IUPAC Nomenclature: Name → Formula

CHENomenclature & Numbers

The reverse task: build the formula from an IUPAC name. You translate ligand names back to symbols, read the oxidation state from the Roman numeral, balance charges to find the number of counter-ions, and write the coordination sphere inside square brackets.

Write the central metal symbol first inside the brackets, then the ligands, then close the bracket and add counter-ions outside: $[M(\text{ligands})]^{charge}$ .
Reverse the ligand names: cyanido $\rightarrow CN$ , chlorido $\rightarrow Cl$ , aqua $\rightarrow H_2O$ , ammine $\rightarrow NH_3$ , hydroxido $\rightarrow OH$ , carbonyl $\rightarrow CO$ , ethane-1,2-diamine $\rightarrow$ en.
Read the oxidation state from the Roman numeral, then balance: metal charge $+$ ligand charges $=$ charge on the complex ion; counter-ions make the whole compound neutral.
Multiplying prefixes give subscripts; group a polyatomic or chelating ligand in parentheses when it carries its own prefix: e.g. tris(ethane-1,2-diamine) $\rightarrow (en)_3$ .
Worked patterns: potassium hexacyanidoferrate(III) $\rightarrow K_3[Fe(CN)_6]$ ; tetraamminecopper(II) sulphate $\rightarrow [Cu(NH_3)_4]SO_4$ ; hexaaquachromium(III) chloride $\rightarrow [Cr(H_2O)_6]Cl_3$ .
Worked patterns: tris(ethane-1,2-diamine)cobalt(III) chloride $\rightarrow [Co(en)_3]Cl_3$ ; dicarbonyldichloridonickel(II) $\rightarrow [Ni(CO)_2Cl_2]$ .
Charge-balance method: with metal oxidation state $+m$ and total ligand charge $L$ , the complex-ion charge is $m + L$ ; add counter-ions of opposite charge to neutralise (e.g. an ion of charge $+3$ needs three $Cl^-$ ).
The -ate suffix tells you the complex ion is an anion, so the named cation (potassium, sodium) goes OUTSIDE the brackets: potassium hexacyanidoferrate(II) $\rightarrow K_4[Fe(CN)_6]$ .
When no counter-ion is named and the name has no -ate suffix, the complex is neutral: tetracarbonylnickel(0) $\rightarrow [Ni(CO)_4]$ and dicarbonyldichloridonickel(II) $\rightarrow [Ni(CO)_2Cl_2]$ .
Translate ambidentate-ligand names to the correct donor end: nitrito-N $\rightarrow NO_2$ (M–N), nitrito-O $\rightarrow ONO$ (M–O); thiocyanato-S $\rightarrow SCN$ , isothiocyanato (thiocyanato-N) $\rightarrow NCS$ .
Keep ligands inside the brackets and counter-ions outside; the bracket content (metal + ligands) is the coordination sphere whose charge you balance externally.
Common Latin-stem decodings to recognise anionic metals: ferrate = Fe, cuprate = Cu, argentate = Ag, aurate = Au, plumbate = Pb, stannate = Sn, cobaltate = Co.

Where the marks go

Placing the named cation (e.g. potassium) inside the brackets — in an -ate (anionic) complex the alkali-metal cation sits OUTSIDE the coordination sphere.
Mis-balancing counter-ions: forgetting that the complex-ion charge is the metal oxidation number PLUS the total ligand charge before adding counter-ions.
Writing the wrong number of waters or counter-ions because a chelating ligand's denticity was overlooked (en occupies two sites but is one molecule).
Confusing nitrito-N ( $NO_2$ ) with nitrito-O ( $ONO$ ), or thiocyanato-S ( $SCN$ ) with thiocyanato-N ( $NCS$ ), when translating ambidentate names to formulas.
Decoding the -ate stem to the wrong element (e.g. 'ferrate' to anything but Fe, 'plumbate' to anything but Pb) and so writing the wrong metal symbol.

How the board asks it

Structure / namingname-to-formula translation
Write the formula of the coordination compound tris(ethane-1,2-diamine)cobalt(III) chloride, grouping the chelating ligand correctly inside the coordination sphere.
Structure / naming-ate stem and anionic-complex decoding
Give the formula corresponding to the IUPAC name potassium hexacyanidoferrate(II), showing clearly which ion lies outside the coordination sphere.
Give reasons-ate suffix and charge logic
Account for the fact that in $[Cu(NH_3)_4]SO_4$ the sulphate ion is written outside the square brackets and not inside the coordination sphere.
Numericaloxidation state and counter-ion balancing
For hexaaquachromium(III) chloride, determine the charge on the complex ion and hence the number of $Cl^-$ counter-ions, then write the complete formula.
Structure / namingambidentate donor-end translation
Write the formula of pentaamminenitrito-N-cobalt(III) chloride and state the atom of the ambidentate ligand bonded to the central metal.

Practise this topic

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.