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Coordination Compounds: Werner's Theory, Terms and IUPAC Names

A coordination compound has a central metal atom or ion joined to a fixed number of ions or molecules called ligands. Each ligand gives one pair of electrons to the metal. The metal and its ligands sit together inside square brackets, the coordination sphere. The number of donor atoms bonded to the metal is the coordination number. Werner said a metal shows two kinds of valence: primary (ionisable, outside the bracket) and secondary (fixed, inside the bracket).

🎬 Step-by-step story

  1. Meet the centre: a cobalt ion, Co³⁺. It has lost 3 electrons, so it has empty spaces (orbitals) that can take in electron pairs.
  2. Six ammonia (NH₃) molecules come close. Each one gives a spare pair of electrons (a lone pair) to the cobalt. This bond, where one side gives both electrons, is a coordinate bond. The givers are called ligands.
  3. Now count the atoms that touch the metal: 1, 2, 3, 4, 5, 6. This count is the coordination number. Here it is 6, and six ligands around a metal make an octahedron shape.
  4. The metal and its ligands stay together as one unit inside square brackets. This unit is the coordination sphere. The three Cl⁻ ions sit outside. They are counter ions and they break away in water. Inside = secondary valence (6). Outside = primary valence (3).
  5. Now we name it, piece by piece: first the ligands with a number prefix (hexa + ammine), then the metal (cobalt), then its oxidation state in Roman numerals (III), then the outside ion (chloride).
  6. Your turn. Pick a complex. Count the ligands, see which ions are outside, and read its full IUPAC name. Try the one with 'en', where one ligand grabs the metal with two hands.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

Why does the metal ion accept electrons at all?

A positive metal ion has empty orbitals of low enough energy. A lone pair can move into them, forming a coordinate bond. See the empty Co³⁺ in step 1.

Is a coordinate bond weaker than a normal covalent bond?

Not necessarily. Once formed, it is a shared pair like any covalent bond. Only its origin is different: both electrons came from the ligand.

Why is CN of [Co(en)₃]³⁺ six and not three?

Each en touches the metal with two N atoms. CN counts donor atoms, not ligands. Pick [Co(en)₃]³⁺ in free play and count the donor balls.

Do the counter ions touch the metal?

No. They are outside the coordination sphere and held only by ionic attraction to the whole complex ion. In water they move away.

Why is K₄[Fe(CN)₆] called ferrate and not iron?

The complex part is a negative ion, and negative complex ions take '-ate'. For iron the Latin root 'ferr' is used.

Is the oxidation state the same as the coordination number?

No. In [Co(NH₃)₆]³⁺ the oxidation state is +3 but CN is 6. Werner called them primary and secondary valence.

What is a coordination compound?

Some metal ions like to hold on to molecules or ions that have spare electron pairs. The group formed is called a complex. A compound that contains a complex is a coordination compound.

A double salt like Mohr's salt breaks fully into simple ions in water. A complex like [Fe(CN)₆]⁴⁻ does not; it stays as one unit. That is the easy test to tell them apart.

Werner's theory

Alfred Werner (1893) studied cobalt chloride and ammonia compounds. He added silver nitrate and counted how much AgCl came out. From this he said:

  1. A metal shows two kinds of valence. Primary valence is ionisable, is shown by negative ions outside the bracket and equals the oxidation state. Secondary valence is non-ionisable, is fixed for a metal and equals the coordination number.
  2. Ligands satisfying secondary valence point in fixed directions in space. So complexes have shapes: octahedral for 6, tetrahedral or square planar for 4.

Example: CoCl₃·6NH₃ gives 3 AgCl, so it is [Co(NH₃)₆]Cl₃. CoCl₃·5NH₃ gives only 2 AgCl, so one Cl is inside: [Co(NH₃)₅Cl]Cl₂. CoCl₃·4NH₃ gives 1 AgCl: [Co(NH₃)₄Cl₂]Cl.

Limits: Werner could not explain why only some metals form complexes, why the bonds point in fixed directions, or why complexes are coloured and magnetic. Later theories (VBT, CFT) answered these.

Ligands and their types

Ligands are sorted by how many donor atoms they use (their denticity, 'number of teeth'):

A di- or polydentate ligand that forms a ring with the metal is a chelating ligand. Chelate complexes are more stable than similar complexes with unidentate ligands.

An ambidentate ligand has two different atoms that can donate, but it uses only one at a time: NO₂⁻ (through N or O), SCN⁻ (through S or N).

Homoleptic complex: only one kind of ligand, like [Co(NH₃)₆]³⁺. Heteroleptic: more than one kind, like [Co(NH₃)₄Cl₂]⁺.

Coordination number, coordination sphere and oxidation state

Coordination number (CN)

The number of donor atoms directly bonded to the metal. Count atoms, not ligands: in [Co(en)₃]³⁺ there are 3 ligands but 6 donor N atoms, so CN = 6.

Coordination sphere and counter ions

The metal plus its ligands, written in square brackets, is the coordination sphere. Ions outside the bracket are counter ions. In K₄[Fe(CN)₆], [Fe(CN)₆]⁴⁻ is the sphere and K⁺ are counter ions.

Coordination polyhedron

The shape made by the donor atoms around the metal: octahedral, square planar or tetrahedral.

Oxidation number of the metal

The charge the metal would have if all ligands were taken away with their electron pairs. Sum of charges = charge of the complex. Neutral ligands (NH₃, H₂O, CO, en) count 0.

