Coordination Compounds · Lesson 3 of 12
Naming and Writing Coordination Compounds
“Convert between names and formulas using ligand identity, alphabetical order and charge balance.”
• Assemble systematic names for cationic, neutral and anionic entities. • Write charge-balanced formulas from names. • Distinguish ligand names from counter-ion names. • Specify alternative donor atoms in ambidentate ligands.
A chemical name is a structured description
A useful coordination name identifies the ligands, their numbers, the metal and its oxidation state. A salt name additionally identifies its counter ion. Start by drawing an imaginary boundary around the bracketed entity. Name a positive ion before a negative ion in the whole salt. Within a coordination entity, name ligands before the metal, in alphabetical order of the ligand names, irrespective of ligand charge.
| Formula | Ligand name | Charge |
|---|---|---|
| NH₃ | ammine | 0 |
| H₂O | aqua | 0 |
| CO | carbonyl | 0 |
| Cl⁻ / Br⁻ / F⁻ | chlorido / bromido / fluorido | −1 |
| OH⁻ / CN⁻ | hydroxido / cyanido | −1 |
| C₂O₄²⁻ / CO₃²⁻ | oxalato / carbonato | −2 |
| en | ethane-1,2-diamine | 0 |
| NO₂⁻ via N / O | nitrito-κN / nitrito-κO | −1 |
| SCN⁻ via S / N | thiocyanato-κS / thiocyanato-κN | −1 |
Ammine has two m letters and denotes coordinated NH₃; methanamine is CH₃NH₂, a different ligand. Inside brackets Cl⁻ is chlorido, whereas an external Cl⁻ is chloride. A change of ending distinguishes the ligand’s role from a simple counter ion. Nitrito-N and nitrito-O are familiar donor-indicating forms; κN and κO make the attachment explicit. A bare NO₂ formula can be ambiguous unless the donor or structure is specified.
Numbers do not change alphabetical order
Use di-, tri-, tetra-, penta- and hexa- for simple ligand names. Ignore these multiplicative prefixes when sorting names. Use bis-, tris- and tetrakis- with parentheses for complex ligand names or to avoid ambiguity, as in tris(ethane-1,2-diamine). Oxalato may be written trioxalato in familiar names, or tris(oxalato) with an explicit multiplying enclosure. Both represent three oxalate ligands. Do not alphabetise tris ahead of aqua just because t follows a.
For a positive or neutral entity use the ordinary metal name, followed by the oxidation state in Roman numerals. For a negative entity use the -ate form: cobaltate, nickelate, platinate and zincate. Some stems change: iron gives ferrate, copper cuprate, silver argentate and gold aurate. The entity’s negative charge does not imply that the metal itself has a negative oxidation state.
Problem
Name [Cr(NH₃)₃(H₂O)₃]Cl₃ and [Co(en)₃]₂(SO₄)₃.
- 1.Three chloride ions imply that the chromium entity is +3. All six ligands are neutral, so chromium is +3. Ammine precedes aqua: triamminetriaquachromium(III) chloride.
- 2.Three sulfate ions contribute −6; two identical cobalt entities must each be +3. en is neutral, giving Co +3. The name is tris(ethane-1,2-diamine)cobalt(III) sulfate.
- 3.The ionic stoichiometry belongs in the formula. The salt name does not need the misleading prefixes dicobalt or trisulfate to reproduce the two-to-three charge balance.
Problem
Name K₃[Cr(C₂O₄)₃], [Ni(CO)₄] and [Ag(NH₃)₂][Ag(CN)₂].
- 1.In the chromium salt, x − 6 = −3 gives +3. Name the cation first: potassium tris(oxalato)chromate(III).
- 2.CO is neutral and Ni(CO)₄ is neutral, so Ni is in oxidation state 0. Write tetracarbonylnickel(0).
- 3.The complex cation is [Ag(NH₃)₂]⁺ and the anion [Ag(CN)₂]⁻. Both silvers are +1, but only the negative entity uses argentate: diamminesilver(I) dicyanidoargentate(I).
Construct the formula before adding counter ions
Write the metal symbol first inside square brackets, followed by ligand formulas or abbreviations. Parentheses group polyatomic ligands and allow subscripts to indicate multiplicity. For a consistent systematic line formula, order ligands alphabetically by their displayed formulas or abbreviations; names are independently alphabetised by ligand names. Familiar formulas sometimes retain a conventional ligand order. Neither alphabetical rearrangement changes connectivity, but donor atoms and stereochemistry must remain unambiguous.
Problem
Write formulas for tetraammineaquachloridocobalt(III) chloride; potassium tetrahydroxidozincate(II); potassium tris(oxalato)aluminate(III); and dichloridobis(ethane-1,2-diamine)cobalt(III).
- 1.The cobalt entity contains four NH₃, one H₂O and one Cl⁻. +3 − 1 = +2, requiring two external chlorides: [CoCl(H₂O)(NH₃)₄]Cl₂, equivalently the familiar [Co(NH₃)₄(H₂O)Cl]Cl₂.
