IUPAC nomenclature of inorganic chemistry 2005

IUPAC nomenclature of inorganic chemistry 2005

The IUPAC Recommendations 2005, Nomenclature of Inorganic Chemistry replaces their previous recommendations "Nomenclatureof Inorganic Chemistry, IUPAC Recommendations 1990 (Red Book I)", and "where appropriate" (sic) "Nomenclature of Inorganic Chemistry II, IUPAC Recommendations 2000 (Red Book II)". The recommendations take up over 300 pages NOMENCLATURE OF INORGANIC CHEMISTRY IUPAC Recommendations 2005 ed. N. G. Connelly et al. RSC Publishing http://www.chem.qmul.ac.uk/iupac/bioinorg/] and the full text can be downloaded from IUPAC. [ [http://old.iupac.org/publications/books/rbook/Red_Book_2005.pdf Nomenclature of Inorganic Chemistry IUPAC Recommendations 2005] - Full text (PDF)] Corrections have been issued. [ [http://www.chem.qmul.ac.uk/iupac/bibliog/RBcorrect.html Corrections to Nomenclature of Inorganic Chemistry: IUPAC Recommendations 2005] ]

Apart from a reorganisation of the content, there is a new section on organometallics and a formal element list to be used in place of electronegativity lists in sequencing elements in formulae and names. The concept of a preferred IUPAC name (PIN), a part of the revised blue book for organic compound naming, has not yet been adopted for inorganic compounds. There are however guidelines as to which naming method should be adopted.

Naming methods

The recommendations describe a number of different ways in which compounds can be named. These are:
*compositional naming (e.g. sodium chloride)
*substitutive naming based on parent hydrides (GeCl2Me2 dichlorodimethylgermane)
*additive naming ( [MnFO3] fluoridotrioxidomanganese)Additionally there are recommendations for the following:
*naming of cluster compounds
*allowed names for inorganic acids and derivatives
*naming of solid phases e.g. non-stoichiometric phasesFor a simple compound such as AlCl3 the different naming conventions yield the following:
*compositional: aluminium trichloride (stoichiometrically) or dialuminium hexachloride (dimer)
*substitutional: trichloralumane
*additive: trichloridoaluminium; hexachloridoaluminium (dimer without structural information); di-μ-chlorido-tetrachlorido-1κ2"Cl",2κ2"Cl"-dialuminium (dimer with structural information)

equencing elements - the "electronegativity" list

Throughout the recommendations the use of the electronegativity of elements for sequencing has been replaced by a formal list which is loosely based on electronegativity. The recommendations still use the terms electropositive and electronegative to refer to an elements relative position in this list.
A simple rule of thumb ignoring lanthanides and actinides is:
*for two elements in different groups - then the element in the higher numbered group has higher "electronegativity"
*for two elements within the same group the element with the lower the atomic number has the higher "electronegativity"
*Hydrogen is fitted in to be less electronegative than polonium and more electronegative than nitrogen. Hence the formulae of water and ammonia can be written NH3 and H2O respectively.

The full list, from highest to lowest "electronegativity":-
*Group 17 in atomic number sequence i.e. F-At "followed by"
*Group 16 in atomic number sequence i.e. O-Po "followed by"
* H, hydrogen "followed by"
* Group 15 in atomic number sequence i.e. N-Bi "followed by"
* Group 14 in atomic number sequence i.e. C-Pb "followed by"
* Group 13 in atomic number sequence i.e. B-Tl "followed by"
* Group 12 in atomic number sequence i.e. Zn-Hg "followed by"
* Group 11 in atomic number sequence i.e. Cu-Rg "followed by"
* Group 10 in atomic number sequence i.e. Ni-Ds "followed by"
* Group 9 in atomic number sequence i.e. Co-Mt "followed by"
* Group 8 in atomic number sequence i.e. Fe-Hs "followed by"
* Group 7 in atomic number sequence i.e. Mn-Bh "followed by"
* Group 6 in atomic number sequence i.e. Cr-Sg "followed by"
* Group 5 in atomic number sequence i.e. V-Db "followed by"
* Group 4 in atomic number sequence i.e. Ti-Rf "followed by"
* Group 3 in atomic number sequence i.e. Sc-La "followed by"
* the lanthanides in atomic number sequence i.e. La-Lu "followed by"
* the actinides in atomic number sequence i.e. Ac-Lr "followed by"
* Group 2 in atomic number sequence i.e. Be-Sg "followed by"
* Group 1 (excl H) in atomic number sequence i.e. Li-Fr "followed by"
* Group 18 in atomic number sequence i.e. He-Rn

