ambidentate ligand
/ am-bee-DEN-tate, 'ambi' = both /
Some ligands have two different atoms that could each grab the metal, but the ligand is too small or too rigid to use both at once — so it must pick one. Like a key that could fit either of two locks but only one at a time, an ambidentate ligand can bind through either of two donor atoms, giving two genuinely different compounds.
Precisely, an ambidentate ligand is a monodentate ligand that has more than one potential donor atom but binds through only one of them at a time. The classic case is the nitrite ion, NO2-, which can attach through its nitrogen (called nitro, M-NO2) or through an oxygen (called nitrito, M-ONO). Thiocyanate, SCN-, is another: it binds through nitrogen (isothiocyanato, M-NCS) or sulfur (thiocyanato, M-SCN). Cyanide can in principle bind through carbon or nitrogen. Because the two donor atoms sit at different ends, choosing one versus the other places different atoms next to the metal and gives different bond lengths, colors, and reactivity, even though the formula looks identical.
Ambidentate ligands are the source of linkage isomerism — two compounds with exactly the same atoms but differing only in which donor atom of the ambidentate ligand is bound. Which end binds is often set by the hard-soft match: soft metals tend to prefer the soft sulfur end of thiocyanate, harder metals the nitrogen end. This was historically important: one of the first linkage isomer pairs Werner studied was the nitro and nitrito cobalt ammines, distinguishable by their different colors.
The two compounds [Co(NH3)5(NO2)]2+ and [Co(NH3)5(ONO)]2+ have identical formulas, but in the first the nitrite binds through nitrogen (nitro, yellow) and in the second through oxygen (nitrito, red). They are linkage isomers — same atoms, different donor atom bound.
The same nitrite ligand bound through N (nitro) or O (nitrito) gives two differently colored isomers.
An ambidentate ligand is still monodentate — it binds through only one atom at a time, just with a choice of which. Do not confuse it with a bidentate or bridging ligand, which use two atoms at once.