Chemistry 2e · Transition Metals and Coordination Chemistry

Coordination Chemistry of Transition Metals

8 min read

Numerical values (molar masses) are standard reference values; verify against current sources before relying on them in assessments.

On this page 9 sections

In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

A coordination compound is built from a metal ion (a Lewis acid, an electron-pair acceptor) surrounded by ligands — molecules or ions (Lewis bases) that donate lone pairs through coordinate covalent bonds. The assembly is a complex ion, written in brackets, e.g. [Co(NH₃)₆]³⁺. What makes this chemistry distinctive: one metal ion can bind several ligands with a fixed and geometry, and the same atoms can be arranged differently to give isomers with very different properties. This topic covers how complexes are held together, common geometries, IUPAC nomenclature, and isomerism. Werner's coordination theory (1890s) first explained these structures, and biology and medicine run on the same principles: hemoglobin (iron), chlorophyll (magnesium), vitamin B₁₂ (cobalt), the anticancer drug cisplatin (platinum), and EDTA chelation therapy for heavy-metal poisoning.

Why this matters

Biology: Hemoglobin's heme is an iron–porphyrin complex; chlorophyll is a magnesium complex; vitamin B₁₂ is a cobalt–corrin complex. cages tune metal reactivity in living systems.
Medicine: Cisplatin (cis-[Pt(NH₃)₂Cl₂]) treats cancer; EDTA chelates lead and other heavy metals so they can be excreted.
Industry: Rhodium and titanium complexes catalyze industrial reactions; complex ions underlie electroplating, metal recovery, and dyes.
Exams: Nomenclature, oxidation-state determination, coordination numbers, and identification are reliable test questions.

The college version

Core Concepts

What holds a complex together

The metal cation is a Lewis acid that accepts electron pairs; each ligand is a Lewis base that donates one or more pairs, forming a (both electrons from the ligand). Ligands may be neutral (H₂O, NH₃, CO) or anionic (Cl⁻, CN⁻, OH⁻, C₂O₄²⁻). Denticity is the number of donor atoms a ligand uses: monodentate (one: H₂O, NH₃, Cl⁻), bidentate (two: ethylenediamine, en; oxalate), or polydentate — EDTA grips a metal with six donor atoms. A polydentate ligand wrapping a metal forms a ring called a (Greek chele, "claw"). Chelates are unusually stable: replacing six water molecules with one EDTA greatly increases the particle count (entropy), which drives binding — the chelate effect.

Coordination number and geometry

The coordination number (CN) is the number of donor atoms attached to the metal — not necessarily the number of ligands, since a bidentate ligand contributes two. Common geometries:

CN = 2, linear: [Ag(NH₃)₂]⁺ (Tollens' reagent).
CN = 4, tetrahedral: [Zn(NH₃)₄]²⁺, [CoCl₄]^2-.
CN = 4, square planar: [Pt(NH₃)₂Cl₂] (cisplatin), [Ni(CN)₄]^2-.
CN = 6, octahedral: most common for 3d metals — [Fe(CN)₆]^4-, [Co(NH₃)₆]³⁺, [Cu(H₂O)₆]²⁺.

Werner's theory: the historical key

Alfred Werner (Nobel Prize 1913) explained puzzling compounds such as CoCl₃·6NH₃. He proposed a primary valence (oxidation state, satisfied by ionizable ions) and a secondary valence (coordination number, satisfied by ligands bonded directly to the metal). In [Co(NH₃)₆]Cl₃ all six NH₃ bind Co³⁺ and all three Cl⁻ are free — silver nitrate precipitates all three chlorides instantly. In [Co(NH₃)₄Cl₂]Cl only one Cl⁻ is free, so only one chloride precipitates. Modern terms: the bracketed part is the ; species outside are counterions.

Naming coordination compounds (IUPAC)

Name the cation first, then the anion (ionic compound order).
Within the complex, name ligands alphabetically (ignoring di-/tri- prefixes), then the metal.
Anionic ligands take -o endings: chloride → chloro, cyanide → cyano, hydroxide → hydroxo, oxalate → oxalato. Neutral ligands keep their names except water → aqua, ammonia → ammine, carbon monoxide → carbonyl.
Add the metal's oxidation state as a Roman numeral in parentheses.
If the complex is an anion, the metal name ends in -ate (iron → ferrate, copper → cuprate, cobalt → cobaltate).

