Isolobal principle

The isolobal principle in organometallic chemistry devised by Roald Hoffmann aims to improve the understanding of chemical bonding in molecules by identifying molecules that share a common frontier orbital type, approximate energy and occupancy just as isoelectronic molecules share the same number of valence electrons and structure. When representing pairs of isolobal structures, they are connected through a two-pointed array with half an arrow below.

The methyl radical is isolobal with the metal carbonyl species manganese pentacarbonyl Mn(CO)5 because both molecules have a single electron in a hybrid orbital pointing away from the plane of the molecule (though they are not isoelectronic). This resemblance is reflected in the chemistry of both molecules. As the methyl radical can dimerize to ethane, Mn(CO)5 can dimerize to (CO)5Mn-Mn(CO)5 and both radicals can even form Mn(CO)5CH3.

In the same way iron tetracarbonyl Fe(CO)4 is isolobal with the carbene CH2 because they both have two electrons contained in two hybrid orbitals. methylene dimerizes to ethylene and both fragments can couple to a carbene complex.