Determining the nomenclature to use

equence and position of ligands and central atoms

Ligands are ordered alphabetically by name and precede the central atom name. The number of ligands coordinating is indicated by the prefixes di-, tri-, tetra- penta- etc for simple ligands or bis-, tris-, tetrakis-,etc for complex ligands. For example:
* [CoCl(NH3)5] Cl2 pentaamminechloridocobalt(2þ) chloride where ammine (NH3)precedes chloride.The central atom name(s) come after the ligands. Where there is more than one central atom it is preceded by di- tr-, tetra- etc.
* Os3(CO)12, decacarbonyldihydridotriosmiumWhere there are different central atoms they are sequenced using the electronegativity list.
* [ReCo(CO)9] nonacarbonylrheniumcobalt

Bridging ligands- use of μ symbol

Ligands may bridge two or more centres. The prefix μ is used to specify a bridging ligand in both the formula and the name. For example the dimeric form of aluminium trichloride:

: Al2Cl4(μ-Cl)2

: di-μ-chlorido-tetrachlorido-1κ2"Cl",2κ2"Cl"-dialuminium

This example illustrates the ordering of bridging and non bridging ligands of the same type. In the formula the bridging ligands follow the non bridging whereas in the name the bridging ligands precede the non bridging. Note the use of the kappa convention to specify that there are two terminal chlorides on each aluminium.

Bridging index

Where there are more than two centres that are bridged a bridging index is added as a subscript. For example in basic beryllium acetate which can be visualised as a tetrahedral arrangement of Be atoms linked by 6 acetate ions forming a cage with a central oxide anion, the formula and name are as follows:

: [Be44-O)(μ-O2CMe)6]


The μ4 describing the bridging of the central oxide ion. (Note the use of the kappa convention to describe the bridging of the acetate ion where both oxygen atoms are involved.)In the name where there a ligand is involved in different modes of bridging, the multiple bridging is listed in decreasing order of complexity,e.g. μ3 bridging before μ2 bridging.

Kappa,κ, convention

The kappa convention is used to specify which ligand atoms are bonding to the central atom and in polynuclear species which atoms , both bridged and unbridged link to which central atom. For monodentate ligands there is no ambiguity as to which atom is forming the bond to the central atom. However when a ligand has more than one atom that can link to a central atom the kappa convention is used to specify which atoms in a ligand are forming a bond. The element atomic symbol is italicised and preceded by kappa,κ. These symbols are placed after the portion of the ligand name that represents the ring, chain etc where the ligand is located. For example:

*pentaamminenitrito-κ"O"-cobalt(III) specifies that the nitrite ligand is linking via the oxygen atomWhere there is more than one bond formed from a ligand by a particular elemnt aq numerical superscript gives the count. For example:
* aqua [(ethane-1,2-diyldinitrilo-κ2"N","N"’)tris(acetato-κ"O")acetato] cobaltate(1-), the cobalt anion formed with water and pentadentate edta, which links via two nitrogen atoms and three oxygen atoms. There are two bonds from nitrogen atoms in edta which is specified by -κ2"N","N"’. The three bonds from oxygen are specified by tris(acetato-κ"O"), where there is one ligation per acetate.