Examples: [Co(NH₃)₅Cl]Cl₂ = pentaamminechloridocobalt(III) chloride; K₃[Fe(CN)₆] = potassium hexacyanoferrate(III); [Cu(NH₃)₄]²⁺ = tetraamminecopper(II).

Isomerism: same formula, different compounds

Structural isomers (different connectivity):

Ionization: a ligand and counterion trade places — [Co(NH₃)₅Br]SO₄ vs [Co(NH₃)₅SO₄]Br (bromide vs sulfate is free; precipitation tests distinguish them).
Coordination: two metals swap ligands — [Co(NH₃)₆][Cr(CN)₆] vs [Cr(NH₃)₆][Co(CN)₆].
Linkage: an binds through different atoms — nitro (N-bound) vs nitrito (O-bound).
Hydrate: water inside vs outside the sphere — [Cr(H₂O)₆]Cl₃ vs [Cr(H₂O)₅Cl]Cl₂ · H₂O.

Stereoisomers (same connectivity, different shape):

Geometric (cis/trans): square planar [Pt(NH₃)₂Cl₂] has cis (Cl adjacent) and trans (opposite) forms; octahedral [Co(NH₃)₄Cl₂]⁺ likewise; [Co(NH₃)₃Cl₃] shows facial (fac) and meridional (mer) isomers.
Optical: nonsuperimposable mirror images (left vs right hand), e.g. [Co(en)₃]³⁺.

Isomerism is not academic: cisplatin kills cancer cells while its trans isomer is essentially inactive — same formula, different geometry, opposite medical outcome.

How It Works / Step-by-Step Process

Finding a metal's oxidation state in a complex:

Write the complex's overall charge (from the formula — K₃ means the complex is 3⁻).
Sum the ligand charges (NH₃, H₂O, en are neutral; Cl⁻, CN⁻, OH⁻ are –1; oxalate is –2).
Solve: metal oxidation state + Σ(ligand charges) = complex charge.

Naming a complex (formula → name):

Identify the metal and its oxidation state (charge balance).
Count each ligand; add prefixes di-, tri-, tetra-, penta-, hexa-.
List ligands alphabetically; give anionic ligands their –o names.
Write metal + (Roman numeral); if the complex is an anion, end the metal name in –ate.

Common Confusions

Do Not Confuse	With	Difference
Coordination number	Oxidation state	CN = donor atoms attached; oxidation state = charge bookkeeping. [Co(NH₃)₆]³⁺ has CN 6 but Co is +3
"Ligands are negative ions"	Ligands can be neutral or anionic	NH₃, H₂O, CO are neutral and contribute 0 to charge balance
Number of ligands	Coordination number	One bidentate en contributes 2 to CN: [Co(en)₃]³⁺ has 3 ligands but CN = 6
Cisplatin (cis)	Transplatin (trans)	Same formula, different geometry; only the cis isomer is an effective anticancer drug
"Metal always named –ate"	–ate only for anionic complexes	[Cu(NH₃)₄]²⁺ is tetraamminecopper(II), but [CuCl₄]^2- is tetrachlorocuprate(II)

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine a central magnet (the metal ion) surrounded by smaller magnets (ligands) that stick to it by sharing their "electron arms." How many arms grab on — two, four, or six — is the coordination number, and it shapes the cluster: four arms might make a flat square or a pyramid, six make a box-like octahedron. If the same set of magnets grabs on in different spots or directions, you get different "cluster-buildings" from the same parts — like the same LEGO bricks making a house or a castle. One arrangement (cisplatin) is a medicine; its mirror twin (transplatin) is not — which is why chemists care exactly how the pieces are arranged.

Worked example

Example 1: Oxidation state of cobalt in a complex ion

What is the oxidation state of Co in [Co(NH₃)₅Cl]²⁺?

NH₃ is neutral (0); Cl⁻ contributes –1; the ion carries +2:

$x + 5(0) + (-1) = +2 \Rightarrow x = +3$

Answer: Co is +3, so the name is pentaamminechloridocobalt(III). Neutral ligands contributing zero is the fastest shortcut in these problems.

Example 2: Charge and oxidation state from a salt formula

Determine (a) the charge on the complex and (b) the oxidation state of Fe in potassium ferricyanide, K₃[Fe(CN)₆].

