4.01 | MO-Diagrams of Metal Complexes

Wednesday June 26th 2024 | In this lesson, we start reiterating the importance of the 18-electron rule. We quickly cover again the different types of ligands: X-type (radical, one electron donor ligands), and L-type (neutral, two electron donor). With that in mind, we learn how to perform the electron count on metal complexes using the covalent model. By using this simple methodology we are able to quickly establish the electron count on the metal, the oxidation state of the metal, and the d-electron count. Have learned how to perform the electron count, we moved next to the different models for metal complexes. Starting from the simple crystal field approximation, we discover that for octahedral metal complexes from a d4-d8 electron count high-spin and low-spin complexes could arise. Hereafter, we presented the empirical observation (spectrochemical series) that certain ligands lead to high-spin complexes and that certain ligand lead to low-spin complexes, an observation that could not be explained by crystal field theory. We then moved to describe the bonding in metal complexes with MO-theory. First we looked at an octahedral complex by establishing σ-interactions only. We realize that that the ordering of the d-orbitals is similar to that predicted by crystal field theory. When we analyze the same MO-diagram, but we now include π* interactions, we realize the the energy difference between the T2g and Eg orbitals increased resulting in low-spin complexes. Conversely, when including π-interaction, we realize that the energy difference between the T2g and Eg orbitals decreases resulting in high-spin complexes. With this knowledge in mind, we completed the MO-diagram of an octahedral complex in tetragonal field. By using a simple analogy of sequently removing one-ligand, the MO-diagram of square pyramidal and square planar metal complexes could be derived. The in-class exercises for next week are located in the supplementary materials for this lecture.

The link to the video can be found below: