Coordination Compounds • Topic 1 of 3

Werner's Theory & Nomenclature

A coordination compound contains a central metal atom or ion bonded to a fixed number of ions or neutral molecules called ligands through coordinate (dative) bonds. Such compounds — e.g. [Co(NH3)6]Cl3 and K4[Fe(CN)6] — retain their identity in solution, unlike double salts (e.g. carnallite KCl·MgCl2·6H2O) which dissociate completely into their ions.

Werner's coordination theory (1893) explained these compounds with three postulates:

  • Every metal shows two kinds of valence. The primary valence is ionisable, satisfied by negative ions and equal to the oxidation state. The secondary valence is non-ionisable, satisfied by ligands and equal to the coordination number.
  • Primary valences are non-directional; secondary valences are directional and fix the geometry of the complex (e.g. coordination number 6 → octahedral, 4 → square planar or tetrahedral).
  • Ions in the secondary sphere (ligands) lie inside square brackets and are not precipitated easily; ions outside are free.

For [Co(NH3)6]Cl3, all three Cl− are primary (ionisable) and the six NH3 are secondary; adding AgNO3 precipitates all three chlorides. In [Co(NH3)5Cl]Cl2 only two Cl− are ionisable.

Ligands are classified by the number of donor atoms (denticity): monodentate (one donor, e.g. Cl−, NH3, H2O, CN−); bidentate (two donors, e.g. ethane-1,2-diamine 'en', oxalate C2O42−); polydentate (e.g. EDTA4− is hexadentate). A polydentate ligand that forms a ring with the metal gives a chelate; such rings give extra stability (the chelate effect). An ambidentate ligand can attach through either of two donor atoms, e.g. NO2− (nitrito-N or nitrito-O) and SCN− (S- or N-bonded).

The coordination number (CN) is the number of donor atoms directly bonded to the metal (count donor atoms, not ligands). The coordination sphere is the metal plus ligands written inside the brackets and behaves as a single unit. The oxidation number of the metal is the charge it would carry if all ligand pairs were removed: for K4[Fe(CN)6], $x+6(-1)=-4\Rightarrow x=+2$.

IUPAC naming rules: (i) name the cation first, then the anion. (ii) Within the sphere, name ligands alphabetically before the metal. (iii) Anionic ligands end in -o (chlorido, cyanido, sulfato, hydroxido); neutral ligands keep their name except aqua (H2O), ammine (NH3), carbonyl (CO), nitrosyl (NO). (iv) Use di, tri, tetra for simple ligands and bis, tris, tetrakis for those whose own name has a multiplier (e.g. bis(ethane-1,2-diamine)). (v) The metal's oxidation state is a Roman numeral in parentheses. (vi) If the complex is an anion, the metal name ends in -ate (ferrate, cuprate, argentate, plumbate).

Common ligands and IUPAC naming components
LigandFormulaDenticityName in complex
ChlorideCl−Monochlorido
CyanideCN−Mono (ambidentate)cyanido / isocyanido
WaterH2OMonoaqua
AmmoniaNH3Monoammine
NitriteNO2−Mono (ambidentate)nitrito-N / nitrito-O
Ethane-1,2-diamineenBi (chelating)ethane-1,2-diamine
OxalateC2O42−Bi (chelating)oxalato
EDTAEDTA4−Hexa (chelating)ethylenediaminetetraacetato
Solved Examples
1
Worked Example
Write the IUPAC name of [Co(NH3)6]Cl3.
Solution
  1. Cation is the complex ion, named first; anion (chloride) last.
  2. Ligand: six NH3 = hexaammine.
  3. Oxidation state of Co: $x+6(0)+3(-1)=0\Rightarrow x=+3$.
  4. Metal is in a cation, so keep name cobalt with (III).

Answer: Hexaamminecobalt(III) chloride.

2
Worked Example
Name K4[Fe(CN)6] and state the oxidation number and coordination number of iron.
Solution
  1. Cation potassium first; complex anion last.
  2. Six cyanido ligands = hexacyanido.
  3. Oxidation number: $x+6(-1)=-4\Rightarrow x=+2$.
  4. Anionic complex → metal becomes ferrate(II). CN = 6.

Answer: Potassium hexacyanidoferrate(II); oxidation number $+2$, coordination number $6$.

3
Worked Example
Write the formula of the complex pentaamminechloridocobalt(III) chloride.
Solution
  1. Co(III) with five NH3 (ammine) and one Cl− (chlorido) inside the sphere.
  2. Charge of complex ion: $+3+5(0)+(-1)=+2$.
  3. Balance with two outer Cl−.

Answer: [Co(NH3)5Cl]Cl2.

4
Worked Example
How many ions does 1 mol of [Co(NH3)5Cl]Cl2 give in water, and how many AgCl moles precipitate with excess AgNO3?
Solution
  1. Complex ion [Co(NH3)5Cl]2+ stays intact; two Cl− are ionisable.
  2. Total ions = 1 complex cation + 2 chloride = 3 ions.
  3. Only ionisable (outer-sphere) Cl− reacts with Ag+; coordinated Cl is not precipitated.

Answer: 3 ions per formula unit; 2 mol of AgCl precipitate.

5
Worked Example
Name the complex [Pt(NH3)2Cl2].
Solution
  1. Neutral complex; name as one word.
  2. Ligands alphabetical: ammine (a) before chlorido (c) → diamminedichlorido.
  3. Oxidation state of Pt: $x+2(0)+2(-1)=0\Rightarrow x=+2$.

Answer: Diamminedichloridoplatinum(II).

6
Worked Example
Identify the secondary valence (coordination number) and the primary valence of cobalt in [Co(en)2Cl2]Cl.
Solution
  1. en is bidentate: 2 en supply 4 donor atoms; 2 Cl inside supply 2 more → CN = 6 (secondary valence).
  2. Oxidation state: $x+2(0)+2(-1)=+1\Rightarrow x=+3$ (primary valence).

Answer: Secondary valence (coordination number) $= 6$; primary valence (oxidation state) $= +3$.

Key Points

  • A coordination compound has a central metal bonded to ligands by coordinate bonds; it keeps its identity in solution, unlike a double salt.
  • Werner: primary valence = ionisable = oxidation state; secondary valence = non-ionisable = coordination number; secondary valences are directional and fix geometry.
  • Ligands are mono-, bi- or polydentate; chelating ligands form rings (extra stability), and ambidentate ligands (NO2−, SCN−) bind through either of two atoms.
  • Coordination number counts donor atoms (not ligands); oxidation number is found by balancing charges, e.g. K4[Fe(CN)6] gives Fe as $+2$.
  • IUPAC: cation before anion; ligands alphabetical with -o for anions; metal oxidation state in Roman numerals; anionic complexes take the -ate suffix (ferrate, cuprate).
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.In [Co(NH3)5Cl]Cl2, the number of chloride ions precipitated immediately by AgNO3 is:
Explanation: Only the two outer-sphere (ionisable) chlorides react; the coordinated Cl stays bound.
Q2.The coordination number of platinum in [Pt(en)2]2+ is:
Explanation: en is bidentate; two en give 4 donor nitrogen atoms, so CN = 4.
Q3.Which ligand is ambidentate?
Explanation: SCN− can bind through S (thiocyanato-S) or N (thiocyanato-N).
Q4.The oxidation state of nickel in [Ni(CO)4] is:
Explanation: CO is neutral, so $x+4(0)=0\Rightarrow x=0$.
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