IUPAC Nomenclature of Coordination Compounds

Contents

1 Rules for Writing Formula
2 The ligands in coordination entity are arranged as
3 Rules for naming the coordination compounds

Rules for Writing Formula

The formula of a compound is a shorthand method used to provide basic information about the constitution of a compound in a concise and convenient manner.

(1) The formula of the cation whether simple or complex is written first followed by that of the anion.

(2) The coordination entity is written in square brackets.

(3) The sequence of symbols within the coordination entity is : first the symbol of the central metal atom followed by ligands in alphabetical order.

The ligands in coordination entity are arranged as

(a) The different ligands are arranged alphabetically according to the first symbol of their formulae. For example, H₂O , NH₃, NO₃¯, SO₄^2-and OH¯ etc. are cited at H, N, N, O, S and O.

(b) When the two ligands have same defining atom, the ligand with fewer such atoms is cited first followed by the ligand having more atoms. For example, NH₃ precedes N₂

For example NH₂¯ precedes NO₂¯ because H comes before O.

(d) Polydentate ligands are also listed alphabetically. In case of abbreviated ligand, the first letter of the abbreviation is used to determine the position of the ligand in alphabetical order.

(e) The formula for the co-ordination entity, whether charged or not, is enclosed in square brackets. Polyatomic ligands are enclosed in parentheses (), but all ligands are written without any separation in between.

(f) There should be no space between the representations of ionic species within the formula.

(g) Sometimes abbreviations are used for formulae of the ligands. These abbreviations should be in lower case and enclosed in parenthesis.

For example, py is used for pyridine and en is used for ethane-1, 2-diamine or ethylene diamine.

(h) The number of cations or anions to be written in the formula is calculated on the basis that total positive charge must be equal to total negative charge.

(i) When the formula of the charged coordination entity is written without the formula of the counter ion, the charge is indicated outside the square brackets as a right superscript with the number before the sign (+ or -).

For example,

[Ni(NH₃)₆]²⁺, [Co(CN)₆]^3-, [CoCl(NH₃)₅]²⁺, etc.

Some common examples are :

K₃[Fe(CN)₆]

[CoCl₂(NH₃)₄]Cl Cl is cited first than NH₃

[Co(H₂O)₂(NH₃)₄]Cl₂ H is cited before N

[Pt Cl₂(C₅H₅N)NH₃] Among neutral ligands C₅H₅N and NH₃, C₅H₅N precedes NH₃

Rules for naming the coordination compounds

(1) Order of naming ions

In ionic complexes, the cation is named first and then the anion.

NaCl : sodium chloride

Non-ionic complexes are given a one word name.

(2) Naming the coordination entity

In naming the coordination entity, the ligands are named first and then the central metal ion.

(3) Names of ligands

The names of anionic ligands (organic or inorganic) end in o-. In general, if the anionic ligand name ends in -ide, -ite or -ate, the final ‘e’ is replaced by ’o’ giving-ido,-ito and-ato respectively. For inorganic anionic ligands containing numerical prefixes such as triphosphate enclosing marks () are added. The names of positive ligands end in -ium. The neutral ligands are named as such.

For example :

(i) Negative ligands end in -o :

F- fluorido

Cl- chlorido

Br¯ bromido

NO2¯ nitrito-N

ONO¯ nitrito-O

SO₄^2¯ sulphato

OH- hydroxo

C₂O₄^2_‾ oxalato

CN^– cyano

CH₃COO¯ acetato

SCN- thiocyanato

O₂^2– peroxo

O^2-oxo

N^3–nitrido

P^3- phosphido

N₃¯ azido

NCS¯ isothiocyanato

H¯ hydrido

CO₃^2– carbonato

NO₃¯ nitrato

NH2¯ amido

NH^2- imido

ClO3¯ Chlorato

S₂O₃^2–thiosulphato

(ii) positive ligands end in -ium

NO⁺nitrosonium

NH₂NH₃⁺ hydrazinium

NO₂⁺ nitronium

(iii) neutral ligands are named as such

NH₂CH₂CH₂NH₂ethane-1,2-diamine or ethylenediamine

C₆H₅N pyridine (py)

(C₆H₅)₃P triphenylphosphine

PH₃phosphine

CS thiocarbonyl

H₂NCSNH₂ thiourea (tu)

dipyridyl (dipy)

However, there are a few exceptions in naming neutral ligands. For example,

H₂O aqua

NH₃ ammine

NO nitrosyl

CO carbonyl

(4) Order of naming ligands

When more than one type of ligands are present, they are named in alphabetical order of preference without any consideration of charge.

For example : In the complex [CoCl(NO₂)(NH₃)₄]⁺, the ligands are named in the order :

ammine, chlorido and nitrito-N. Similarly, in the complex K₃[Fe(CN)₅NO], the ligands are named as cyano and nitrosyl.

(5) Numerical prefixes to indicate number of ligands

When more than one ligands of a particular kind are present in the complex, the di-, tri-, tetra-, penta-, hexa-, etc. are used to indicate their number: two, three, four, five and six respectively.

