MHT-CET Chemistry · Coordination Compounds

Ligands: Denticity, Donor Atoms, Charge and Field Strength

A ligand is an ion or molecule that donates one or more electron pairs to the central metal; its denticity is the number of donor atoms it uses at once (monodentate chloro and cyano, bidentate oxalato and ethylenediamine, hexadentate EDTA), an ambidentate ligand such as nitrito or thiocyanato can bind through either of two atoms, its charge is neutral (ammine, aqua, carbonyl) or anionic (chloro, sulphato, oxalato), and its position in the spectrochemical series says how strongly it splits the metal's d orbitals.

All Coordination Compounds notes MHT-CET Chemistry strategy

Why this matters

33 PYQs, 2 HARD. Seventeen are denticity and donor-atom counts — oxalate and en bidentate, EDTA hexadentate, cyanide monodentate, the total donor atoms in tetracyanonickelate (4), trioxalatocobaltate (6) or pentaammineaquacobalt (6), and the two HARD counts on ambidentate SCN⁻ and free NO₂; ten are charge — the neutral ligand from a list (ammine, aqua, carbonyl, asked five times) and which complex has only anionic, only neutral or both kinds of ligand; six are the spectrochemical series — the strongest or weakest field ligand from a list, and one increasing order. Three cards.

Concept 1 of 3

Denticity and Counting Donor Atoms

Intuition

Denticity is the number of donor atoms a ligand uses on the SAME metal at the same time. Cl⁻, CN⁻, OH⁻, NH₃, H₂O, CO each use one atom: monodentate. Oxalate uses two oxygens, ethylenediamine two nitrogens: bidentate. EDTA uses two nitrogens and four carboxylate oxygens: hexadentate. An ambidentate ligand has two possible donor atoms but uses only one at a time — nitrite through N (nitro) or O (nitrito), thiocyanate through S (thiocyanato) or N (isothiocyanato) — so it is monodentate in any one complex. Donor atoms in a complex = sum over ligands of (number of ligands × denticity).

Definition

Monodentate: Cl⁻, Br⁻, I⁻, F⁻, CN⁻, OH⁻, NH₃, H₂O, CO, NO₂⁻, SCN⁻ (one donor atom each). Bidentate: oxalato $\text{C}_2\text{O}_4^{2-}$ (2 O), ethylenediamine en (2 N). Hexadentate: EDTA (2 N + 4 O = 6).
Ambidentate (two possible donor atoms, one used): NO₂⁻ (nitro, N / nitrito, O), SCN⁻ (thiocyanato, S / isothiocyanato, N), CN⁻ (cyano / isocyano). The paper's count for an ambidentate ion counts BOTH atoms: 2n mol SCN⁻ → 4n mol donor atoms.
Donor atoms in a complex: $[\text{Ni(CN)}_4]^{2-}$ → 4 (one C each); $[\text{Co(C}_2\text{O}_4)_3]^{3-}$ → 3 × 2 = 6; $[\text{Co(H}_2\text{O)(NH}_3)_5]\text{I}_3$ → 1 + 5 = 6 (counter ions do not count); one mole oxalate → 2 moles donor atoms.
Free $\text{NO}_2$ (the neutral molecule, not the nitrite ion) is keyed 0 donor atoms — it is not a ligand.
Complexes with bidentate ligands in a list: those with oxalato or en (trioxalatocobaltate, bis(ethylenediamine)…platinum); cyano and fluoro complexes have none.

Donor-atom count

\text{donor atoms} = \sum (\text{number of ligands} \times \text{denticity});\quad \text{EDTA} = 6,\ \text{ox} = \text{en} = 2

Worked example

Count the donor atoms in [Cr(en)₂(ox)]⁺ and give the coordination number of Cr.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1Coordination CompoundsHARD

Identify number of moles of donor atoms in

2n

mole of

SCN^-

[Q82 · 9th May Shift 1 · 2024]

Counting all four oxygens of oxalate

Oxalate has four O atoms but coordinates through two — it is bidentate, and one mole gives 2 moles of donor atoms. Cyanide has C and N but donates through carbon only: [Ni(CN)₄]²⁻ has 4 donor atoms, not 8.

