3.4.2 · D5Coordination Chemistry
Question bank — Ligands — classification (mono, bi, poly, ambidentate, chelating); denticity
Before the traps, look at the pictures that anchor every idea below — refer back to them line by line.
True or false — justify
is a bidentate ligand because it has both N and O donor atoms.
False. Having two potential donor atoms is not the same as using two at once; normally binds through N or O, one at a time — so it is monodentate and ambidentate, not bidentate.
Every polydentate ligand is a chelating ligand.
Mostly true, with a catch. A polydentate ligand chelates when all its donors grip one metal (forming a ring, Figure 2 left). If those donors instead reach out to different metals it is bridging (Figure 2 right), not chelating — so "polydentate" describes the tooth-count, "chelating" describes the geometry of the bite.
A ligand's denticity equals the number of lone pairs it possesses.
False. Water has two lone pairs on oxygen yet is monodentate (Figure 1, left) — denticity counts donor atoms bonding simultaneously to the same metal, not the total lone pairs available.
Ethylenediamine (en) and two separate molecules give the same stability because both make Ni–N bonds of the same strength.
False. Bond enthalpies are similar, but tying two donors into one molecule releases more free particles in the substitution, giving a positive ; via that raises — the chelate effect.
A 3-membered chelate ring is more stable than a 5-membered one because it is tighter.
False. A 3-membered ring forces bond angles far from their natural ~109° or 120° (Figure 3, red), so it is strained; 5- and 6-membered rings sit near natural angles (teal) and are the most stable.
bound through sulfur and bound through nitrogen are the same complex.
False. They are linkage isomers — identical formula but different donor atom (thiocyanato-S vs isothiocyanato-N), often with different colour and reactivity. See Linkage Isomerism.
The denticity of EDTA is always 6 in every complex it forms.
False. EDTA is capable of hexadentate binding (2 N + 4 O, Figure 1 right), but with a metal that cannot fit all six or a smaller coordination number it may bind through fewer donors — denticity describes what actually bonds, not the maximum possible.
A bridging ligand and a chelating ligand can never be the same species.
False. Oxalate can chelate one metal (5-membered ring) or bridge two metals (Figure 2); the role changes with the situation, so the same ligand carries different denticity descriptions in different complexes.
is a pentadentate ligand because five atoms touch the metal.
False. Five contiguous carbons donate shared π-electrons — that is hapticity (), not denticity. Denticity counts separate lone-pair donor atoms; hapticity counts a bonded π-set acting as one unit.
Spot the error
" has a coordination number of 3 because there are three en ligands."
The error: forgetting denticity. Each en donates 2 atoms, so coordination number , not 3. Always compute (denticity × count) and sum.
"In as a ligand, the donor atom is hydrogen."
The error: the donor atom is the one carrying the lone pair and forming the coordinate bond — that is oxygen. Hydrogen has no lone pair to give.
" is ambidentate, therefore it is bidentate."
Two mistakes fused. Ambidentate means it can choose between C and N donors; it still uses only one at a time, so it is monodentate, and ambidentate ≠ bidentate.
"Chelation makes complexes stronger because chelate bonds are stronger than normal coordinate bonds."
The error: the individual M–donor bonds are essentially the same strength. The extra stability is entropic (positive from releasing more free molecules), not a stronger bond.
" and are geometrical isomers."
The error: they differ in which atom of binds (N vs O), not in spatial arrangement — that makes them linkage isomers, a subtype of structural isomerism.
"EDTA is polydentate, so it must bridge several metal ions at once."
The error: polydentate says many teeth, not many metals. EDTA wraps its six donors around one metal like a cage (Figure 1 right) — it chelates a single centre, it does not normally bridge.
"Denticity 2 always means the ligand is chelating."
The error: a ligand using 2 donors could be bridging two metals instead of ringing one (Figure 2). Denticity-2 chelating requires the same metal and a closed ring.
Why questions
Why does the definition of denticity insist on the words "simultaneously" and "same metal"?
Because a ligand may have several lone pairs or donor atoms, but only those bonding at the same time to one centre form a chelate ring — the wording separates chelating from ambidentate (one at a time) and from bridging (different metals).
Why is entropy, not bond strength, the source of the chelate effect?
Replacing 6 separate monodentate ligands with 3 bidentate ones releases more free particles into solution (); since (all quantities at the same standard state, marked by the ) and is similar, the entropy term drives more negative and larger.
Why can an ambidentate ligand produce two differently coloured complexes?
Because the metal's environment changes with which donor atom bonds (N-bound vs O-bound). A different donor alters the ligand-field splitting, so the absorbed wavelength — and hence the colour — shifts.
Why do we call a ligand a Lewis base and the metal a Lewis acid?
The ligand donates a lone pair and the metal accepts it into an empty orbital; donating electron density is the definition of a Lewis base, accepting it defines the Lewis acid.
Why is EDTA so useful in titrations and in treating metal poisoning?
Its six donors wrap one metal into an exceptionally stable 1:1 chelate (huge ), so it "traps" ions cleanly and quantitatively — ideal for EDTA Titrations and for sequestering toxic ions like .
Why does higher denticity per ligand tend to lower the number of ligands needed for a given coordination number?
Coordination number counts donor atoms bonded, not molecules; a hexadentate ligand alone supplies all six, whereas six monodentate ligands are needed for the same count.
Edge cases
A ligand has three donor atoms but the metal only offers two free coordination sites. What is its denticity here?
Denticity describes what actually bonds, so it acts as bidentate in this complex even though it is potentially tridentate — the third donor stays free.
Can a monodentate ligand ever be ambidentate and a bridging ligand at once?
Not simultaneously in the same bond. can bind one metal via S (ambidentate choice) or bridge two metals using S on one and N on the other — but each described role is a distinct bonding situation.
What is the denticity of oxalate when it bridges two metal centres instead of chelating one?
It still uses two donor atoms, but toward two different metals (Figure 2 right), so on each metal it contributes one donor — it is bridging, not a chelate, and no ring forms on a single centre.
If a bidentate ligand's two donor atoms are held rigidly too far apart to both reach one metal, how does it behave?
It can only reach with one donor, so it effectively acts monodentate (or bridges two metals) — geometry, not just donor count, decides real denticity.
Is a 7-membered chelate ring impossible, or just unfavourable?
Not impossible, but unfavourable — the long, floppy chain rarely brings both donors to the metal at once, so such rings are entropically disfavoured and rare compared to 5- and 6-membered ones.
What happens to the "chelate effect" argument if the reaction releases fewer particles than it consumes?
Then would be negative, weakening or reversing the effect; the chelate effect specifically relies on more free particles being released than taken in.
How is (hapticity 5) different from a genuine pentadentate ligand?
The cyclopentadienyl ring donates one delocalised π-system from five bonded, contiguous carbons (hapticity), whereas a pentadentate ligand has five separate lone-pair donor atoms each forming an independent bond (denticity).
Recall
Recall One-line self-checks
Ambidentate vs bidentate — the single distinguishing word? ::: "Simultaneously" — bidentate uses two donors at once, ambidentate uses one at a time. The thermodynamic quantity behind the chelate effect? ::: A positive (entropy), which makes more negative and larger. Why is oxygen, not hydrogen, the donor in aqua ligands? ::: Oxygen carries the lone pairs; hydrogen has none to donate. Denticity vs hapticity in one line? ::: Denticity counts separate lone-pair donor atoms; hapticity () counts contiguous π-bonded atoms acting as one unit.