A theoretical study has been carried out on 13C and 17O NMR shielding tensors in M(CO)6 (M=Cr, Mo, and W); Fe(CO)5 and Ni(CO)4 . The study was based on modern density functional theory (DFT) and gauge-including atomic orbitals (GIAO). The calculated shielding constants and tensors are in good agreement with the available experimental data. The difference in the isotropic shielding DÓ = Ó CO - Ó M(CO)71 between free CO, Ó CO, and CO as a ligand, Ó M(CO)71 was analyzed in detail. It was shown that the stretch of CO on coordination as well as the back-donation from dp on the metal to p*COof the CO ligand have positive contribution to the coordination shift Dd whereas the donation from the Ó CO HOMO of CO to the metal orbitals has a negative contribution to Dd.