Successful derivation of an element’s electron configuration depends on a thorough understanding of three main principles, namely the Aufbau principle, Pauli principle, and Hund’s rule. In a molecule, Hund’s rule is particularly challenging for students to realize as the orbital degeneracy varies with the structure. To help elucidate the aforementioned principles, we exploit the non-interacting nature of the 4f orbitals within lanthanides. Using acetylacetonate (acac) as the coordinating ligand, students observe that the valence 4f orbitals in lanthanides remain effectively degenerate and that electron occupation follows the occupation principles in their purest form. To further elaborate on electron configuration, the developed student laboratory intertwines rudimentary synthetic inorganic techniques in the synthesis of M(acac)3 compounds (where M = FeIII, CoIII) and the measurement of magnetic susceptibility. Transition metals provide a stark contrast to lanthanides in that they experience a significant ligand field effect and subsequent loss of d-orbital degeneracy. This is illustrated beautifully by the experimentally observed differences in magnetic susceptibilities of Co(acac)3 and Fe(acac)3 in which the former results in a low spin (S=0) complex and the latter in a high spin (S=5/2) configuration. It is our belief that this lab illustrates the theory behind electron configurations as well as essential general and inorganic principles making it suitable to various levels of baccalaureate education.

Using lanthanides to illustrate Hund's rule

251st National Meeting (2016)

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