ISSN:
0947-6539
Keywords:
carbonyl complexes
;
density-functional theory
;
NMR chemical shifts
;
pseudopotentials
;
transition-metal complexes
;
Chemistry
;
General Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
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.
Additional Material:
5 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/chem.19960020317
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