Isolobal Principle - Construction of Isolobal Fragments

Construction of Isolobal Fragments

To begin to generate an isolobal fragment, the molecule needs to follow certain criteria. Molecules based around main group elements should satisfy the octet rule when all bonding and nonbonding molecular orbitals (MOs) are filled and all antibonding MOs are empty. For example methane is a simple molecule from which to form a main group fragment. The removal of a hydrogen atom from methane generates a methyl radical. The molecule retains its molecular geometry as the frontier orbital points in the direction of the missing hydrogen atom. Further removal of hydrogen results in the formation of a second frontier orbital. This process can be repeated until only one bond remains to the molecule's central atom. Figure 2 demonstrates this example of step-by-step generation of isolobal fragments.

The isolobal fragments of octahedral complexes, such as ML₆, can be created in a similar fashion. Transition metal complexes should initially satisfy the eighteen electron rule, have no net charge, and their ligands should be two electron donors (Lewis bases). Consequently, the metal center for the ML₆ starting point must be d6. Removal of a ligand is analogous to the removal of hydrogen of methane in the previous example resulting in a frontier orbital, which points toward the removed ligand. Cleaving the bond between the metal center and one ligand results in a ML₅− radical complex. In order to satisfy the zero charge criteria the metal center must be changed. For example, a MoL₆ complex is d6 and neutral. However, removing a ligand to form the first frontier orbital would result in a MoL₅− complex because Mo has obtained an additional electron making it d7. To remedy this, Mo can be exchanged for Mn, which would from a neutral d7 complex in this case, as shown in Figure 3. This trend can continue until only one ligand is left coordinated to the metal center.