classification of organometallic ligands
When you walk into a busy kitchen, it helps to sort the helpers by what they bring to the counter. Some arrive carrying a full bowl of ingredients and simply set it down; others arrive empty-handed but hold out one hand to shake, expecting you to bring something too. Ligands around a metal can be sorted the same way: by how many electrons they bring to the shared bond and whether they share evenly or donate the whole pair.
The most useful scheme, due to Malcolm Green, sorts ligands into a few simple types. An L-type ligand donates a lone pair, contributing both electrons of the bond while staying neutral; carbon monoxide, a phosphine, an amine and an alkene are all L-type. An X-type ligand forms a normal shared bond in which the metal and the ligand each supply one electron; a hydride, a halide, a methyl group and an aryl group are X-type. A Z-type ligand is the rare opposite, accepting a pair from the metal. Many real ligands are combinations, written as LnXm: a cyclopentadienyl ring acting through all five carbons is treated as an L2X donor, and an allyl group bound through three carbons as an LX donor. This bookkeeping is deliberately a formalism, not a claim about real charges; its only job is to let you count consistently.
Why bother sorting? Because the entire electron-counting machinery of organometallic chemistry rests on it. Once you know each ligand's type you can add up the valence electrons, test the 18-electron rule, and read off the metal's oxidation state. The same complex can be counted by either the ionic or the neutral convention and give the same total, but only if you classify every ligand consistently first. The classification is the grammar; the electron count is the sentence you build with it.
In CpMn(CO)3, the cyclopentadienyl ring (Cp) is an L2X ligand and each of the three CO molecules is an L ligand. So manganese sees one X-type and five L-type donor functions in all, a tidy way to bookkeep before counting electrons.
Labelling each ligand as L, X, or a combination is the first step before any electron count.
The L/X labels are a counting formalism, not a statement about real electron distribution; an X-type bond is usually a perfectly ordinary covalent bond, not an ionic one.