• Here, the information required to develop the IUPAC name of a complex and to write the structural formula when the IUPAC name is given will be discussed. Only complexes formed by elements of the d block are considered.
The complexes are considered simply under two categories.
(i) Cations are simple while the anions are complex.
(ii) Cations are complex while the anions are simple.
• Whatever the complex considered, a common set of rules has to be followed stepwise in their nomenclature.
(1) As in the case of a simple inorganic compound, first the cation is named and then the anion. A space is left between the name of the cation and the name of the anion.
(2) The complex ion in the compound can be either positively charged or negatively charged. First, identify the metal ion and the ligand/s in the complex ion.
(3) The ligands could be negatively charged, neutral or positively charged (rarely). When naming the ligands, the charge of the ligand is considered.
(i) Neutral ligands have no special ending.
(ii) Some ligands have special names.
Examples:
H2O aqua
NH3 ammine
CO carbonyl
NO nitrosyl
(iii) For negatively charged ligands an “o” is added to their English name.
Examples:
Cl– chlorido
CN– cyanido
NO2– nitrito
OH– hydroxido
SCN– thiocyanato
H– hydrido
O2- oxido
(iv) For positively charged ligands the suffix” ium” is added to their English name.
Example: +NH3-NH2 hydrazinium
(4) When there is more than one ligand of the same type, in order to indicate the number of such ligands, the name of the relevant number is used as a prefix to the name of the ligand. When there are 2, 3, 4, 5 and 6 ligands of the same type, the prefixes di-, tri,- tetrapenta- and hexa- are used respectively.
(5) When several ligand types are present in a complex ion, in naming the ligands they are listed in the alphabetical order (English) of the first letter of the ligand.
Note: The first letter of the prefix used to denote the number of ligands is NOT considered when deciding the alphabetical order.
No space is left between the names of the ligands.
Example: [Fe(CN)2(NH3)4]+
Ligands are named as; tetraamminedicyanido
(6) When writing the name of the complex ion, the ligands are named first and then the metal ion. The oxidation number of the metal ion is given in capital Roman numerals, within parentheses, after the name.
No space is left between the words when writing the name.
Examples:
[Co(NH3)6]3+ hexaamminecobalt(III) ion
[Fe(H2O)6]2+ hexaaquairon(II) ion
[Cu(NH3)4]2+ tetraamminecopper(II) ion
(7) The complex may be positively charged, negatively charged or neutral. Depending on this, the name also changes.
(i) When the complex is positively charged or neutral, the name of thecomplex ends with the name of the metal.
Reminder
The name of the metal is followed by the oxidation number of the metal ion, in capital Roman numerals, within parentheses. No space is left between the name of the metal ion and the oxidation number given in parantheses.
Example:
[Fe(CN)3(NH3)3]
The complex is neutral. Hence, its name is triamminetricyanidoiron(III).
Example:
[Cu(H2O)6 ]2+
The complex is positively charged. Its name is hexaaquacopper(II) ion.
(ii) When the complex ion is negatively charged, the suffix ‘ate’ is added to the end of the name of the metal. Here also the oxidation number of the metal ion should be indicated in capital Roman numerals within parentheses. No space is left between the name of the metal ion and the oxidation number given in parentheses.
Examples:
[CoCl4]2- tetrachloridocobaltate(II) ion
[Co(CN)6]3- hexacyanidocobaltate(III) ion
[CuCl4]2- tetrachloridocuprate(II) ion
[Fe(CN)6]4- hexacyanidoferrate(II) ion
[Fe(CN)6]3- hexacyanidoferrate(III) ion
[Ag(CN)2]– dicyanidoargentate(I) ion
[Cr(Br)6]3- hexabromidochromate(III) ion
The IUPAC name of any compound can be developed by systematically following the rules studied so far.
(8) When writing the name of a coordination compound, a space should be left between the name of the positively charged species and the negatively charged species.
Examples :
• Simple cation and complex anion
K3[Fe(CN)5NO] potassium pentacyanidonitrosylferrate(II)
Na2[ZnCl4] sodium tetrachloridozincate(II)
• Complex cation and simple anion
[Ag(NH3)2]Cl diamminesilver(I) chloride
[Fe(OH)2(H2O)4]Br tetraaquadihydroxidoiron(III) bromide
[CoCl(NH3)5](NO2)2 pentaamminechloridocobalt(III) nitrate
[CoCl(NH3)5](NO2)2 pentaamminechloridocobalt(III) nitrite
1. The positively charged species is written first, followed by the negatively charged species. No space is left between them.
2. The complex part of the compound is always written within square brackets.
3. When the formula of the complex ion is written, the metal should be indicated first and then the ligands. In writing the ligands, the charge on the ligand is NOT considered. The ligands are written in the alphabetical order of the ligating atoms. (i.e. Atom through which the ligand coordinates to the metal ion).
Note: In multiatomic ligands, where possible, it is recommended that the ligating atom is placed first followed by the other atoms in the ligand.
Example : (:OH2) rather than H2O
4. Multi atomic ligands are given in parentheses. The number of each type of ligand is given in Arabic numerals as a subscript on the right hand side immediately after the symbol of the ligand. If parentheses are present, this number is written as a subscript on the right hand side without leaving a space, just outside the parentheses.
5. The formula of a complex ion should be written within square brackets. If the complex ion has a charge, it should be indicated outside the square bracket as a superscript on the right side. The numerical value should be given first followed by the sign of the charge.
Note: No space is left between the formulae of the ligands or between the formulae of the ligands and symbol of the metal ion.
Example 1
Write the chemical formula of pentacyanidonitrosylferrate(II) ion.
Step 1: Write the symbol of the metal.
Fe
Step 2: Decide on the order of the ligands. Include the number of ligands of each type when writing their symbols/formulae. Then write the symbols / formulae of the ligands after the symbol of the metal. The ligands are CN– (ligating atom is C) and NO (ligating atom is nitrogen) hence, the order is CN– followed by NO. Both ligands are multi-atomic. Hence, their formulae are placed within parentheses. The presence of five CN– ligands are indicated.
Fe(CN)5(NO)
Step 3: Place the symbol of the metal ion and the formulae of the ligands within square brackets and show charge of complex.
Oxidation number of metal is +II. Overall charge of the complex ion is + 2 + (-5) = -3
Chemical formula is [Fe(CN)5NO]3-
Example 2
Write the chemical formula of pentaamminechloridocobalt(III) ion.
Step 1: Write the symbol of the metal.
Co.
Step 2: Decide on the order of the ligands. Include the number of ligands of each type when writing their symbol/formula. Then write the symbols/ formulae of the ligands after the symbol of the metal.
Ligands are Cl– (ligating atom is Cl) and NH3 (ligating atom is N). Considering the alphabetical order of the ligating atoms, Cl– is written first followed by NH3. The NH3 ligand is multi-atomic and hence the formula is placed within parentheses. The presence of five NH3 ligands is indicated.
CoCl(NH3)5
Step 3: Place the symbol of the metal and the formulae of the ligands within square brackets and show charge of complex.Oxidation state of the metal ion is +III. Therefore, overall charge of the complex is +3 + (-1) = +2
Chemical formula is; [CoCl(NH3)5]2+
Example 3
Write the chemical formula of pentaamminechloridocobalt(III) bromide.
As shown in Example 2, the chemical formula of the complex ion is;
[CoCl(NH3)5]2+.
Two Br– ions are required to neutralize the charge on this complex ion. Therefore, the formula is; [CoCl(NH3)5]Br2
