Coordination Compounds — MHT-CET Chemistry Formulas and Traps

Ligands: Denticity, Donor Atoms, Charge and Field Strength

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}

Neutral and Anionic Ligands by Name

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.

Common traps

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.

Reading carbonyl or carbonato by the 'carbon'

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

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.

Types of Complexes: Homoleptic or Heteroleptic, Cationic, Anionic or Neutral

Learn this subtopic in the notes

Same Ligands or Mixed, and the Charge on the Sphere

Sphere charge

\text{charge on sphere} = \text{oxidation state of metal} + \sum \text{ligand charges}

Common traps

Calling a salt 'neutral' because the whole compound is

K₄[Fe(CN)₆] is a neutral salt with an ANIONIC complex. 'Neutral complex' means the sphere itself carries no charge — [Co(NO₂)₃(NH₃)₃], cisplatin, the carbonyls.

Oxidation State, Coordination Number, Werner's Counter Ions and IUPAC Names

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Oxidation state and coordination number

x + \sum q_{\text{ligand}} = q_{\text{sphere}};\qquad \text{CN} = \sum (\text{ligands} \times \text{denticity})

Counter Ions: What Precipitates with Silver Nitrate

Werner's precipitation count

n(\text{AgCl}) = n(\text{Cl}^- \text{ outside } [\ ])\ \text{per mole of complex}

From Name to Formula and Back

Name to formula

q_{\text{sphere}} = \text{(Roman numeral)} + \sum q_{\text{ligand}};\quad \text{counter ions} \times \text{their charge} = -q_{\text{sphere}}

Common traps

Counting ligands instead of donor atoms

[Fe(C₂O₄)₃]³⁻ has three ligands but six donor atoms, so its coordination number is 6, not 3. The coordination number counts bonds to the metal.

Reading -ous and -ic backwards

-ous is the LOWER oxidation state (aurous Au⁺, cuprous Cu⁺, ferrous Fe²⁺, mercurous Hg₂²⁺), -ic the higher (auric Au³⁺, cupric Cu²⁺, ferric Fe³⁺, mercuric Hg²⁺).

Counting every chlorine in the formula

[Co(NH₃)₅Cl]Cl₂ has three chlorines but gives only 2 AgCl — the chloro ligand inside the bracket is held by the metal and never meets the Ag⁺.

Ordering ligands by charge or by prefix

Ligands go alphabetically by the ligand name alone: tetraammine before diaqua because 'ammine' precedes 'aqua' — the tetra- and di- do not count. Neutral-before-anionic is the formula-writing habit, not the naming rule.

Dropping the -ate or the oxidation state

An anionic sphere always ends in -ate (hexacyanoferrate, not hexacyanoiron), and the oxidation state is the metal's, not the sphere's charge: [Fe(CN)₆]⁴⁻ is ferrate(II), not ferrate(IV).

Isomerism in Coordination Compounds: Structural and Geometric

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Geometric (cis–trans) Isomerism and the MA₂BC Type

Square planar geometric isomers

\text{MA}_4, \text{MA}_3\text{B}: 0;\quad \text{MA}_2\text{B}_2, \text{MA}_2\text{BC}: 2 \ (\textit{cis}, \textit{trans});\quad \text{MABCD}: 3

The Four Structural Isomerisms

Isomerism	What differs	Example pair
Ionisation	Anion inside ↔ anion outside	[Co(NH₃)₅Br]SO₄ / [Co(NH₃)₅SO₄]Br
Solvate (hydrate)	Water inside ↔ water of crystallisation	[Cr(H₂O)₆]Cl₃ / [Cr(H₂O)₅Cl]Cl₂·H₂O
Linkage	Donor atom of an ambidentate ligand	–NO₂ / –ONO; –SCN / –NCS Only an ambidentate ligand can do this.
Coordination	Ligands swapped between cation and anion metals	[Co(NH₃)₆][Cr(CN)₆] / [Cr(NH₃)₆][Co(CN)₆]

Find what moved: an anion, a water, a donor atom, or a whole set of ligands.

Common traps

Calling a water swap 'ionisation'

In the chromium hydrates a chloride also moves, but what defines the pair is the WATER moving between sphere and crystal — solvate isomerism. Ionisation isomers exchange two different anions and contain the same water.

Reading MA₂B₂ as MA₂BC

Pt(NH₃)₂Cl₂ also has cis and trans forms, but it is MA₂B₂ — two pairs of identical ligands. MA₂BC needs one pair and two DIFFERENT single ligands.

Bonding in Complexes: Hybridisation, Magnetism, EAN and Stability

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Hybridisation and Geometry from d Electrons and Field Strength

Hybridisation by coordination number

\text{CN } 2: sp;\quad \text{CN } 4: sp^3 \text{ (tetrahedral)},\ dsp^2 \text{ (square planar)};\quad \text{CN } 6: d^2sp^3 \text{ (inner)},\ sp^3d^2 \text{ (outer)}

Unpaired Electrons and the Spin-Only Magnetic Moment

Spin-only magnetic moment

\mu = \sqrt{n(n+2)}\ \text{BM}

Effective Atomic Number (EAN)

EAN

\text{EAN} = Z - x + 2\,\text{CN}

Stability of Complexes: Metal Charge and the Irving–Williams Order

Irving–Williams order (same ligand)

\text{Cu}^{2+} > \text{Ni}^{2+} > \text{Co}^{2+} > \text{Fe}^{2+} > \text{Mn}^{2+} > \text{Cd}^{2+}

Common traps

Calling every four-coordinate complex tetrahedral

Four ligands can be tetrahedral (sp³) OR square planar (dsp²). A d⁸ ion with a strong-field ligand — [Ni(CN)₄]²⁻, every Pt(II) complex — is square planar.

Using the free-ion count for a strong-field complex

Co³⁺ has four unpaired electrons only before the ligands arrive. With NH₃ or CN⁻ they pair — [Co(NH₃)₆]³⁺ has zero. Decide the ligand's field strength before counting.

Adding one electron per ligand, or per bidentate ligand

Each donor atom gives a PAIR, so add 2 × coordination number. With en or oxalate count donor atoms, not ligands: [Co(en)₃]³⁺ is 27 − 3 + 12 = 36.

Ranking by atomic number

Stability rises from Mn²⁺ to Cu²⁺ but it is not 'heavier is more stable': Cd²⁺ is heavier than all of them and forms the least stable complexes.

Coordination Compounds formulas

Ligands: Denticity, Donor Atoms, Charge and Field Strength

Denticity and Counting Donor Atoms

Field Strength: the Spectrochemical Series

Neutral and Anionic Ligands by Name

Common traps

Types of Complexes: Homoleptic or Heteroleptic, Cationic, Anionic or Neutral

Same Ligands or Mixed, and the Charge on the Sphere

Common traps

Oxidation State, Coordination Number, Werner's Counter Ions and IUPAC Names

Counter Ions: What Precipitates with Silver Nitrate

From Name to Formula and Back

Common traps

Isomerism in Coordination Compounds: Structural and Geometric

Geometric (cis–trans) Isomerism and the MA₂BC Type

The Four Structural Isomerisms

Common traps

Bonding in Complexes: Hybridisation, Magnetism, EAN and Stability

Hybridisation and Geometry from d Electrons and Field Strength

Unpaired Electrons and the Spin-Only Magnetic Moment

Effective Atomic Number (EAN)

Stability of Complexes: Metal Charge and the Irving–Williams Order

Common traps

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