Drill 16 more on denticity and counting donor atoms

Concept 2 of 3

Neutral and Anionic Ligands by Name

Intuition

The name tells the charge. Neutral molecules keep a molecule-like name: ammine (NH₃), aqua (H₂O), carbonyl (CO), nitrosyl (NO), ethylenediamine (en). Anions end in -o: chloro, bromo, iodo, fluoro, cyano, hydroxo, nitro/nitrito, thiocyanato/isothiocyanato, sulphato, carbonato, nitrato, oxalato. So a complex named with only -o ligands (potassium trioxalatoaluminate(III)) has only anionic ligands; one named with ammine and aqua only (pentaammineaquacobalt(III) chloride) has only neutral ligands; pentaamminecarbonatocobalt(III) chloride has both.

Definition

Neutral: ammine NH₃, aqua H₂O, carbonyl CO, nitrosyl NO, en, pyridine. Ammine (NH₃) is not amido (NH₂⁻).
Anionic (-o): chloro, bromo, iodo, fluoro, cyano, hydroxo, nitro/nitrito, thiocyanato/isothiocyanato, sulphato, carbonato, nitrato, oxalato.
Only anionic ligands: $\text{K}_3[\text{Al(C}_2\text{O}_4)_3]$ , $[\text{Ni(CN)}_4]^{2-}$ , $\text{Na}_3[\text{Co(NO}_2)_6]$ . Only neutral: $[\text{Co(NH}_3)_5\text{H}_2\text{O}]\text{Cl}_3$ , $\text{Fe(CO)}_5$ , $[\text{Cu(NH}_3)_4]^{2+}$ . Both: $[\text{Co(NH}_3)_5\text{CO}_3]\text{Cl}$ , $[\text{Fe(H}_2\text{O)}_5\text{NCS}]^{2+}$ , $[\text{Pt(en)}_2(\text{SCN})_2]^{2+}$ .
Cisplatin $[\text{Pt(NH}_3)_2\text{Cl}_2]$ : ligands NH₃ and Cl⁻.

Ligand	Formula	Charge	Denticity
Ammine	NH₃	0	1 Ammine (NH₃) is neutral; amido (NH₂⁻) is anionic.
Aqua	H₂O	0	1
Carbonyl	CO	0	1
Ethylenediamine (en)	H₂NCH₂CH₂NH₂	0	2
Chloro / bromo / iodo / fluoro	X⁻	−1	1
Cyano	CN⁻	−1	1 (through C)
Nitro / nitrito	NO₂⁻ / ONO⁻	−1	1, ambidentate
Thiocyanato / isothiocyanato	SCN⁻ / NCS⁻	−1	1, ambidentate
Oxalato	C₂O₄²⁻	−2	2
Sulphato / carbonato	SO₄²⁻ / CO₃²⁻	−2	1 (usually)
EDTA	(edta)⁴⁻	−4	6

Molecule-like names are neutral; names ending in -o are anions.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2Coordination CompoundsMODERATE

Which from following complexes contains only anionic ligands?

[Q94 · 12th May Shift 1 · 2024]

Reading carbonyl or carbonato by the 'carbon'

Carbonyl is CO, neutral; carbonato is CO₃²⁻, anionic. Likewise nitrosyl (NO, neutral) against nitro/nitrato (anions).

Drill 9 more on neutral and anionic ligands by name

Concept 3 of 3

Field Strength: the Spectrochemical Series

Intuition

Ligands are ranked by how far they split the metal's d orbitals. Halides and sulphide split least (weak field, high-spin complexes); CO and CN⁻ split most (strong field, low-spin). The order to carry: I⁻ < Br⁻ < S²⁻ < SCN⁻ < Cl⁻ < NO₃⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < EDTA⁴⁻ < NH₃ < en < NO₂⁻ < CN⁻ < CO. Any question that lists CO or CN⁻ wants it as strongest; any that lists a halide wants it as weakest.

