AB-INITIO ECP DFT CALCULATION AND INTERPRETATION OF CARBON AND OXYGENNMR CHEMICAL-SHIFT TENSORS IN TRANSITION-METAL CARBONYL-COMPLEXES/

Carbon and oxygen NMR chemical shift tensors for Group 6 hexacarbonyl complexes M(CO)(6) (M = Cr, Mo, W) have been calculated by using a com bination of quasirelativistic metal effective-core potentials and dens ity-functional theory. Comparison with high-resolution solid-state shi ft tensors indicates excellent agreement between theory and experiment . The sensitivity of the shifts to the W-C distance in W(CO)(6) is dis cussed. A breakdown of the shielding tensor components into contributi ons from localized molecular orbitals allows the detailed interpretati on of the trends on going down Group 6, and of differences to free CO. Group trends in the carbon shielding tensors are related largely to c ontributions from M-C sigma-bonding orbitals. The presence of occupied metal (n-1)p and (n-1)d orbitals is partly responsible for the change s on going from free to metal-bound CO. The origin of the less pronoun ced trends in the oxygen shielding tensors is more complicated. The in fluence of scalar relativistic effects on the shift tensors has been s tudied for W(CO)(6) and is found to be relatively small, in spite of c onsiderable changes in the W-C distance.