Without a need for any quantum chemical calculations (only CFT and the number of d electrons are required), we can thus begin to anticipate the shapes of the frontier orbitals of organometallic complexes! Combined with a solid understanding of organic FMO theory and the frontier orbitals of organic ligands, we can recognize important orbital interactions in organometallic complexes that can explain a wide variety of observations.īefore digging in to an example, let’s introduce some terminology commonly used by organometallic chemists related to FMO theory. In the figure above, assuming that the metal is d6, the frontier falls between the dπ and dσ orbitals. When it comes to organometallic complexes, we can gain considerable insight by noticing that orbitals on the frontier are usually the metallic d orbitals. FMO theory uses these foundational ideas to explain the structure and reactivity of molecules, and at least in organic chemistry, the theory has been insanely successful. Furthermore, both chemical reactions and resonance can be explained by interactions (overlap) between a filled HOMO and an empty LUMO on one or more molecules. The HOMO is logically viewed as nucleophilic or electron donating, while the LUMO is electrophilic and electron accepting. The frontier molecular orbitals of a compound are at the “frontier” of electron occupation-the highest-energy occupied and lowest-energy unoccupied molecular orbitals (the HOMO and LUMO). What happens when we take the HOMO and LUMO of the ligands into account? Read on… Until now, we’ve considered only the HOMO of the ligands. Yet this groundwork is necessary to build a truly powerful theory of the electronic structure of organometallic complexes. All we did, it seems, was tack the 4s, 4p and ligand orbitals on to crystal field theory. This may seem like a relatively simple picture at first glance. In the end, we arrive at a critically important, albeit expected insight: the metal is electrophilic (owner of many unfilled MOs) while the ligands are nucleophilic (owners of filled MOs). The guiding principle here is that MOs are composed primarily of those AOs to which they are closest in energy. But more important than the number of MOs is the character of each orbital-where is the electron density primarily located in these MOs? Ligand field theory provides a logical answer: the bonding MOs primarily possess ligand character, while the antibonding MOs are primarily localized on the metal. Notice that the math works out:ġ 4s + 3 4p + 2 3dσ + 6 ligand HOMOs = 6 bonding MOs + 6 antibonding MOsĪs necessary, the number of atomic orbitals in (12) equals the number of molecular orbitals out (12). According to the LFT picture, orbital overlap between the 4s, 4p and symmetry-matched 4d (i.e., the 4dσ) AOs and the six ligand HOMOs results in six bonding and six antibonding MOs. What LFT adds to these CFT ideas is a description of the fate of the remaining unfilled metal valence orbitals and the filled ligand orbitals. The dσ orbitals are destabilized through electrostatic repulsion, while the dπ orbitals essentially remain non-bonding. The labels dσ and dπ will be useful for us later, and indicate how the d orbitals overlap with the incoming ligand orbitals-the dσ orbitals overlap in a head-on, sigma-type manner, and the dπ orbitals overlap in a side-on, pi-type manner. Notice that the perturbations of the metallic d orbitals are consistent with the ideas of crystal field theory for the octahedral geometry. The M–L bonding molecular orbitals mostly have ligand character, while the antibonding orbitals mostly reside on the metal. Notably, we need to address the often forgotten influence of the metal on the ligands-how might a metal modify the reactivity of organic ligands? The last post on geometry touched on these ideas a little, but we’ll dig a little deeper here. The theories described here try to address how the approach of ligands to a transition metal center modifies the electronics of the metal and ligands. Check out Fukui’s Nobel Prize lecture for an introduction to FMO theory. Some background in molecular orbital theory will be beneficial an understanding of organic frontier molecular orbital theory is particularly helpful. In this post, we’ll begin to explore the molecular orbital theory of organometallic complexes.
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