🎯 Coordination Compounds — Nomenclature Made Easy

Hey there! Ready to crack the code behind those square-bracketed formulas and long names? Let’s walk through the must-know ideas step by step and keep things light with some emojis along the way.

1. Core Vocabulary 📚

  • Coordination number: Count only the σ-bonds from ligands to the metal (π-bonds don’t add to the tally). For example, \( [Fe(CN)_6]^{4-} \) has CN = 6. :contentReference[oaicite:0]{index=0}
  • Coordination sphere: Everything inside the square brackets. Anything outside balances charge and is called a counter-ion (e.g., K4\( [Fe(CN)_6] \); K+ is the counter-ion). :contentReference[oaicite:1]{index=1}
  • Coordination polyhedron: 3-D shape formed by ligand atoms around the metal — common ones are octahedral \( [Co(NH_3)_6]^{3+} \), tetrahedral \( [Ni(CO)_4] \), and square-planar \( [PtCl_4]^{2-} \). :contentReference[oaicite:2]{index=2}
  • Oxidation number: Pretend all ligands leave with their electron pairs; the charge left on the metal is its oxidation state (Cu in \( [Cu(CN)_4]^{3-} \) is +1, written Cu(I)). :contentReference[oaicite:3]{index=3}
  • Homoleptic vs heteroleptic: Same-ligand only? Homoleptic (e.g., \( [Co(NH_3)_6]^{3+} \)). Mixed ligands? Heteroleptic (e.g., \( [Co(NH_3)_4Cl_2]^+ \)). :contentReference[oaicite:4]{index=4}

2. Writing Formulas ✍️

  1. Write the metal first. :contentReference[oaicite:5]{index=5}
  2. List ligands alphabetically — ignore their charges for ordering. :contentReference[oaicite:6]{index=6}
  3. Polydentate ligands follow the same rule; abbreviations count by the first letter. :contentReference[oaicite:7]{index=7}
  4. Wrap the whole coordination entity in square brackets [ ]; put polyatomic ligands in parentheses. :contentReference[oaicite:8]{index=8}
  5. No spaces inside the brackets (metal bumps elbows with ligands!). :contentReference[oaicite:9]{index=9}
  6. Add the overall charge as a right-hand superscript, e.g., \( [Cr(H_2O)_6]^{3+} \). :contentReference[oaicite:10]{index=10}
  7. Outside the brackets, counter-ions keep the compound neutral. :contentReference[oaicite:11]{index=11}

Quick example: “Tetraammineaquachloridocobalt(III) chloride” → \( [Co(NH_3)_4(H_2O)Cl]Cl_2 \) 🎉 :contentReference[oaicite:12]{index=12}

3. Naming Rules 🔤

  1. Name the cation first (whether the complex carries the charge or not). :contentReference[oaicite:13]{index=13}
  2. List ligands alphabetically before the metal name. (Opposite order to formulas!) :contentReference[oaicite:14]{index=14}
  3. Ligand endings: anionic → “-o” (e.g., chloro), neutrals keep names except H2O = aqua, NH3 = ammine, CO = carbonyl, NO = nitrosyl. :contentReference[oaicite:15]{index=15}
  4. Use prefixes: mono-, di-, tri-… but switch to bis-, tris-, tetrakis- when the ligand name already has a number (e.g., bis(ethane-1,2-diamine)). :contentReference[oaicite:16]{index=16}
  5. Show the metal’s oxidation state in Roman numerals after its name: nickel(II). :contentReference[oaicite:17]{index=17}
  6. If the complex is an anion, change the metal’s ending to “-ate” (ferrate, cobaltate). :contentReference[oaicite:18]{index=18}

Worked example 1: \( [Cr(NH_3)_3(H_2O)_3]Cl_3 \) → triamminetriaquachromium(III) chloride ✨ :contentReference[oaicite:19]{index=19}

Worked example 2: \( [Ag(NH_3)_2][Ag(CN)_2] \) → diamminesilver(I) dicyanidoargentate(I) ➡️ note how the name shifts when the same metal appears in cation and anion. :contentReference[oaicite:20]{index=20}

4. Practice Corner 📝

Try naming these (answers in white text—highlight to peek):

  • Tetrahydroxidozincate(II) → K2\( [Zn(OH)_4] \) :contentReference[oaicite:21]{index=21}
  • Pentaamminecarbonatocobalt(III) chloride → \( [Co(NH_3)_5(CO_3)]Cl \) :contentReference[oaicite:22]{index=22}

5. High-Yield NEET Nuggets 💡

  1. Alphabetical order of ligands matters — charge doesn’t. :contentReference[oaicite:23]{index=23}
  2. Oxidation state comes from total complex charge + ligand charges — learn to calculate it fast. :contentReference[oaicite:24]{index=24}
  3. Use bis, tris, tetrakis with polydentate or complicated ligand names — a common trick question. :contentReference[oaicite:25]{index=25}
  4. Anionic complexes change the metal’s name to “-ate” (e.g., ferrate, cobaltate). :contentReference[oaicite:26]{index=26}
  5. Coordination number = number of σ-bonds (π-bonds don’t count) — key for structure-based MCQs. :contentReference[oaicite:27]{index=27}

Keep practicing these rules, and naming coordination compounds will feel like solving a fun puzzle. Happy studying! 🎉