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Hartree-Fock-Slater calculations have been carried out on Ni(CO)3L for a number of different ligands (L) in order to investigate the abilities of the ligands to act as donors and x acceptors. The order, based on extent of electron transfer, for donation is CS =* CO > CNCH3 > N2 ~PF3 and for x back-bonding is CNCH3 > CS > CO > PF3 > N2. The contributions to the total bonding energy between Ñi(CO)3 and L from donation and x back-donation are evaluated by the Hartree-Fock-Slater transition-state method, and the same method is used to optimize the Ni-L bond distances.Calculations on the stretching frequency vCo of carbon monoxide complexed to Ni(CO)3 showed that vCo is decreased by the x back-donation but is increased by the steric interaction energy between Ni(CO)3 and CO. Thus the decrease in vCo is not a reliable measure of the extent of x back-bonding in the metal-ligand bond. The calculated influence on vCo from donation was negligible.