] (MV, Nb, Ta) and [M( n 5 ‐C 5 H 5 )(CO) 3 CH 3 ] (MCr, Mo, W) exhibit successively smaller but still significant paramagnetic d‐orbital contributions to σ 33 , consistent with the observed less dramatic deshielding. The three‐legged d 6 piano‐stool complexes [M( n 5 ‐C 5 H 5 )(CO) 3 ] (MMn, Tc, Re) do not exhibit these reductions of the shielding anisotropy, but have carbonyl 13 C 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( n 5 ‐C 5 H 5 ) 2 (CO) 2 ] exhibit comparable deshielding contributions from an occupied metal d orbital. For similar reasons, the 17 O 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 ( n 5 ‐C 5 H 5 )‐ligand atoms and the structures of the complexes are also discussed.