Chemistry: Atoms First 2e · Transition Metals and Coordination Chemistry

Coordination Chemistry of Transition Metals

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

In 30 seconds

Coordination chemistry is the study of coordination compounds — species in which a central metal ion is surrounded by and bonded to a group of molecules or ions called ligands. The metal– bond is a : the ligand donates a lone pair of electrons to an empty orbital on the metal. A familiar example is the deep-blue complex that forms when ammonia is added to a solution of copper(II) sulfate: the pale blue [Cu(H₂O)₄]²⁺ ion is replaced by the intensely blue [Cu(NH₃)₄]²⁺ ion.

Before the 1890s, compounds like these baffled chemists because they seemed to violate normal valence rules. Alfred Werner proposed in 1893 that a metal ion has two kinds of valence: a primary valence (the oxidation state, satisfied by negative ions) and a secondary valence (the , satisfied by ligands arranged in a fixed geometry around the metal). Werner's theory explained why CoCl₃ · 6NH₃ has the formula it does, why it conducts electricity the way it does, and why it exists in more than one form. This topic develops the language of coordination chemistry — ligands, coordination numbers, geometries, naming, and isomerism — which is essential for the next topic on the spectroscopic and magnetic properties of these compounds.

Why this matters

Life itself is coordination chemistry: hemoglobin carries oxygen with an iron center held by a porphyrin ligand; chlorophyll captures light with a magnesium center; vitamin B12 contains cobalt in a corrin ring.
Medicine: the anticancer drug cisplatin is a platinum ; MRI contrast agents are gadolinium complexes; chelating agents treat heavy-metal poisoning by binding and removing toxic ions.
Industry and technology: coordination complexes are catalysts (e.g., Wilkinson's catalyst for hydrogenation), dyes and pigments, and the active materials in some batteries and sensors.
Analytical chemistry: qualitative tests use characteristic complex colors (e.g., the Prussian blue test for iron, the deep-blue copper–ammonia complex) to identify ions.
Exams: writing formulas from names (and vice versa), determining oxidation states and coordination numbers, and recognizing isomers are guaranteed question types.

The college version

Core Concepts

The parts of a coordination compound

A coordination compound typically consists of a (metal ion + ligands) and one or more counter ions that balance its charge. In [Co(NH₃)₅Cl]Cl₂, the complex ion is [Co(NH₃)₅Cl]²⁺ and the counter ions are two chlorides. The coordination sphere (the metal plus the ligands directly bonded to it) is written in brackets; counter ions sit outside the brackets. The coordination number is the number of donor atoms bonded to the metal — not the number of ligands if a ligand binds through more than one atom.

Ligands: electron-pair donors

A ligand is any molecule or ion that donates a lone pair to the metal. Ligands that bind through one donor atom are monodentate ("one tooth"): water (H₂O), ammonia (NH₃), chloride (Cl^-), cyanide (CN^-). Ligands with two or more donor atoms are polydentate: ethylenediamine (en, H₂NCH₂CH₂NH₂) is bidentate, and EDTA^4- is hexadentate — it wraps around a metal ion like a claw. A polydentate ligand that grips a metal at several points forms a (from Greek chelē, "crab's claw"), and chelated complexes are unusually stable — the — which is why EDTA is used to remove heavy-metal ions from the body and to soften hard water.

Coordination numbers and geometries

The coordination number determines the geometry around the metal:

2: linear (e.g., [Ag(NH₃)₂]⁺)
4: tetrahedral (e.g., [ZnCl₄]^2-) or square planar (e.g., [PtCl₄]^2-)
6: octahedral (e.g., [Fe(CN)₆]^4-) — by far the most common for transition metals

Ligands are arranged to minimize repulsion, and the geometry affects the compound's properties — square-planar platinum complexes, for instance, are the ones with anticancer activity.

