JEE Mains Chemistry · Coordination Compounds

Werner's Theory and Ionisable Ligands

Only the ions written outside the square bracket are free in solution; they decide how much AgCl or BaSO₄ precipitates, how many ions the salt gives and its primary valency, while the ligands inside fix the coordination number.

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Why this matters

Nineteen PYQs, eight of them multiple choice, and four from 2026. Fourteen count the ions outside the bracket, from the silver chloride or barium sulphate they precipitate, from a cation exchanger, from conductivity or from a freezing-point depression. Five turn a formula into primary and secondary valency or tell a double salt from a complex. Eleven of the nineteen ask for a number.

Concept 1 of 2: Counting the ions outside the coordination sphere

A ligand inside the bracket is bonded to the metal and does not leave it in water. An ion outside the bracket is an ordinary counter-ion. So

\mathrm{AgNO_3}

precipitates only the chloride outside the bracket, and

\mathrm{BaCl_2}

only the sulphate outside it. Every question here is the same count: how many free ions does one formula unit give?

Definition

Moles of AgCl = moles of complex × number of $\mathrm{Cl^-}$ outside the bracket.
Number of ions per formula unit = 1 complex ion + the counter-ions. It sets the conductivity type (1:1, 1:2, 1:3) and the van 't Hoff factor $i$ in $\Delta T_f = iK_f m$ .
$\mathrm{CoCl_3\cdot xNH_3}$ with coordination number 6: $x = 6, 5, 4, 3$ give 3, 2, 1, 0 mol AgCl per mol.
A cation exchanger holds back the complex cation and releases the outer anions into the eluate, still one $\mathrm{Cl^-}$ per ionisable chloride.
Ionisation isomers such as $\mathrm{[Co(NH_3)_5SO_4]Br}$ and $\mathrm{[Co(NH_3)_5Br]SO_4}$ each give only their own outer ion: the first precipitates AgBr, the second $\mathrm{BaSO_4}$ .

Precipitate from the ionisable ions

n_{\mathrm{AgCl}} = n_{\text{complex}} \times (\text{number of } \mathrm{Cl^-} \text{ outside } [\;]) \qquad \Delta T_f = i\,K_f\,m

Worked example

Excess

\mathrm{AgNO_3}

is added to 50 mL of a 0.04 M solution of

\mathrm{[Co(NH_3)_6]Cl_3}

. Find the moles and the mass of AgCl formed (molar mass of AgCl = 143.5 g mol

^{-1}

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2026 · 6 Apr 2026 Shift 2 · Q46Moderate

Example 1 · Coordination Compounds · Werner's Theory and Ionisable Ligands

An excess of

{AgNO}_{3}

is added to 100 mL of a 0.05 M solution of tetraaquadichloridochromium (III) chloride. The number of moles of AgCl precipitated will be

\_\_\_\_

\times 10^{- 3}

. (Nearest integer)

Chloride inside the bracket never precipitates

\mathrm{[Cr(H_2O)_4Cl_2]Cl}

only one of the three chlorides reaches the silver ion. Counting all three triples the answer, and that value is always among the options.

Use the portion that was actually tested

When a mixed solution is split in two and each half meets a different reagent, each half carries only half the moles. Working from the whole volume doubles every answer.

Read which ratio the question wants

Moles of complex per mole of AgCl and moles of AgCl per mole of complex are reciprocals. For

\mathrm{[Cr(H_2O)_6]Cl_3}

the first is 1/3 and the second is 3.

Drill 13 more on counting the ions outside the coordination sphere

Concept 2 of 2: Primary valency, secondary valency and double salts

Werner saw two kinds of valency in one metal. The primary valency is the oxidation state: it is balanced by negative ions and can be ionisable. The secondary valency is the coordination number: the ligands that hold a fixed shape around the metal. A double salt has no such inner sphere, so in water it breaks up completely into simple ions.

Definition

Primary valency = oxidation state of the metal. It stays the same whether the balancing anions are inside or outside the bracket.
Secondary valency = coordination number = number of donor atoms bonded to the metal (a bidentate ligand counts 2, EDTA⁴⁻ counts 6).
Oxidation state: $x + \sum(\text{ligand charges}) = \text{charge on the complex ion}$ . A complex counter-cation also carries charge: in $\mathrm{Hg[Co(SCN)_4]}$ mercury is +2, so cobalt is +2.
Double salts such as Mohr's salt $\mathrm{FeSO_4\cdot(NH_4)_2SO_4\cdot 6H_2O}$ and potash alum $\mathrm{K_2SO_4\cdot Al_2(SO_4)_3\cdot 24H_2O}$ give all their simple ions in water.
A complex keeps its complex ion: $\mathrm{Fe(CN)_2\cdot 4KCN}$ is $\mathrm{K_4[Fe(CN)_6]}$ and gives no free $\mathrm{Fe^{2+}}$ or $\mathrm{CN^-}$ .

Oxidation state of the central metal

x + \sum q_{\text{ligands}} = q_{\text{complex ion}}

Worked example

Find the primary and secondary valency of iron in

\mathrm{K_3[Fe(C_2O_4)_3]}

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2022 · 27 July 2022 · Q57Moderate

Example 2 · Coordination Compounds · Werner's Theory and Ionisable Ligands

The conductivity of a solution of complex with formula

CoCl_{3}\left( NH_{3} \right)_{4}

corresponds to

1:1

electrolyte, then the primary valency of central metal ion is

Primary valency is the oxidation state, not the ionisable count

\mathrm{[Co(NH_3)_4Cl_2]Cl}

has only one ionisable chloride, but cobalt is still +3, so its primary valency is 3. The number of ions outside the bracket tells you the formula, not the valency.

A complex counter-ion changes the metal's charge

\mathrm{Hg[Co(SCN)_4]}

the mercury ion is +2, so the complex anion is 2− and cobalt is +2, with coordination number 4. A 2025 key gave this complex primary valency 3; the charge balance gives 2.

Drill 4 more on primary valency, secondary valency and double salts

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)

Counting the ions outside the coordination sphere
Precipitate from the ionisable ions
$n_{\mathrm{AgCl}} = n_{\text{complex}} \times (\text{number of } \mathrm{Cl^-} \text{ outside } [\;]) \qquad \Delta T_f = i\,K_f\,m$
Primary valency, secondary valency and double salts
Oxidation state of the central metal
$x + \sum q_{\text{ligands}} = q_{\text{complex ion}}$

Watch out for (5)

Chloride inside the bracket never precipitates
→ Counting the ions outside the coordination sphere
Use the portion that was actually tested
→ Counting the ions outside the coordination sphere
Read which ratio the question wants
→ Counting the ions outside the coordination sphere
Primary valency is the oxidation state, not the ionisable count
→ Primary valency, secondary valency and double salts
A complex counter-ion changes the metal's charge
→ Primary valency, secondary valency and double salts

Test yourself on Coordination Compounds

20 past JEE Mains questions from this chapter, timed at 48 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

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