The maximum possible denticities of EDTA towards a common transition metal ion and an inner-transition metal ion,…

🧠 EDTA's Six Donor Atoms — But Steric Reach Differs

EDTA (ethylenediaminetetraacetate) has 6 donor atoms total: 2 amine N + 4 carboxylate O.

For a regular transition metal ($\mathrm{Co^{3+}, Fe^{3+}, Ca^{2+}}$, etc.), the metal is small enough that all 6 donor atoms wrap around it → denticity 6 (hexadentate).

For a larger inner-transition metal ($\mathrm{Ln^{3+}, Ac^{3+}}$), the metal radius is so large that EDTA cannot fill the coordination sphere alone — the metal can additionally accept water molecules. EDTA still uses 6 of its donors AND allows extra ligands. But the maximum possible denticity of EDTA itself is sometimes quoted as 8 in lanthanide contexts when both amine N atoms and both O atoms of each $-\mathrm{COO^-}$ chelate (one to the same metal, one to a neighbouring one) — extending its bridging reach.

So:

• Transition metal: max denticity = 6.
• Inner-transition (lanthanide/actinide): max denticity = 8.

🗺️ Why the Lanthanide Case Reaches 8

Lanthanides routinely have CN = 8–10. EDTA, with its flexible carboxylate arms, can use both oxygens of some carboxylate groups (η²-COO chelation) when bound to a large $\mathrm{Ln^{3+}}$, raising its effective denticity. This is the textbook reasoning for the 6 vs 8 distinction.

⚡ The "EDTA = 6 (small) / 8 (large)" Anchor

Two numbers, one ligand:

• Common transition metals → 6.
• Lanthanides/actinides → 8.

⚠️ Don't Quote 4 or 7

EDTA never coordinates as 4 (you'd be ignoring half its donors) or 7 (no consistent geometry). The valid numbers are 6 and 8 only.

$\boxed{\text{Answer: (4) 6 and 8}}$