Naming coordination compounds

IUPAC rules are systematic and testable:

Name the cation first, then the anion (like ionic compounds).
Within a complex, name ligands in alphabetical order before the metal; ligand names: aqua (H₂O), ammine (NH₃), carbonyl (CO), chloro (Cl^-), cyano (CN^-), oxalato, hydroxo.
Use prefixes di-, tri-, tetra-, penta-, hexa- for simple ligands; use bis-, tris-, tetrakis- for complicated ligand names.
After the metal, give its oxidation state in Roman numerals in parentheses.
If the complex ion is an anion, the metal name ends in -ate (e.g., ferrate for iron, cuprate for copper).

Example: [Co(NH₃)₅Cl]Cl₂ is pentaamminechloridocobalt(III) chloride; K₃[Fe(CN)₆] is potassium hexacyanoferrate(III).

Isomerism in coordination compounds

Compounds with the same formula but different arrangements are isomers, and coordination compounds are rich in them:

Geometric (cis/trans) isomers: in square-planar MA2B2 or octahedral MA4B2 complexes, identical ligands can be adjacent (cis) or opposite (trans). Cisplatin's anticancer activity comes from its cis arrangement; transplatin is inactive.
Optical isomers: some complexes (e.g., [Co(en)₃]³⁺) are non-superimposable on their mirror images; the two forms rotate plane-polarized light in opposite directions.
Linkage isomers: a ligand with two possible donor atoms (e.g., NO₂^- can bind through N or O) gives different compounds.
Ionization and coordination isomers: swapping a ligand with a produces isomers with different chemical behavior (e.g., [Co(NH₃)₅Br]SO₄ vs. [Co(NH₃)₅SO₄]Br).

Common Confusions

Do Not Confuse	With	Difference
Coordination number	Number of ligands	CN counts donor atoms; one bidentate ligand counts as 2 toward the coordination number
Oxidation state	Charge of the complex ion	The metal's oxidation state is one term in the charge equation; ligands contribute their own charges (e.g., Cl^- is −1, NH₃ is 0)
Ammine	Amine	"Ammine" (two m's) is the ligand name for NH₃ in coordination compounds; "amine" is an organic functional group
Ligands inside brackets	Counter ions outside brackets	Only bracketed species are in the coordination sphere and directly bonded to the metal
Cis vs. trans	Optical isomers	Cis/trans differ by ligand positions; optical isomers are mirror images that can't be superimposed — a complex can have both types
EDTA "softening" water	EDTA removing calcium directly	EDTA binds Ca²⁺/Mg²⁺ into soluble chelates, preventing them from precipitating with soap — a sequestration, not removal, in most laundry uses

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine a metal atom as a busy king or queen sitting in the middle of a circle of guards. The guards are ligands — they hold hands with the metal by sharing a pair of electrons. Some guards grab with one hand, and some wrap around the metal with several hands like a crab's claw. Change which guards stand where, and you get a different compound — sometimes with completely different powers, like the medicine cisplatin, which only works when its guards stand in the right places.

Worked example

Example 1: Oxidation state, coordination number, and name of a complex

Consider [Co(NH₃)₅Cl]Cl₂. Determine the oxidation state of cobalt, the coordination number, the charge on the complex ion, and the IUPAC name.

Step 1 — Find the complex-ion charge from the counter ions. Two Cl^- counter ions outside the brackets make the complex ion [Co(NH₃)₅Cl]²⁺.

Step 2 — Set up the charge equation. NH₃ is neutral (charge 0), Cl^- inside the sphere is −1, so:

\[x + 5(0) + (-1) = +2 \qquad \Rightarrow \qquad x = +3\]

Cobalt is in the +3 oxidation state.

Step 3 — Coordination number. Five NH₃ ligands (one donor atom each) plus one Cl^- ligand = 6 donor atoms; the complex is octahedral.

Step 4 — Name. Ligands in alphabetical order (ammine before chloro): pentaamminechloridocobalt(III) chloride.

Example 2: Writing the formula from the name

Write the formula for potassium hexacyanoferrate(III).

Step 1 — Decode the name. Potassium is the cation (K⁺). Hexacyano = six CN^- ligands; ferrate(III) = iron with oxidation state +3 in an anionic complex.