When the name of the ligand, includes the numerical prefix (di, tri, tetra), the prefixes bis, tris, tetrakis are used for two, three, four ligands, respectively. Such ligands are called complexligands.

For example: to indicate two simple ligands such as chloro, bromo, ammine, oxalato, etc., we use the prefix di but to indicate two complex ligands such as ethylenediamine we use the prefix bis (ethylenediamine) or bis (1,2-ethanediamine).

The name of the complex ligand is given in brackets.

For example :

[Co(en)₃]³⁺ tris(ethane-1,2,-diamine)cobalt(III) ion

[NiCl₂(PPh₃)₂] dichloridobis (triphenylphosphine)nickel(II)

(6) Ending of names

When the complex is anionic, the name of the central metal atom ends in -ate. For cationic and neutral complexes, the name of the metal is written without any characteristic ending.

For example, the cationic complex [Co(NH₃)₆]Cl₃ is named without characteristic ending of the name of the metal as :

[Co(NH₃)₆]Cl₃ hexaamminecobalt (III) chloride

The coordination compound, K[PtCl₅(NH₃)] which contains the anionic complex [PtCl₅(NH₃)]¯ is named with ending of the name of the metal as -ate.

K[PtCl₅(NH₃)] potassium amminepentachloridoplatinate (IV)

Similarly, the anionic complex Ca₂[Fe(CN)₆] is named as calcium hexacyanoferrate (II).

[Co(SCN)₄]^2–tetrathiocyanatocobaltate(II) ion.

For anionic complexes the Latin names of certain metals are commonly used.

For example: ferrate for Fe, cuperate for Cu, argentate for Ag, aurate for Au, stannate for

Sn, etc.

However, if the complex is cationic or neutral the name of the metal is given as such e.g., iron for Fe, silver for Ag, gold for Au, copper for Cu, etc.

For example:

K3[Fe(CN)6] Potassium hexacyanoferrate (III)

[Fe(CO)5] Pentacarbonyliron (0)

(7) Oxidation state of central metal ion

The oxidation state of the central metal ion is designated by a Roman numeral (such as II, III, IV) in the brackets at the end of the name of the complex without a gap between the two. Let us discuss some examples:

(i) CoCl(NH₃)₅]Cl₂

In this case the ligands are chloro and ammine. The complex is cation and chloride is anion. The oxidation state of cobalt is III as

x+ 5(0) – 1- 2 = 0

x = +3.

Its name is pentaamminechloridocobalt(III) chloride

(ii) K₃[Fe(CN)₆]

In this case, the ligands are cyano. The complex is anionic. The oxidation state of iron is +3 as 3 (+1) +x + 6(-1) = 0

The name of the complex is potassium hexacyanoferrate(III). If the complex containing the central ion, Fe³⁺ is anionic, the Latin name of metal is used i.e. ferrate.

(iii) [Co(H₂NCH₂CH₂NH₂)₃] (SO₄)₃

In this case, ligands are ethane-1, 2-diamine (or ethylenediamine). The complex is cationic. The oxidation state of cobalt is +3 as

[x + 3 x 0]2 + 3 (-2) = 0 or

2x = +6 or

x = +3

The name of the complex is tris (ethane-1, 2-diamine) cobalt(II) sulphate.

tris is used because the ligand is complex ligand.

(iv) [Ag(NH₃)₂] [Ag(CN)₂]

Both cation and anion are complexes. The oxidation state of silver in both cationic and anionic complexes is +1. The name of the complex is diamminesilver (I) dicyanoargentate (I)

(8) Point of attachment

When a ligand can coordinate through more than one atom, then the point of attachment of the ligand is indicated by putting the symbol of the atom through which coordination occurs after the name of the ligand. For example: NO₂¯ can coordinate through -N or -O. If it coordinates through N, it is called nitrito -N (or simply as nitro). If it coordinates through O, -ONO, it is called nitrito -O or simply as nitrito.

NO₂¯ (through N)

-ONO¯ (through O)

Nitrito – N

Nitrito – O

SCN¯ (through S) thiocyanato

NCS (through N) isothiocyanato

For example,

[Co(NO₂)₃(NH₃)₃] triamminetrinitrito-N-cobalt(III)

[Co(ONO)(NH₃)₅]SO₄ pentaamminenitrito-O-cobalt(III) sulphate

(9) Naming geometrical isomers

Geometrical isomers are named by the use of the terms cis-to designate adjacent positions and trans- to designate opposite positions.

For example: in square planar complexes shown below

(a) the positions 1, 4 ; 1, 2 ; 2, 3 and 3, 4 are cis-

(b) 1,3 and 2, 4 are trans

In naming these complexes, cis or trans is written before the names of these compounds. Similarly, for octahedral complexes, cis- and trans are used in isomerism in coordination compounds.

(10) Naming optical isomers

Optically active compounds are designated by the symbols (+) or d -for dextrorotatory and (-) or l- for laevorotatory.

For example : d- K₃[Cr(C₂O₄)₃] Potassium (+) trioxalatochromate (III)