In polynuclear complexes the use of the kappa symbol is extended in two related ways. Firstly to specify which ligating atoms bind to which central atom and secondly to specify for a bridging ligand which central atoms are involved. The central atoms must be identified, i.e. by assigning numbers to them. (This is formally dealt with in the recommendations). To specify which ligating atoms in a ligand link to which central atom, the central atom numbers precede the kappa symbol, and numerical superscript specifies the number of ligations and this is followed by the atomic symbol. Multiple occurrences are separated by commas.

Examples:: di-μ-chlorido-tetrachlorido-1κ2Cl,2κ2Cl-dialuminium , (aluminium trichloride).:: tetrachlorido-1κ2Cl,2κ2Cl specifies that there are two chloride ligands on each aluminium atom.:decacarbonyl-1κ3"C",2κ3"C",3κ4"C"-di-μ-hydrido-1:2κ2"H";1:2κ2"H"-"triangulo"-(3 " Os"—"Os"), (Decacarbonyldihydridotriosmium).::decacarbonyl-1κ3"C",2κ3"C",3κ4"C" shows that there are three carbonyl groups on two osmium atoms and four on the third. ::di-μ-hydrido-1:2κ2"H";1:2κ2"H" specifies that the two hydride bridge between the osmium atom 1 and osmium atom 2.

Eta, η, convention

The use of η to denote hapticity is systematised. The use of η1 is not recommended. When the specification of the atoms involved is ambiguous the position of the atoms must be specified. This is illustrated by the examples:
* Cr(η6-C6H6)2, named as bis(η6-benzene)chromium as all of the (contiguous) atoms in the benzene ligands are involved there position does not have to be specified
* [(1,2,5,6-η)-cycloocta-1,3,5,7-tetraene] (η5-cyclopentadienyl)cobalt in this only two (at positions 1 and 5) of the four double bonds are linked to the central atom.

Coordination geometry

For any coordination number above 2 more than one coordination geometry is possible. For example four coordinate coordination compounds can be tetrahedral, square planar, square pyramidal or see-saw shaped. The polyhedral symbol is used to describe the geometry. A configuration index is determined from the positions of the ligands and together with the polyhedral symbol is placed at the beginning of the name. For example in the complex ("SP"-4-3)-(acetonitrile)dichlorido(pyridine)platinum(II) the ("SP"-4-3) at the beginning of the name describes a square planar geometry, 4 coordinate with a configuration index of 3 indicating the position of the ligands around the central atom. For more detail see polyhedral symbol.

Organometallic groups 3-12

Additive nomenclature is generally recommended for organometallic compounds of groups 3-12 (transition metals and zinc, cadmium and mercury).


Following on from ferrocene the first sandwich compound with a central Fe atom coordinated to two parallel cyclopentadienyl rings, names for compounds with similar structures such as osmocene and vanadocene are in common usage. The recommendation is that the name-ending ‘ocene’ should be restricted to compounds where there are discrete molecules of bis(η5-cyclopentadienyl)metal (and ring-substituted analogues), where the cyclopentadienyl rings are essentially parallel, and the metal is in the d-block. The terminology does NOT apply to compounds of the s- or p-block elements such as Ba(C5H5)2 or Sn(C5H5)2] .
Examples of compounds that meet the criteria are :
* vanadocene, [V(η5-C5H5)2]
* chromocene, [Cr(η5-C5H5)2]
* cobaltocene, [Co(η5-C5H5)2]
* nickelocene, [Ni((η5-C5H5)2]
* ruthenocene, [Ru(η5-C5H5)2]
* osmocene, [Os(η5-C5H5)2]
*decamethylmanganocene, [Mn(η5-C5H5)2]
* [Re(η5-C5H5)2] .

Examples of compounds that should not be named as metallocenes are:
* C10H10Ti
* [Ti(η5-C5H5)2Cl2] is named dichloridobis(η5-cyclopentadienyl)titanium NOT titanocene dichloride

Polynuclear cluster compounds

Metal-metal bonds

In polynuclear compounds with metal- metal bonds these are shown after the element name as follows:(3 "Os"—"Os") in DecacarbonyldihydridotriosmiumA pair of brackets contain a count of the bonds formed (if greater than 1), followed by the the italicised element atomic symbols separated by an "em-dash".