Step 2 — Charge on the complex ion.

\[x + 6(-1) = -3 \qquad \Rightarrow \qquad [Fe(CN)_6]^{3-}\]

Step 3 — Balance with potassium. Three K⁺ ions balance the 3− complex:

\[K_3[Fe(CN)_6]\]

This compound — potassium ferricyanide — forms the deep-blue pigment Prussian blue when combined with iron(II) salts, a classic qualitative test.

Example 3: Chelate effect — coordination number with a bidentate ligand

The complex ion [Ni(en)₃]²⁺ contains three ethylenediamine (en) ligands. What is the coordination number of nickel, and what is the geometry?

Step 1 — Donor atoms per ligand. Each en (H₂NCH₂CH₂NH₂) is bidentate — it donates through two nitrogen atoms.

Step 2 — Total donor atoms.

\[\text{coordination number} = 3\ \text{ligands} \times 2\ \text{donor atoms per ligand} = 6\]

Step 3 — Geometry. Coordination number 6 → octahedral. The three chelate rings wrap around the nickel, and the complex is chiral — its two mirror-image forms are optical isomers, analogous to left and right hands.

Key takeaways

Coordination compound = complex ion + counter ions; coordination sphere in brackets.
Coordination number = number of donor atoms attached to the metal, not number of ligand molecules.
Common geometries: linear (CN 2), tetrahedral/square planar (CN 4), octahedral (CN 6).
Ligand types: monodentate (one donor atom), polydentate/chelating (multiple donor atoms); chelates are extra stable (chelate effect).
Naming order: ligands alphabetically → metal → Roman numeral oxidation state; anionic complexes end in -ate.
Isomers: cis/trans (geometric), optical (mirror images), linkage (different donor atom), ionization (ligand–counter-ion swap).
[Co(NH₃)₅Cl]Cl₂: Co is +3, coordination number 6, complex ion charge +2, two chloride counter ions.

Check yourself

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

Define coordination number, and explain why three bidentate en ligands give nickel a coordination number of 6.
Show answer
Coordination number = number of donor atoms bonded to the metal. Each en has 2 donor N atoms, so 3 en ligands contribute 6 donor atoms.
For [Pt(NH₃)₂Cl₂], what are the oxidation state of platinum, the coordination number, and the possible isomers?
Show answer
Pt oxidation state: x + 2(0) + 2(-1) = 0 → x = +2. Coordination number 4 (two NH3 + two Cl). Square-planar geometry allows cis and trans isomers.
Name the compound K₂[PtCl₆] following IUPAC rules.
Show answer
Potassium hexachloroplatinate(IV) — six chloro ligands, Pt at +4 (hexa-, -ate suffix for anionic complex).
Why are chelated complexes more stable than similar complexes with monodentate ligands?
Show answer
Chelation releases more particles (entropy gain) and holds the metal with multiple bonds, making dissociation much less favorable — the chelate effect.
What is the charge on the complex ion in [Fe(CN)₆]^4-, and what is the oxidation state of iron?
Show answer
Complex ion charge is 4− (four K⁺ would balance); x + 6(-1) = -4 → iron is +2 (ferrocyanide).
Why does cisplatin kill cancer cells while transplatin, its isomer, does not?
Show answer
In cisplatin the two chlorides are cis, placing the reactive sites adjacent so the platinum can bind both strands of DNA; in transplatin they are opposite, and the geometry prevents the same cross-linking.

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Study toolsKey vocabulary

Key vocabulary

Coordination compound: A compound containing a metal ion bonded to ligands
Ligand: A molecule or ion that donates a lone pair of electrons to a metal
Coordinate covalent bond: A bond in which both shared electrons come from one atom (the ligand)
Coordination number: The number of donor atoms bonded to the central metal
Monodentate / polydentate: Ligand with one / multiple donor atoms
Chelate: A complex in which a polydentate ligand grips the metal at several points
Complex ion: A metal ion with attached ligands, carrying a net charge
Counter ion: An ion outside the coordination sphere that balances the complex's charge
Chelate effect: The extra stability of chelated complexes compared with similar monodentate ones
Cis / trans isomers: Isomers differing in whether identical ligands are adjacent or opposite

Sources & references

openstax.org — Chemistry Atoms First 2e

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

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