IUPAC nomenclature of coordination compounds

Writing the formula

  1. Metal first, then ligands in alphabetical order of their symbols, all inside [ ].
  2. Polyatomic ligands in ( ). Charge of a complex ion written outside as a superscript.

Writing the name

  1. Name the cation first, then the anion (like 'sodium chloride').
  2. Inside the complex, name ligands first in alphabetical order, then the metal.
  3. Anionic ligands end in -o: chlorido, cyanido, hydroxido, oxalato. Neutral ones keep their name, except aqua (H₂O), ammine (NH₃), carbonyl (CO), nitrosyl (NO).
  4. Prefixes di, tri, tetra… tell the number. If the ligand name already has a number (like ethane-1,2-diamine), use bis, tris, tetrakis and brackets.
  5. Oxidation state in Roman numerals in ( ) after the metal.
  6. If the complex is an anion, the metal ends in -ate: cobaltate, zincate. Some use Latin roots: ferrate (Fe), cuprate (Cu), argentate (Ag), plumbate (Pb), stannate (Sn), aurate (Au).
  7. Neutral complex: one word, no ion name, e.g. tetracarbonylnickel(0).

Prefixes like di, tri are ignored while arranging alphabetically: tetraammine comes before dichlorido because 'a' comes before 'c'.

Try it: build a complex at home

Take one big ball of clay (the metal) and six toothpicks with small balls on their ends (ligands). Push them in: up, down, left, right, front, back. You have an octahedron with CN = 6. Now join two neighbouring small balls with a bent straw: that is one 'en' ligand using two hands. Three straws = [M(en)₃]. In the 3D above, pick [Co(en)₃]³⁺ and count: 3 ligands, 6 donor atoms.

Key formulas and definitions

Worked examples

1. Find the oxidation state of Fe in K₃[Fe(CN)₆].

Step 1: 3 K⁺ outside give +3, so the complex ion is [Fe(CN)₆]³⁻. Step 2: Let Fe = x. Six CN⁻ give −6. Step 3: x + (−6) = −3, so x = +3. Answer: Fe is +3.

2. Find the coordination number and oxidation state of Co in [Co(en)₂Cl₂]⁺.

Step 1: Each en has 2 donor N atoms: 2 × 2 = 4. Two Cl give 2. CN = 4 + 2 = 6. Step 2: en is neutral, Cl⁻ is −1. x + 0 + 2(−1) = +1. Step 3: x = +3. Answer: CN = 6, Co is +3.

3. One mole of CoCl₃·5NH₃ gives 2 moles of AgCl with excess AgNO₃. Write its formula and name.

Step 1: 2 AgCl means 2 Cl⁻ are outside (free). Step 2: The third Cl is inside with 5 NH₃, giving CN 6. Step 3: Formula [Co(NH₃)₅Cl]Cl₂. Step 4: Ligands alphabetically: ammine (a) before chlorido (c). Co = +3. Answer: pentaamminechloridocobalt(III) chloride.

4. Name K₂[Zn(OH)₄].

Step 1: Cation K⁺ is named first: potassium. Step 2: Complex is an anion [Zn(OH)₄]²⁻, so zinc becomes zincate. Step 3: x + 4(−1) = −2, x = +2. Answer: potassium tetrahydroxidozincate(II).

5. Write the formula of tetraamminediaquacobalt(III) chloride.

Step 1: Ligands: 4 NH₃, 2 H₂O. Metal Co(III). Step 2: Charge of complex = +3 + 0 + 0 = +3. Step 3: Needs 3 Cl⁻ outside. Answer: [Co(NH₃)₄(H₂O)₂]Cl₃.

6. Name [Pt(NH₃)₂Cl(NO₂)] and give the number of ions it makes in water.

Step 1: Ligands: ammine (a), chlorido (c), nitrito-N (n) — already alphabetical. Step 2: Pt: x + 0 − 1 − 1 = 0, x = +2. Step 3: No brackets outside, so it is neutral: one word. Answer: diamminechloridonitrito-N-platinum(II). It gives 0 ions (non-electrolyte).

Common mistakes

Practice quiz

1. The coordination number of Fe in [Fe(C₂O₄)₃]³⁻ is:
2. Which ligand is ambidentate?
3. EDTA⁴⁻ is a ____ ligand.
4. In Werner's theory, the secondary valence equals:
5. IUPAC name of [Ni(CO)₄]:

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

What is the difference between a double salt and a complex?

A double salt (like potash alum) breaks fully into simple ions in water. A complex keeps its bracket unit together, so [Fe(CN)₆]⁴⁻ does not give Fe²⁺ or CN⁻ tests.

Why are ligands called Lewis bases?

Because they give (donate) an electron pair to the metal. The metal accepts it, so the metal acts as a Lewis acid.

How do I decide the order of ligands in the name?

Alphabetical order of the ligand names, ignoring number prefixes like di, tri, bis. So aqua comes before chlorido, and ammine before cyanido.

Where this is taught

RomaniaClasa a XII-aClassification of chemical reactions
RomaniaClasa a XII-aClassification of chemical reactions
RomaniaClasa a XII-aClassification of chemical reactions
Ukraine11 класGeneral review of chemistry
CBSE (India)Class 12Coordination Compounds
China高二Selective 2 Ch.3 Crystals

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