What Is a Coordination Compound?

Some of the most important molecules in nature and industry are coordination compounds: haemoglobin (which carries oxygen), chlorophyll (which captures sunlight), vitamin B12_{12}, and countless catalysts and pigments. In each, a central metal atom or ion is surrounded by a set of bound molecules or ions called ligands.

Coordination compound: a compound in which a central metal atom/ion is bonded to a fixed number of ligands by coordinate (dative) bonds, forming a coordination entity that retains its identity even in solution.

A classic example is [Co(NH3)6]Cl3[\text{Co(NH}_3)_6]\text{Cl}_3. Here Co3+^{3+} is the central ion, six NH3_3 molecules are ligands, and the whole [Co(NH3)6]3+[\text{Co(NH}_3)_6]^{3+} unit stays intact in solution — only the three Cl−^- outside the square brackets ionise.

Double salt vs complex: a double salt (e.g. Mohr's salt, FeSO4⋅_4\cdot(NH4_4)2_2SO@@GYANGHAR_MATH@@96H2_2O) dissociates completely into all its ions in water. A complex (e.g. K4_4[Fe(CN)6_6]) does not give all ions — the complex ion [Fe(CN)6_6]4−^{4-} stays intact.

Werner's Theory

Alfred Werner (Nobel Prize, 1913) explained coordination compounds with a brilliant idea: metals have two kinds of valency.

  1. Primary valency (ionisable):
  • Satisfied by negative ions (anions).
  • Non-directional — does not fix geometry.
  • Corresponds to the modern oxidation state of the metal.
  1. Secondary valency (non-ionisable):
  • Satisfied by ligands (negative ions or neutral molecules).
  • Directional — fixes the geometry of the complex.
  • Corresponds to the modern coordination number.

Werner theory diagram of primary and secondary valencies in a cobalt ammine complex

Worked logic: for CoCl3⋅6_3\cdot6NH3_3, all 3 Cl−^- are precipitated by AgNO3_3 — so all three are primary (ionisable, outside the sphere), and the six NH3_3 are held by secondary valencies. The formula is [Co(NH3)6]Cl3[\text{Co(NH}_3)_6]\text{Cl}_3.

How Werner Deduced Structures

Werner studied a series of cobalt-ammine chlorides and counted how many Cl−^- ions precipitated with excess AgNO3_3 (only ionisable, outside-sphere chloride precipitates):

Compound Cl−^- precipitated Modern formula Ions in solution
CoCl3⋅6_3\cdot6NH3_3 3 [Co(NH3)6]Cl3[\text{Co(NH}_3)_6]\text{Cl}_3 4
CoCl3⋅5_3\cdot5NH3_3 2 [Co(NH3)5Cl]Cl2[\text{Co(NH}_3)_5\text{Cl}]\text{Cl}_2 3
CoCl3⋅4_3\cdot4NH3_3 1 [Co(NH3)4Cl2]Cl[\text{Co(NH}_3)_4\text{Cl}_2]\text{Cl} 2

A Cl−^- inside the coordination sphere (bonded to Co) is not precipitated; only the outside (ionisable) Cl−^- are.

[NEET Important] The number of ions a complex gives in solution (and hence its molar conductivity) reveals how many ions are outside the coordination sphere. [Co(NH3)6]Cl3[\text{Co(NH}_3)_6]\text{Cl}_3 gives 4 ions; [Co(NH3)4Cl2]Cl[\text{Co(NH}_3)_4\text{Cl}_2]\text{Cl} gives only 2 — a classic exam discriminator.

Solved Examples

Example 1: Double salt vs complex

How does a double salt differ from a complex compound?

Solution: A double salt dissociates completely into all its constituent ions in water (e.g. Mohr's salt gives Fe2+^{2+}, NH4+_4^+, SO42−_4^{2-}). A complex retains its coordination entity in solution (e.g. K4_4[Fe(CN)6_6] gives K+^+ and the intact [Fe(CN)6_6]4−^{4-}, not free Fe2+^{2+} or CN−^-).

Example 2: Primary and secondary valency

In Werner's theory, what do primary and secondary valencies correspond to today?

Solution: Primary valency = oxidation state (ionisable, satisfied by anions); secondary valency = coordination number (non-ionisable, directional, fixes geometry).

Example 3: Deduce the formula

CoCl3⋅5_3\cdot5NH3_3 precipitates 2 mol of AgCl per mole with excess AgNO3_3. Write its coordination formula.

Solution: 2 Cl−^- are ionisable (outside), so 1 Cl−^- is inside the sphere with the 5 NH3_3. Formula: [Co(NH3)5Cl]Cl2[\text{Co(NH}_3)_5\text{Cl}]\text{Cl}_2.

Example 4: Ions in solution

How many ions does [Co(NH3)6]Cl3[\text{Co(NH}_3)_6]\text{Cl}_3 give in solution?

Solution: It gives 4 ions: one [Co(NH3)6]3+[\text{Co(NH}_3)_6]^{3+} cation and three Cl−^- ions.

Example 5: Chloride inside the sphere

Why is the chloride in [Co(NH3)4Cl2]Cl[\text{Co(NH}_3)_4\text{Cl}_2]\text{Cl} that is inside the brackets not precipitated by AgNO3_3?

Solution: Chloride inside the coordination sphere is bonded to cobalt by a secondary (coordinate) valency and is non-ionisable, so it does not give free Cl−^- ions and is not precipitated. Only the one outside (ionisable) Cl−^- is precipitated.

Example 6: Directional valency

Which of Werner's valencies determines the geometry of the complex?

Solution: The secondary valency is directional and fixes the geometry (e.g. octahedral, square planar). Primary valency is non-directional.

Example 7: Count primary and secondary valencies

For [Co(NH3)6]Cl3[\text{Co(NH}_3)_6]\text{Cl}_3, state the primary and secondary valencies of cobalt.

Solution: Primary valency = 3 (satisfied by 3 Cl−^-, the +3 oxidation state). Secondary valency = 6 (six NH3_3 ligands, coordination number 6).

Example 8: Identify the complex ion

In K3_3[Fe(CN)6_6], what is the complex ion and what ionises off?

Solution: The complex ion is [Fe(CN)6_6]3−^{3-}; the three K+^+ ions ionise. The complex ion stays intact, so free CN−^- is not released.

Example 9: AgCl from a complex

How many moles of AgCl will [Co(NH3)4Cl2]Cl[\text{Co(NH}_3)_4\text{Cl}_2]\text{Cl} give with excess AgNO3_3?

Solution: Only the one chloride outside the sphere is ionisable, so 1 mol of AgCl is precipitated. The two chlorides inside the sphere are not precipitated.

Example 10: Mohr's salt behaviour

When FeSO4_4 and (NH4_4)2_2SO4_4 are mixed in 1:1 ratio (Mohr's salt), what ions form in solution?

Solution: Mohr's salt is a double salt, so it dissociates completely into Fe2+^{2+}, NH4+_4^+ and SO42−_4^{2-} ions (it gives the normal tests of Fe2+^{2+}). It is not a complex.