Formulas, Names of Complexes and d-Configurations
The following summary is based on the IUPAC Red Book (377 pages pdf file, 4.1 MB) rules operating since 2005. Earlier versions of the rules are still used in many textbooks but these have now been superseded and should be avoided. Like the rules of grammar in any language, these rules have been developed to prevent ambiguity. Just as when writing peoples' names, you should be very careful with spelling and the correct use of capitals letters and spaces.
Video tutorial
 | To write the formula of a complex from its name, apply the following rules in the order written. |
- The complex is enclosed within square brackets with the charge, if any, shown as a superscript outside the brackets.
- The metal atom is listed first in the formula, followed by the ligands in alphabetical order. This alphabetical order is determined by the symbol of the ligand donor atom. Subscripts indicate the number of each ligand present in the formula and parentheses are used around polyatomic ligands. For example,
- [CoCl(NH3)5]2+ contains Cl and five NH3 bonded to the Co2+ cation. As 'C' is before 'N' in tha alphabet, the ligands are listed in the order given.
- [Pt(NH3)3(NO2)]+ contains three NH3 and one NO2-. Both ligands begin with 'N' so the next letter is used and as 'H' is alphabetically before 'O', the ligands are listed in the order given.
- For polyatomic ligands, the donor atom is listed first. This makes it easy to see which atom is actually bonded to the metal. For example,
- In [Co(NH3)3(OH2)3]3+, the N atom of ammonia and the O atom of water is bonded to the metal ion.
- [Co(NH5)(NO2)]2+ and [Co(NH5)(ONO)]2+ both contain five NH3 and one NO2- bonded to Co3+. In the former, the NO2- is bonded to the metal using a N atom. In the latter, it is bonded to the metal through one of the O atoms.
| To write the name of a complex from its formula, apply the following rules in the order written. |
- The name consists of two parts: the complex ion and the counter ions. If the complex ion is negatively charged, the positive counter ions are named before the complex ion. If the complex ion is positively charged, the negative counter ions are named after the complex ion.
- The number of counter ions is not specified, as it can be inferred from the charge of the complex. There is a space between the names of the cation and anion.
- The name of the complex ion is written as one word, with no space between its parts. To name the complex ion, follow steps 4-9.
- Neutral ligands use the name of neutral molecule. Thus, the molecule NH2CH2CH2NH2 is ethane-1,2-diamine and this is also used when it is a ligand. There are three common exceptions to this rule:
- H2O - aqua
- NH3 - ammine (note the double m to distinguish it from RNH2 amine ligands)
- CO - carbonyl
- Negatively charged ligands use the name of the anion with the last letter changed to 'o'.
This is the most significant change in the 2005 nomenclature. Note that for the anion Cl-, for example, it is called chloride when acting as a counter ion and chlorido when coordinated to the metal.Examples:
anion name name when coordinated to a metal acetate acetato azide azido bromide bromido bromidechloride chlorido chloridecyanide cyanido cyanofluoride fluorido fluoridehydroxide hydroxido hydroxohydrogenperoxide hydrogenperoxido hydroperoxoiodide iodido iodidenitrate nitrato oxide oxido oxoperoxide peroxido peroxophenoxide phenoxido phenoxotetrafluoridoborate tetrafluoridoborato tetrafuoroboratethiocyanate thiocyanato sulfide sulfido
- The ligands are named first, in alphabetical order. This alphabetical order is determined by the name of the ligand.
- Numerical prefixes based on the Greek, di-, tri-, tetra-, penta-, hexa- are used to indicate the presence of more than one of any given ligand. These prefixes are disregarded for the purpose of determining the order of listing ligands in the name. Double vowels such as in "hexaaqua" and "pentaammine" are retained.
- The name of the complex ion is completed with that of of the metal ion. The name of the metal ion includes its oxidation number and this is given by Roman numerals written in brackets. There is no space between any part of the name of the complex ion, including the metal atom and its oxidation number.
- If the complex ion is negatively charged, the name of the metal ion is changed with 'ate' added to the name of the metal atom. When the metal name ends in 'um', the metal's name is first stripped to the second last consonant.
For historic reasons, some elements are given their Latin names (stripped to the second last consonant) when present in complex anions:
Note that the oxidation number is written after 'ate'. For example, [CoCl4]2- is the tetrachloridocobaltate(II) ion and [FeCl4]2- is the tetrachloridoferrate(II) ion.Common exceptions:
metal metal name in an anionic complex iron ferrate ironatecopper cuprate copperatetin stannate tinategold aurate goldatelead plumbate leadateantimony stibnate antimonate
| Work through these examples carefully to see how the rules are applied and then test yourself with the self test quiz. |
1. The formula of hexaaquanickel(II) sulfate is [Ni(OH2)6]SO4:
- The cation and anion are written as separate words with a space inbetween: the cation is hexaaquanickel(II) sulfate and the anion is sulfate.
