HOMO and LUMO
/ HOH-moh and LOO-moh /
When you fill a molecular orbital diagram from the bottom up, two levels stand out by their position. The highest one that still has electrons in it is the HOMO — the highest occupied molecular orbital. The lowest one that is still empty, sitting just above, is the LUMO — the lowest unoccupied molecular orbital. They are the topmost rung with people standing on it and the next empty rung above. Almost everything a molecule does chemically happens at these two rungs.
Why these two and not the buried lower orbitals? Because chemistry is about giving and taking electrons. The HOMO holds a molecule's most loosely bound, easiest-to-donate electrons — so it is what gets attacked by electron-poor species and what determines how good an electron donor or reductant the molecule is. The LUMO is the cheapest empty slot the molecule has — so it is where incoming electrons go, which sets how good an acceptor or oxidant it is. The energy gap between HOMO and LUMO is a kind of chemical stiffness: a wide gap means a stable, sluggish, often colorless molecule, while a narrow gap means a soft, reactive one that can absorb visible light.
These two frontier orbitals do an astonishing amount of explanatory work. The HOMO-LUMO gap sets the color of dyes and the photons a molecule absorbs; a small gap is why long conjugated systems are colored. In reactions, a good reaction is often one where one molecule's HOMO overlaps well in energy and symmetry with another's LUMO — the donor's filled orbital pours electrons into the acceptor's empty one. In inorganic chemistry this frontier-orbital view underlies how Lewis bases bind metals (base HOMO into metal LUMO) and the whole picture of charge-transfer and electron-transfer reactions.
Carbon monoxide bonds to a metal using its HOMO — a carbon-based lone pair — donated into an empty metal orbital, while the metal pushes electron density back into CO's LUMO, an empty pi-star orbital. This two-way HOMO-LUMO traffic is exactly the bonding in metal carbonyls.
HOMO donates, LUMO accepts; the gap between them governs color and reactivity.
HOMO and LUMO are powerful but approximate guides — they capture where reactions tend to start, but real reactivity also depends on the orbitals just below and above them and on the reaction's full energy landscape.