How to Write the Formula of a Complex From Its Name
Chemistry · Coordination Compounds · NEET
To write a formula from a name, read the name backwards. Put the metal first, then its ligands inside square brackets [ ], and write the counter ion outside. Then balance the total charge to zero to find how many counter ions you need. Memory hook: "Name reads outside-in, but the formula is built inside-out."
Split the name into ligands + metal + counter ion. Build the complex ion inside [ ] with the metal first, find its charge (+2 here), then add counter ions outside to make the total zero.
Your doubts, answered
How do I even start? The name is one long word.
Break the name into 3 parts. Part 1 = the ligands (the middle chunks like tetraammine, aqua, chlorido). Part 2 = the central metal + its oxidation state in brackets like cobalt(III). Part 3 = the ion written as a separate word at the end (like 'chloride' or 'sulphate') which is the counter ion outside the sphere. Once you split it, each part maps to a piece of the formula.
What goes inside the square brackets and what goes outside?
Everything in the coordination sphere goes inside [ ]: the metal first, then all its ligands. The counter ion (the last separate word of the name) goes OUTSIDE the brackets. Example: in 'potassium hexacyanoferrate(II)', ferrate + cyanide are inside, potassium is outside, giving K4[Fe(CN)6].
In what order do I write the ligands inside the bracket?
NCERT rule: metal symbol first, then ligands in alphabetical order (by ligand name, ignoring the multiplying prefix). Neutral and anionic ligands are just listed; prefixes like di, tri, tetra tell you how many. Put polyatomic ligand formulas in round brackets, e.g. (NH3)4, (H2O), (en)2.
How do I know how many counter ions to write outside?
First find the charge of the complex ion inside the bracket. Add up: (metal oxidation state) + (sum of ligand charges). Neutral ligands like NH3 and H2O add 0; anionic ligands like Cl- add -1 each. Then add just enough counter ions outside so the whole formula is neutral (total charge = 0).
The prefix says 'chlorido' inside AND the last word is 'chloride' outside. Are they the same Cl?
No, they are different chlorines. 'Chlorido' is a Cl- bonded to the metal INSIDE the bracket (a ligand). 'Chloride' at the end is a free Cl- counter ion OUTSIDE the bracket. This is the exact trap in tetraammineaquachloridocobalt(III) chloride: one Cl is inside, and two more Cl- are outside.
Does 'di, tri, tetra' vs 'bis, tris' change the formula?
No, they mean the same count; only the ligand type differs. Use di/tri/tetra for simple ligands (dichlorido = 2 Cl). Use bis/tris for ligands that already contain a prefix or are complex, like bis(ethylenediamine) = (en)2. Both just tell you the number to write in the formula.
⚠️ The NEET trap ✗ For tetraammineaquachloridocobalt(III) chloride, students write [Co(NH3)4Cl2]·H2O or put all the chlorides inside/outside wrongly, forgetting that 'chlorido' is a ligand and 'chloride' is a separate counter ion. ✓ The complex ion is [Co(NH3)4(H2O)Cl]2+ (Co is +3, one coordinated Cl- makes charge +2), so you need 2 free Cl- outside: [Co(NH3)4(H2O)Cl]Cl2. 🧠 '-ido' lives INSIDE the bracket; '-ide' as a last word lives OUTSIDE. Count charge, then balance to zero.
Real NEET questions
ReNEET 2026
The formula of tetraammineaquachloridocobalt(III) chloride is:
A · [Co(NH3)4Cl2]·H2O
B · [Co(NH3)4]Cl3·H2O
C · [Co(NH3)4(H2O)Cl]Cl
D · [Co(NH3)4(H2O)Cl]Cl2 ✓
Solution: Read the name into pieces. Ligands inside the sphere: four ammine (NH3)4, one aqua (H2O), one chlorido (Cl-). Metal = cobalt(III), so Co is +3. Now find the charge of the complex ion: (+3) + 4(0 for NH3) + (0 for H2O) + (-1 for Cl-) = +2. So the complex ion is [Co(NH3)4(H2O)Cl]2+. The last word 'chloride' is the counter ion Cl- outside. To balance +2, you need two Cl-. Formula = [Co(NH3)4(H2O)Cl]Cl2. Answer (D). Trap: the one 'chlorido' Cl is inside, the two 'chloride' Cl- are outside.
NEET 2022
The IUPAC name of the complex [Ag(H2O)2][Ag(CN)2] is (reverse skill: name to formula check):
A · dicyanidosilver(II) diaquaargentate(II)
B · diaquasilver(II) dicyanidoargentate(II)
C · dicyanidosilver(I) diaquaargentate(I)
D · diaquasilver(I) dicyanidoargentate(I) ✓
Solution: This tests the same name-formula link both ways. The cation is named first: [Ag(H2O)2]+ has two aqua ligands and Ag in +1 = diaquasilver(I). The anion is second and takes the -ate ending with the Latin stem argent-: [Ag(CN)2]- has two cyanido and Ag +1 = dicyanidoargentate(I). Both silvers are +1 (each 0-charge and -1 ligand set balances to give +1), so the (II) options are wrong. Answer (D). To go the reverse way (name to formula), you would place the cation sphere first, then the anion sphere, exactly as shown.
Solved Coordination Compounds NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Do I write the metal or the ligand first inside the bracket?
Always the metal symbol first, then the ligands. In the NAME the ligands come first, but in the FORMULA the metal leads. That is why you 'flip' the order when converting name to formula.
How do I decide the number of counter ions outside?
Compute the charge on the complex ion = metal oxidation state + total ligand charge. Then add counter ions outside until the overall charge is zero. If the complex is +2, you need two -1 counter ions (or one -2 counter ion).
When do I use round brackets ( ) inside the square brackets?
Use round brackets around any ligand that is a group or has more than one atom shown, like (NH3)4, (H2O)2, (en)2, (CN)6. Single-atom ligands like Cl need no round bracket. This keeps the formula clear for NEET.
Why is this concept important for NEET?
NEET often gives a long IUPAC name and asks for the correct formula (like ReNEET 2026). If you cannot balance the charge and separate inside vs outside ligands, you pick a wrong option. It is a quick, guaranteed mark if you follow the steps.
What is the difference between 'chlorido' and 'chloride' in a name?
'Chlorido' is a Cl- ligand bonded to the metal, written inside the bracket. 'Chloride' written as the final separate word is a counter ion, written outside the bracket. They look similar but sit in different places in the formula.