Polynuclear cluster geometry

The geometries of polynuclear clusters can range in complexity. A descriptor e.g. tetrahedro or the CEP descriptor e.g. "Td"-(13)-Δ4-"closo"] can be used. this is determined by the complexity of the cluster. Some examples are shown below of descriptors and CEP equivalents. (The CEP descriptors are named for Casey, Evans and Powell who described the system. [ A descriptor system and principles for numbering closed boron polyhedra with at least one rotational symmetry axis and one symmetry plane Casey J.B., Evans W.J., Powell W.H. Inorg. Chem., 20, 5,(1981), 1333–1341 DOI: 10.1021/ic50219a001]


decacarbonyldimanganese bis(pentacarbonylmanganese)("Mn"—"Mn")

dodecacarbonyltetrairidium tri-μ-carbonyl-1:2κ2"C";1:3κ2"C";2:3κ2"C"-nonacarbonyl-1κ2"C",2κ2"C",3κ2"C",4κ3"C"- ["T"d-(13)-Δ4-"closo"] -tetrarhodium(6 "Rh"—"Rh")
or tri-μ-carbonyl-1:2κ2"C";1:3κ2"C";2:3κ2"C"-nonacarbonyl-1κ2"C",2κ2"C",3κ2"C",4κ3"C"-tetrahedro-tetrarhodium(6 "Rh"—"Rh")

Inorganic acids

Hydrogen names

The recommendations include a description of hydrogen names for acids. The following examples illustrate the method:

* HNO3 hydrogen(nitrate)
* H2SO4 dihydrogen(sulfate)
* HSO4 hydrogen(sulfate)(2−)
*H2S dihydrogen(sulfide)Note that the difference from the compositional naming method (hydrogen sulfide) as in hydrogen naming there is NO space between the electropositive and electronegative components.
This method gives no structural information regarding the position of the hydrons (hydrogen atoms). If this information is to be conveyed then the additive name should be used (see the list below for examples).

List of acceptable names

The recommendations give a full list of acceptable names for common acids and related anions. A selection from this list is shown below.


Stoichiometric phases are named compositionally. Non-stoichiometric phases are more difficult. Where possible formulae should be used but where necessary naming such as the following may be used:
*iron(II) sulfide (iron deficient)
*molybdenum dicarbide (carbon excess)

Mineral names

Generally mineral names should not be used to specify chemical composition. However a mineral name can be used to specify the structure type in a formula e.g.
*BaTiO3 ( perovskite type)

Approximate formulae &variable composition

A simple notation may be used where little information on the mechanism for variability is either available or is not required to be conveyed:
*~FeS (circa or approximately)Where there is a continuous range of composition this can be written e.g., K(Br,Cl) for a mixture of KBr and KCl and (Li2,Mg)Cl2 for a mixture of LiCl and MgCl2.The recommendation is to use trhe following generalised method e.g.
* CuxNi1-x for (Cu,Ni)
* KBrxCl1-x for K(Br,Cl)Note that cation vacancies in CoO could be described by CoO1-x

Point defects(Kröger-Vink) notation

Point defects, site symmetry and site occupancy can all be described using Kröger-Vink Notation, note that the IUPAC preference is for vacancies to be specified by "V" rather than V (the element vanadium).

Phase nomenclature

To specify the crystal form of a compound or element the Pearson symbol may be used. The use of "Strukturbericht" (e.g. A1 etc) or Greek letters is not acceptable. The Pearson symbol may be followed by the space group and the prototype formula. Examples are:

* carbon(c"F" 8), diamond
* RuAl(C"P2"2, Pm3m )("CsCl" type)


It is recommended that polymorphs are identified, (e.g. for ZnS where the two forms zincblende (cubic) and wurtzite (hexagonal))as ZnS("c") and ZnS("h") respectively.


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