The magnetic moments of neutral Sc, V, and Ti atoms supported on a benzene molecule are found to be substantially enhanced over their free-atom value while those of neutral Mn, Fe, and Co atoms experience a similar reduction. The magnetic moment of the neutral Cr atom, the highest (6 mu(B)) in the 3d-transition metal series, remains unchanged and that of neutral Ni is completely quenched. These results, based on density functional theory and the generalized gradient approximation, are shown to be a simple consequence of the Pauli exclusion principle. Similar calculations on positively and negatively charged 3d-metal-benzene complexes are carried out.