ISC 12 Chemistry: Naming & Bonding of Coordination Compounds

Why coordination compounds matter in everyday chemistry

Ever wondered why your blood carries oxygen or why some dyes change colour? Both rely on coordination compounds – the unsung heroes of many everyday processes.

💡 In Simple Words: A coordination compound is a metal atom or ion that holds onto a group of molecules or ions called ligands, kind of like a central magnet surrounded by tiny metal‑shaped stickers.

What is a coordination compound?

A coordination compound (also called a complex) consists of a central metal ion and one or more ligands. The metal provides a positively charged centre, while ligands are neutral molecules or anions that donate a pair of electrons to the metal.

Basic parts: central metal ion and ligands

• Central metal ion: any transition metal (Fe, Cu, Co, etc.) that can accept electron pairs.
• Ligand: a donor atom or group that supplies a lone‑pair of electrons. Common examples are water (H₂O), ammonia (NH₃), cyanide (CN⁻), and ethylenediamine (en).

Bonding in coordination compounds

The bond formed between a ligand and the metal is called a coordinate covalent bond (also known as a dative bond). Think of it like a water pipe: the ligand is a faucet that pushes water (electron pair) into the pipe (metal) which then carries it away.

How ligands donate electrons

Each ligand uses a lone pair from an atom such as N, O, S, or P to form a bond. The metal doesn’t give any electrons back – it just accepts. This is why the bond is called "coordinate" – the electron pair comes from one side only.

Naming coordination compounds – step‑by‑step

Naming can feel like solving a puzzle, but follow the same order every time and you’ll never get lost.

Here’s the checklist:

1. List ligands first, in alphabetical order, ignoring any prefixes (di‑, tri‑, etc.). Use the ligand name list (see table below).
2. Attach prefixes (di‑, tri‑, tetra‑) to indicate how many of each ligand are present.
3. Enclose the ligand part in square brackets [] if the complex is an anion, or just write it if it’s neutral.
4. State the oxidation number of the metal in Roman numerals inside parentheses.
5. Write the metal name. If the overall complex is an anion, change the metal’s ending to “‑ate” (e.g., Fe³⁺ becomes ferrate).
6. Attach any counter‑ions outside the brackets.

Ligand naming table

Worked example 1

Write the name of [Co(NH₃)₆]Cl₃.

1. Ligand list: only ammine, six of them → hexaammine.
2. Metal centre: cobalt.
3. Oxidation state: each NH₃ is neutral, overall charge of complex ion is +3 (because three Cl⁻ are outside). So cobalt is +3 → (III).
4. Complex is a cation, so metal name stays “cobalt”.
5. Combine: hexaamminecobalt(III) chloride.

Worked example 2

Name K₄[Fe(CN)₆].

1. Ligand: cyanide, six of them → hexacyano.
2. Metal: iron, overall complex charge is –4, so iron oxidation state = +2 (since 6×(–1) = –6, +2 gives –4).
3. Because the complex is an anion, change “iron” to “ferrate”.
4. Write metal with oxidation state: ferrate(II).
5. Combine inside brackets and attach counter‑ion K⁺: potassium hexacyanoferrate(II).

Common pitfalls and quick tips

• Never count the prefixes (di‑, tri‑) when alphabetising ligands.
• Remember that “ammine” has two “m’s” – it’s not “amine”.
• If the complex ion itself carries a charge, write that charge as a superscript after the closing bracket (e.g., [Cu(NH₃)₄]²⁺).
• For bidentate ligands (those that bind through two atoms, like ethylenediamine), use the prefix “bis‑” (e.g., bis(ethylenediamine)copper(II)).
• When the metal is in a negative oxidation state (rare), still use Roman numerals, e.g., nickel(–I).

📝 Likely Exam Questions

1. Write the IUPAC name of the complex [Cr(H₂O)₄Cl₂]Cl·2H₂O.
   Answer: tetraaquadichloro­chromium(III) chloride dihydrate.
2. Give the oxidation state of the metal in K₃[Co(CN)₆].
   Answer: +3 (each CN⁻ is –1, total –6, overall charge –3, so Co must be +3).
3. Explain why a coordinate covalent bond is different from a normal covalent bond.
   Answer: In a coordinate bond both electrons come from the same atom (the ligand), whereas in a normal covalent bond each atom contributes one electron.
4. Predict the colour change when [Ni(H₂O)₆]²⁺ is replaced by [NiCl₄]²⁻.
   Answer: The aqua complex is typically green, while the tetrachloro complex is usually yellow‑brown due to different crystal‑field splitting.
5. Write the formula for the complex named “pentaamminecobalt(III) nitrate”.
   Answer: [Co(NH₃)₅(NO₃)]NO₃ (the nitrate inside the brackets acts as a ligand).