Exercises — Nomenclature (IUPAC) — naming complex ions and compounds

These four are the memorised exceptions (mnemonic AANC):

• ${\text{H}}_2\text{O}$ → aqua
• ${\text{NH}}_3$ → ammine (double m — one m is an organic amine)
• $\text{CO}$ → carbonyl
• $\text{NO}$ → nitrosyl

Anionic ligands take an -o ending (older texts use chloro, current IUPAC uses -ido):

• ${\text{Cl}}^{-}$ → chlorido
• ${\text{CN}}^{-}$ → cyanido
• ${\text{SO}}_4^{2-}$ → sulfato
• ${\text{OH}}^{-}$ → hydroxido

Charge is given: $+3$. Ligand: water is neutral, so ox. state $x$ satisfies $x+6(0)=+3\Rightarrow x=+3.$ Ligand name: aqua; six of them → hexaaqua (yes, two a's meet — keep both). Metal: complex is a cation → chromium keeps its name. Name: hexaaquachromium(III) (the ion). As it stands it is a cation, so no "-ate."

Split: $3K^{+}$ outside ⇒ complex ion is $3-$ (an anion). Ox. state: each ${\text{CN}}^{-}=-1$, six of them $=-6$: $x+6(-1)=-3\Rightarrow x=+3.$ Ligand: cyanido; six → hexacyanido. Metal: the complex ion is negative → Latin root + -ate → ferrate, oxidation state III → ferrate(III). Cation named first, space, then anion. Name: potassium hexacyanidoferrate(III).

Split: one ${\text{Cl}}^{-}$ outside ⇒ complex ion is $1+$. Ox. state: ammine $=0$ (×4), inside chloride $=-1$ (×2): $x+4(0)+2(-1)=+1\Rightarrow x=+3.$ Ligands alphabetical: "ammine" (a) before "chlorido" (c). Prefixes: four ${\text{NH}}_3$ → tetraammine; two Cl → dichlorido. Metal: cation → cobalt stays cobalt, III. Name: tetraamminedichloridocobalt(III) chloride.

Split: one outside ${\text{Cl}}^{-}$ ⇒ complex ion is $1+$. Ox. state: en $=0$ (×2), inside Cl $=-1$ (×2): $x+2(0)+2(-1)=+1\Rightarrow x=+3.$ Prefix problem: "ethylenediamine" already contains "di." Using "di" would collide → use bis + parentheses: bis(ethylenediamine). For chloride (simple) keep dichlorido. Alphabetical order: alphabetise by the ligand name inside the parentheses — "ethylenediamine" (e) vs "chlorido" (c). "c" < "e", so chlorido first. Name: dichloridobis(ethylenediamine)cobalt(III) chloride.

(en is a chelating bidentate ligand — see Ligands — types, denticity, chelation.)

No counter ions ⇒ overall charge $0$. Ox. state: ammine $0$ (×2), chloride $-1$ (×2): $x+2(0)+2(-1)=0\Rightarrow x=+2.$ Alphabetical: ammine (a) before chlorido (c). Prefixes: diammine, dichlorido. Metal: neutral complex → platinum keeps its name, still write ox. state. Name: diamminedichloridoplatinum(II).

Note: this formula has cis and trans geometric isomers — same name unless you add the prefix (see Isomerism in Coordination Compounds).

Charge given: $2-$ (anion). Ox. state: each ${\text{CN}}^{-}=-1$ (×4): $x+4(-1)=-2\Rightarrow x=+2.$ Ligand: tetracyanido. Metal: anion → nickelate (nickel has no Latin root, just add -ate), II. Name: tetracyanidonickelate(II).

