Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2021 / Paper 1 / 1

The isolobal analogy compares fragments through their frontier molecular orbitals: their number, approximate energies, shapes, symmetries and electron occupancies should correspond. It does not require the fragments to have the same total number of electrons or the same constituent elements. If the orbitals available for forming new bonds correspond, analogous ways of joining the fragments become plausible.

For example, $\mathrm{C}{\mathrm{H}}_3^{\bullet }$ and $\mathrm{Mn}(\mathrm{CO}{)}_5^{\bullet }$ are isolobal. The methyl radical has one singly occupied approximately outward-pointing orbital; removing one carbonyl ligand from an octahedral molecular geometry leaves an analogous frontier orbital on the 17-electron $\mathrm{Mn}(\mathrm{CO}{)}_5$ fragment. Joining two such fragments pairs the two electrons: the organic example is the C–C bond of ethane, and the organometallic example is the Mn–Mn bond of $\mathrm{M}{\mathrm{n}}_2(\mathrm{CO}{)}_{10}$. The analogy predicts a bonding pattern, rather than equality of bond energies or reaction rates.^[ref]