Definition

Weak field (small Δ, high spin): I⁻ < Br⁻ < S²⁻ < SCN⁻ < Cl⁻ < NO₃⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O.
Strong field (large Δ, low spin): NCS⁻ < EDTA⁴⁻ < NH₃ < en < NO₂⁻ < CN⁻ < CO (strongest).
Keyed answers: strongest of {H₂O, OH⁻, ox, CO} → CO; strong among {CN⁻, SCN⁻, I⁻, ox} → CN⁻; weak among {EDTA, CO, F⁻, NH₃} → F⁻; weak among {NH₃, NC⁻, Br⁻, CO} → Br⁻; increasing among I⁻, Cl⁻, S²⁻, OH⁻ keyed I⁻ < Cl⁻ < S²⁻ < OH⁻.
One paper keyed EDTA above en as the strongest of {S²⁻, OH⁻, EDTA, en}; the textbook series puts en above EDTA. Either way both sit far above OH⁻ and S²⁻.

Spectrochemical series

\text{I}^- < \text{Br}^- < \text{S}^{2-} < \text{SCN}^- < \text{Cl}^- < \text{F}^- < \text{OH}^- < \text{C}_2\text{O}_4^{2-} < \text{H}_2\text{O} < \text{NH}_3 < \text{en} < \text{CN}^- < \text{CO}

Worked example

Arrange NH₃, Cl⁻, CN⁻ and H₂O in increasing field strength, and say which would give a low-spin d⁶ complex.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 3Coordination CompoundsMODERATE

Identify the correct increasing order of field strength of ligands from following.

[Q89 · 25 April Shift II · 2025]

Ranking by charge

A −2 oxalate is a WEAKER field ligand than neutral NH₃ or CO. Field strength follows the donor atom and π-bonding, not the charge: C-donors (CO, CN⁻) top the series, halides sit at the bottom.

Drill 5 more on field strength: the spectrochemical series

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Denticity and Counting Donor Atoms
Donor-atom count
$\text{donor atoms} = \sum (\text{number of ligands} \times \text{denticity});\quad \text{EDTA} = 6,\ \text{ox} = \text{en} = 2$
Field Strength: the Spectrochemical Series
Spectrochemical series
$\text{I}^- < \text{Br}^- < \text{S}^{2-} < \text{SCN}^- < \text{Cl}^- < \text{F}^- < \text{OH}^- < \text{C}_2\text{O}_4^{2-} < \text{H}_2\text{O} < \text{NH}_3 < \text{en} < \text{CN}^- < \text{CO}$

Reference tables (1)

Neutral and Anionic Ligands by Name11 rows

Ligand	Formula	Charge	Denticity
Ammine	NH₃	0	1 Ammine (NH₃) is neutral; amido (NH₂⁻) is anionic.
Aqua	H₂O	0	1
Carbonyl	CO	0	1
Ethylenediamine (en)	H₂NCH₂CH₂NH₂	0	2
Chloro / bromo / iodo / fluoro	X⁻	−1	1
Cyano	CN⁻	−1	1 (through C)
Nitro / nitrito	NO₂⁻ / ONO⁻	−1	1, ambidentate
Thiocyanato / isothiocyanato	SCN⁻ / NCS⁻	−1	1, ambidentate
Oxalato	C₂O₄²⁻	−2	2
Sulphato / carbonato	SO₄²⁻ / CO₃²⁻	−2	1 (usually)
EDTA	(edta)⁴⁻	−4	6

Molecule-like names are neutral; names ending in -o are anions.

Watch out for (3)

Counting all four oxygens of oxalate
→ Denticity and Counting Donor Atoms
Reading carbonyl or carbonato by the 'carbon'
→ Neutral and Anionic Ligands by Name
Ranking by charge
→ Field Strength: the Spectrochemical Series

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