A principle used to discuss the electronic structure of complex molecules in terms of simpler molecules. This has enabled Wade’s rules to be extended from boranes to organometallic compounds. Two fragments (or molecules) are said to be isolobal if their frontier orbitals have similar symmetry, a similar number of bonding electrons, and approximately similar energy. For example, methylene (CH₂) is isolobal to M(CO)₄ and BH is isolobal to M(CO)₃, where M is Fe, Ru, or Os. The isolobal principle can also be used to predict the reactivity and reaction mechanism of complexes.