The π-bonding ability of several ligands bonding through N, P, and S to group 6 transition metals was studied via non-empirical molecular orbital calculations. The ability of the ligands to donate or accept π electrons to or from the metal was compared in 1,2-Mo 2 [P(t-Bu) 2 ] 2 [NMe 2 ] 4 [I] and 1,2-W 2 [P(t-Bu) 2 ] 2 [NMe 2 ] 4 [II]; (PSO)Cr(CO) 4 [III] and (PSO)W(CO) 4 [IV] (PSO = 2-diphenyl phosphino methyl-2-phenylthiomethyl-l-methoxy propane); and [W(2-Spy)(CO) 4 ] - [V] and [Mo(2-Spy)(CO) 4 ] - [VI] (2-Spy = 2-thiol pyridine). Mulliken population analyses were used to study how two different atoms compete for π-bonding with the metal. Both NMe 2 and PMe 2 are π donors to the metal in complexes [I] and [II]. Overall, the phosphido ligand is a better π donor than the amido ligand. The PSO ligand in complexes [III] and [IV] is a good π acceptor and phosphorus is a better π acceptor than sulfur in both complexes. The 2-Spy ligand in the two anionic complexes, [V] and [VI], is a good π acceptor. π back-donation from the metal d orbitals to the ligand is mostly through the nitrogen of the pyridine ring rather than the sulfur external to the pyridine ring. The electron density withdrawn from the metal through the nitrogen is delocalized throughout the π-conjugated ring. The π-accepting ability of the 2-Spy ligand in the two complexes is about the same.The observed differences in the π-bonding ability of the ligands are explained in terms of the accessibility of the ligand molecular oribitals and the geometry of the complexes. A comparison of π-bonding between the two linking atoms in the same bidentate ligand is also made in terms of the type and energy of ligand orbitals (compound [III] vs [IV] and [V] vs [VI]). A direct comparison is made between electron density distribution and crystallographic bond lengths.