Demonstrating Hund’s Rule in Action by Exploring the Magnetic Properties of Metal Complexes with 3d n and 4f n Configurations

Students collect magnetic susceptibility data to verify that Hund’s rule correctly predicts electronic configurations. Systems examined include three commercially available lanthanide­(III)-containing complexes of the form M­(acac)3(H2O)2 (where M = La­(III), Nd­(III), and Gd­(III), and acac denotes the [CH3C­(O)­CHC­(O)­CH3]− anion, i.e., deprotonated acetylacetone) as well as student-synthesized Co­(III)- and Fe­(III)-containing M­(acac)3 analogues. The spin-only formula for the paramagnetic susceptibility cleanly reveals the number of unpaired electrons present in all metal ions investigated herein, except Nd­(III), because it also has an orbital magnetic moment. In addition to illustrating the key role that the Pauli principle has in shaping electronic structure, the exercise contrasts the behavior of high- vs low-spin complexes and 3d versus 4f orbitals. Finally, students find that oxidation–reduction and acid–base reactions can be useful in chemical synthesis.