In discrete molecules and extended solids, magnetic ions M are surrounded with ligands L to form ML_n (typically, n = 2 - 6) complexes. The spin moments of M exhibit preferential orientations with respect to the local coordinates defining the structures of ML_n complexes. Elaborate experimental measurements and sophisticated first principles computational efforts are necessary to find the preferred spin orientations of magnetic ions M. However, they can be readily predicted on the basis of qualitative perturbation theory in which the spin-orbit coupling (SOC) of M is taken as the perturbation with the magnetic orbitals of ML_n in the absence of SOC as the unperturbed states. This analysis gives rise to simple and powerful selection rules with which to predict the preferred spin orientations of magnetic ions M based on the minimum difference in the magnetic quantum numbers of M in the HOMO and LUMO of ML_n.