Carbon-13 and oxygen-17 NMR spin-lattice relaxation time measurements were used to investigate pi bonding in the following group 6 transition metal pentacarbonyl amines : WL(CO)(5), L = pyrazine, pyridine, quinuclidine, trimethylamine; M(quinuclidine)(CO)(5), M = Cr, Mo. Values of T-1(O-17) and T-1(13C) [at two field strengths] of axial and radial carbonyls permitted calculation of the O-17 quadrupole coupling constants (chi) and the CO pi antibonding orbital populations ([2 pi](chi)) at both sites. It was observed that the ratio of increases of the axial/radial orbital populations (relative to the metal hexacarbonyls) was substantially greater than the theoretical maximum of 2 and approximately independent of ligand, indicating that CO pi bond strength enhancements induced by the amine ligands are (a) very strongly trans directed and (b) virtually independent of the amine’s sigma basicity. The above conclusions were supported by the results of Fenske-Hall MO calculations. Both experimental and calculated values of the 2 pi orbital populations of the two aromatic amines, pyrazine and pyridine, were virtually identical to results for the aliphatic ligands, from which it was concluded that, in contrast to some other reports, the pi* orbital systems in these aromatic amines are not capable of accepting electrons from the metal d(pi) orbitals.