- 2.Four hydroxido ligands give Zn: +2 − 4 = −2. Add two K⁺ to obtain K₂[Zn(OH)₄]. Three oxalates give Al: +3 − 6 = −3, so K₃[Al(C₂O₄)₃].
- 3.For the last name, Co +3 minus two chlorides gives entity charge +1. Write [CoCl₂(en)₂]⁺. No counter ion was named, so do not invent one.
Check several ligand environments
| Formula | Charge reasoning | Name |
|---|---|---|
| [PtCl(NH₃)₂(NO₂)] with N-bound nitrite | x − 1 − 1 = 0; Pt +2 | diamminechlorido(nitrito-κN)platinum(II) |
| [CoCl₂(en)₂]Cl | x − 2 = +1; Co +3 | dichloridobis(ethane-1,2-diamine)cobalt(III) chloride |
| [Co(CO₃)(NH₃)₅]Cl | x − 2 = +1; Co +3 | pentaamminecarbonatocobalt(III) chloride |
| [PtCl(NH₃)₂(NH₂CH₃)]Cl | x − 1 = +1; Pt +2 | diamminechlorido(methanamine)platinum(II) chloride |
| Hg[Co(SCN)₄], with Hg(II), S-bound SCN | Co +2 − 4 = −2 | mercury(II) tetrakis(thiocyanato-κS)cobaltate(II) |
For the carbonate example, formula and name establish charge and composition; denticity is a separate question. In the usual six-coordinate pentaammine entity carbonate contributes one donor. For the mercury example the formula alone does not supply unique metal oxidation states. The stated Hg(II)/Co(II) assignment is a charge-consistent interpretation: +2 balances −2. Assigning Hg(I) and Co(III) without additional structural evidence is not justified; common mercury(I) chemistry also involves the dimer Hg₂²⁺.
A formula-writing checklist
Verify four things independently: ligand multiplicities, entity charge, neutral-salt charge balance and oxidation-state label. For iron(III) hexacyanidoferrate(II), the inner Fe(II) gives [Fe(CN)₆]⁴⁻ and the outer Fe(III) is +3. The smallest charge-balanced formula is Fe₄[Fe(CN)₆]₃. The two iron sites must not be merged into one oxidation-state calculation. In hydrates, water written after a dot is not automatically a ligand; this distinction will matter in isomerism.
Quick Quiz
Quiz
What is the ligand name for coordinated NH₃?
The special name ammine denotes NH₃; amine refers to a broader organic class.
What metal name is used for an anionic iron entity?
The traditional stem ferr- is used with -ate for negatively charged iron entities.
Which formula represents hexaamminecobalt(III) sulfate?
Two +3 entities balance three −2 sulfates.
What determines ligand order in a systematic name?
Ammine precedes chlorido even when its multiplier is penta.
Which notation specifically identifies O-bound nitrite?
κO identifies oxygen as the donor of nitrite; nitrate and nitrosyl are different ligands.
Practice Problems
- Write formulas for tetraamminediaquacobalt(III) chloride, potassium tetracyanidonickelate(II), tris(ethane-1,2-diamine)chromium(III) chloride and iron(III) hexacyanidoferrate(II).
- Write formulas for tetrahydroxidozincate(II), potassium tetrachloridopalladate(II), diamminedichloridoplatinum(II), hexaammineplatinum(IV) and tetrabromidocuprate(II).
- Write formulas for amminebromidochlorido(nitrito-κN)platinate(II), dichloridobis(ethane-1,2-diamine)platinum(IV) nitrate and pentaammine(nitrito-κO)cobalt(III).
- Name [Co(NH₃)₆]Cl₃, [CoCl(NH₃)₅]Cl₂, K₃[Fe(CN)₆], K₃[Fe(C₂O₄)₃] and K₂[PdCl₄].
- Name [Ti(H₂O)₆]³⁺, [Mn(H₂O)₆]²⁺, [NiCl₄]²⁻, [Ni(NH₃)₆]Cl₂, [Co(en)₃]³⁺ and [CoCl(NH₃)₄(NO₂)]Cl with N-bound nitrite.
- Check the name mercury(I) tetrathiocyanatocobaltate(III) against Hg[Co(SCN)₄].
- Why does swapping the order of ligands in a formula not automatically produce an isomer?
1. Neutral NH₃/H₂O around Co +3 require three chloride ions: [Co(H₂O)₂(NH₃)₄]Cl₃. 2. Ni +2 with four CN⁻ gives K₂[Ni(CN)₄]. 3. Three neutral en around Cr +3 give [Cr(en)₃]Cl₃. 4. Fe³⁺ and [Fe(CN)₆]⁴⁻ balance in a 4:3 ratio: Fe₄[Fe(CN)₆]₃.
Key Takeaways
• Name the cation before the anion in a salt. • Within an entity, name ligands alphabetically before the metal. • Multiplicative prefixes do not control alphabetical order. • Anionic entities use metal names ending in -ate. • Roman numerals specify metal oxidation state rather than entity charge. • Donor labels distinguish linkage forms of ambidentate ligands. • Every written salt must be electrically neutral. • Formula order alone does not define stereochemistry.