Part (a): each K⁺ is +1 and the salt is neutral, so the complex balances it:

$3(+1) + z = 0 \Rightarrow z = -3$

Part (b): each CN⁻ is –1; six give –6:

$x + 6(-1) = -3 \Rightarrow x = +3$

Answer: [Fe(CN)₆]^3- with Fe(III), hence "hexacyanoferrate(III)." The old ferri-/ferro- distinction is exactly this: ferricyanide = Fe(III), ferrocyanide [Fe(CN)₆]^4- = Fe(II).

Example 3: EDTA chelation stoichiometry

EDTA binds metal ions 1:1. How many grams of disodium EDTA dihydrate (Na₂H₂EDTA·2H₂O, molar mass 372.24 g/mol) chelate all the Ca²⁺ in 250.0 mL of 0.0500 M CaCl₂?

Step 1 — moles of Ca²⁺ (molarity definition M = n/V, so n = M × V):

$n Ca 2+ = 0.0500 mol L \times 0.2500 L = 0.01250 mol$

Step 2 — moles of EDTA (1:1): n_EDTA = 0.01250 mol.

Step 3 — mass of EDTA:

$m EDTA = 0.01250 mol \times 372.24 g mol = 4.65 g$

Answer: 4.65 g of Na₂H₂EDTA·2H₂O — the same calculation a chelation-therapy protocol rests on: one EDTA molecule captures one metal ion.

Key takeaways

Complex = metal ion (Lewis acid) + ligands (Lewis bases) joined by coordinate covalent bonds.
Coordination number = number of donor atoms; 6 (octahedral) most common; 4 = tetrahedral or square planar; 2 = linear.
Bidentate/polydentate ligands (en, oxalate, EDTA) form chelates; chelate effect = entropy-driven extra stability.
Werner: primary valence = oxidation state; secondary valence = coordination number.
Nomenclature: ligands alphabetically + metal + (oxidation state); anionic complex → metal + -ate; anionic ligands end in -o; H₂O = aqua, NH₃ = ammine.
Isomers: ionization, coordination, linkage, hydrate; cis/trans; optical. Cisplatin vs transplatin — same formula, different activity.
EDTA binds metal ions 1:1; basis of chelation therapy for heavy-metal poisoning.
Biological complexes: heme (Fe), chlorophyll (Mg), vitamin B₁₂ (Co).

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

Define ligand, coordination number, and chelate, each in one sentence.
Show answer
Ligand: an ion or molecule that donates an electron pair to a metal. Coordination number: the number of donor atoms bonded to the metal. Chelate: a ring formed when a polydentate ligand binds a metal with two or more donor atoms.
What is the oxidation state of platinum in cisplatin, [Pt(NH₃)₂Cl₂]?
Show answer
x + 2(0) + 2(-1) = 0 ⇒ x = +2.
Name K₄[Fe(CN)₆].
Show answer
Potassium hexacyanoferrate(II) — four K⁺ make the complex 4⁻; six CN⁻ (–6) leave Fe at +2.
Why can EDTA displace six water molecules from a metal ion in solution?
Show answer
The chelate effect: one EDTA molecule replacing six waters greatly increases the number of free particles (entropy gain), stabilizing the complex far more than six separate monodentate bindings.
[Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅SO₄]Br have the same formula. What kind of isomers are they, and how could you tell them apart?
Show answer
Ionization isomers. Adding AgNO₃ precipitates AgBr from the first (free Br⁻) but Ag₂SO₄ from the second (free SO₄²⁻).
How many grams of Na₂H₂EDTA·2H₂O (372.24 g/mol) chelate the Ca²⁺ in 100.0 mL of 0.100 M CaCl₂?
Show answer
n = 0.100 × 0.1000 = 0.0100 mol; mass = 0.0100 × 372.24 = 3.72 g.

Keep learning

Ready to build on this? Continue to the next lesson.

Study toolsKey vocabulary

Key vocabulary

Ligand: An ion or molecule that donates a lone pair of electrons to a metal ion
Coordinate covalent bond: A bond where both shared electrons come from one partner (the ligand)
Coordination number: Number of donor atoms attached to the metal
Chelate: A ring formed when a polydentate ligand grips a metal with multiple donor atoms
Coordination sphere: The metal plus its directly bonded ligands, written in brackets
Ambidentate ligand: A ligand that can bind through either of two donor atoms (NO₂⁻)
Isomer: Same formula, different arrangement of atoms

Sources & references

openstax.org — Chemistry 2e

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.