- The cation contains 6 ("hexa") water ("aqua") ligands bonded to a Ni2+ ("nickel(II)") cation. As water is a neutral ligand, the charge on this comes from the Ni2+ and hence the formula of the cation is [Ni(OH2)6]2+. Note that this complex ion is written in square brackets and the ligands are written OH2 to show that it is the oxygen atom that is connected to the metal.
- The counter ion is SO42- ("sulfate").
- As the cation has a charge of +2 and the anion has a charge of -2, the compound has a 1:1 stoichiometry.
2. The name of [Co(NH3)5(OH2]Cl2 is pentaammineaquacobalt(II) chloride:
- The complex ion is written in square brackets: the complex ion is [Co(NH3)5(OH2]2+ where the charge of +2 arises because there are 2Cl- counter ions.
- The complex ion contains 5 ("penta") ammonia ligands ("ammine") and 1 water ligand ("aqua"). As these are both neutral ligands, the charge of the complex ion corresponds to the oxidation number of the metal. The metal ion is cobalt(II) (with no space between the element and its oxidation number.
- Hence, the complex ion is pentaammineaquacobalt(II). This is written as one word with no spaces. Note that ammine is written before aqua as "am" is before "aq" in the alphabet. Note also the double a and double m in the name.
- Counter ions are not bonded to the complex ion and are written as a separate word. The complex ion is a cation and thus proceeds the anion in the name. There is a space between cation and anion so they can be distinguished.
3. The formula of sodium hexacyanidoferrate(II) is Na4[Fe(CN)6]:
- The cation and anion are written as separate words with a space inbetween: the cation is sodium and the anion is hexacyanidoferrate(II).
- The anion contains 6 ("hexa") CN- ("cyanido") ligands bonded to a Fe2+ ("ferrate(II)") cation. As this complex is an anion, the ending "ate" is used. For iron(II), the Latin stem "ferr" is used. Note that this complex ion is written in square brackets and the ligands are written CN- to show that it is the carbon atom that is connected to the metal.
- As the complex contains 6CN- and Fe2+, it has an overall charge of -4 ( = +2 + 6 × -1).
- As the complex ion has a charge of -4, four Na2+ counter ions are required for charge balance. The formula is written, as normal, with cation followed by anion.
4. The name of Cs2[CuCl4] is caesium tetrachloridocuprate(II):
- The complex ion is written in square brackets: the complex ion is [CuCl4]2- where the charge of -2 arises because there are 2Cs+ counter ions.
- The complex ion contains 4 ("tetra") Cl- ligands ("chlorido"). The complex ion contains 4Cl- and Cu2+ so has an overall charge of -2 ( = +2 + 4 × -1). As the complex ion is an anion, the ending "ate" is used. For copper(II), the Latin stem "cupr" is used.
- Hence, the complex ion is tetrachloridocuprate(II). This is written as one word with no spaces.
- Counter ions are not bonded to the complex ion and are written as a separate word. The complex ion is a anion and thus the counter cation proceeds the anion in the name. There is a space between cation and anion so they can be distinguished.
5. The formula of tetraamminedibromidocobalt(III) chloride is [CoBr2(NH3)4]Cl:
- The cation and anion are written as separate words with a space inbetween: the cation is tetraamminedibromidocobalt(III) and the anion is chloride.
- The cation contains 4 ("tetra") NH3 ("ammine") ligands and 2 ("di") Br- ("bromido") ligands bonded to a Co3+ ("cobalt(III)") cation. As "a" ("ammine") proceeds "b" ("bromido") alphabetically, the ligands are listed in this order in the name. As "B" ("Br-") proceeds "N" ("NH3"), however, the formula is written with Br before NH3: [CoBr2(NH3)4]+.
- As the complex contains 2Br-, 4 neutral NH3 and Co3+, it has an overall charge of +1 ( = +3 + 4 × 0 + 2 × -1).
- As the complex ion has a charge of +1, one Cl- counter ion is required for charge balance. The formula is written, as normal, with cation followed by anion.
- Note that Br- is a ligand - it is attached to the Co3+ so is given the name "bromido". The Cl- is a counter - it is not attached to the Co3+ and is written as "chloride".
6. The name of [PtCl2(NH3)2] is diamminedichloridoplatinum(II):
- The complex ion is written in square brackets: the complex ion is [PtCl2(NH3)2]. There are no counter ions and so the complex must have no charge
- The complex contains 2 ("di") NH3 ("ammine") and 2 Cl- ("chlorido") ligands. The complex contains 2Cl- and 2 neutral NH3 ligands. As it has no charge, it must contain Pt2+ ("platinum(II)"). As the complex ion is not an anion, the ending "ate" is not used.
- As "a" ("ammine") proceeds "c" ("chlorido") alphabetically, the ligands are listed in this order in the name.
- Hence, the complex is diamminedichloridoplatinum(II). This is written as one word with no spaces.
Self test quiz