Work the name backwards:

• cuprate = copper as an anion → Latin root cupr- + -ate → metal is $\text{Cu}$, and the complex is negative.
• (II) → copper oxidation state $+2$.
• tetrachlorido → four ${\text{Cl}}^{-}$, total ligand charge $4(-1)=-4$.
• Complex ion charge $=x+\sum =(+2)+(-4)=-2$ → $[{\text{CuCl}}_4{]}^{2-}$.
• potassium is the cation, ${\text{K}}^{+}$; need 2 to balance $2-$. Formula: $K_2[{\text{CuCl}}_4].$

• cobalt(III) → ${\text{Co}}^{3+}$, cation (no -ate).
• hexaammine → six neutral ${\text{NH}}_3$, contributing $0$ charge.
• Complex ion charge $=+3+6(0)=+3$ → $[\text{Co}({\text{NH}}_3{)}_6{]}^{3+}$.
• chloride counter ion ${\text{Cl}}^{-}$; need 3 to balance $3+$. Formula: $[\text{Co}({\text{NH}}_3{)}_6]{\text{Cl}}_3.$

• cobaltate(III) → cobalt as an anion, ${\text{Co}}^{3+}$, negative complex.
• hexanitrito → six ${\text{NO}}_2^{-}$, total $6(-1)=-6$.
• Complex charge $=+3+(-6)=-3$ → $[\text{Co}({\text{NO}}_2{)}_6{]}^{3-}$.
• sodium cation ${\text{Na}}^{+}$; need 3. Formula: $N{a}_3[\text{Co}({\text{NO}}_2{)}_6].$

Two complexes, no simple counter ion. They must have equal and opposite charges. Cation $[\text{Cr}({\text{NH}}_3{)}_6{]}^{?}$: ammine neutral, so the charge equals $x_{\text{Cr}}$. Chromium is commonly $+3$ here. $x_{\text{Cr}}+6(0)=\text{charge}\Rightarrow \text{charge}=+3.$ Anion $[\text{Co}(\text{CN}{)}_6{]}^{?}$: must balance $+3$ → charge $=-3$. $x_{\text{Co}}+6(-1)=-3\Rightarrow x_{\text{Co}}=+3.$ Name the cation (complex) first: hexaamminechromium(III). Then the anion (complex, gets -ate): hexacyanidocobaltate(III). Name: hexaamminechromium(III) hexacyanidocobaltate(III).

Split: one ${\text{SO}}_4^{2-}$ outside ⇒ complex ion is $2+$. Ox. state: water $0$ (×5), nitrosyl treated as neutral $0$: $x+5(0)+0=+2\Rightarrow x=+2.$ Alphabetical: "aqua" (a) before "nitrosyl" (n). Prefixes: pentaaqua, (one) nitrosyl. Metal: cation → iron stays iron (English name for cation form), II. Name: pentaaquanitrosyliron(II) sulfate. (Historically written as iron(I) with ${\text{NO}}^{+}$; with neutral NO the balance gives iron(II). This ambiguity is exactly why we always state the convention.)

Cation $[\text{Pt}({\text{NH}}_3{)}_4{]}^{?}$: ammine neutral → charge $=x_1$. Anion $[{\text{PtCl}}_6{]}^{?}$: six ${\text{Cl}}^{-}=-6$ → charge $=x_2-6$. For a neutral salt the charges cancel; the standard assignment is $x_1=+2$, $x_2=+4$:

• cation charge $=+2$, anion charge $=+4-6=-2$. They cancel. ✓ Cation (complex, no -ate): tetraammineplatinum(II). Anion (complex, gets -ate): hexachloridoplatinate(IV). Name: tetraammineplatinum(II) hexachloridoplatinate(IV). The same element is named "platinum" (cation part) and "platinate" (anion part) — the ending flags which one is the negative ion, removing all ambiguity.

How do you find the complex-ion charge with no simple counter ion? ::: The two complex parts must have equal and opposite charges — solve one, the other follows. Which complex part gets "-ate"? ::: The anionic (net-negative) part only. When do bis/tris/tetrakis appear? ::: When the ligand name already contains di/tri or is otherwise complex (e.g. ethylenediamine). Do you alphabetise by "bis"? ::: No — ignore all multiplying prefixes; alphabetise by the ligand root. Oxidation state vs complex-ion charge — same thing? ::: No. Ox. state = metal only; complex charge = metal ox. state + sum of ligand charges.