Coordination compounds are formulated and named according to the system set up by the Inorganic Nomenclature Committee of the International Union of Pure and Applied Chemistry (IUPAC). It is called the IUPAC System of Nomenclature. According to the latest (2004) IUPAC system, the following rules are observed for writing formulas and naming mononuclear coordination compounds.

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. The following rules are applied while writing the formulas:

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

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

(iii) The sequence of symbols within the coordination entity is: first the symbol of the central metal atom followed by ligands in alphabetical order. According to the latest IUPAC recommendations, the placement of a ligand in the list does not depend on its charge. The ligands in the 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, S, and O. The ligands containing carbon and hydrogen are cited only under C.

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

(c) If the numbers of defining atoms are equal, the subsequent symbol decides the sequence. For example, NH₂^– precedes NO₂^– because H comes before O.

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

(v) The formula for the coordination 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.

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

(vii) Sometimes abbreviations are used for formulae of the ligands. These abbreviations should be in lowercase and enclosed in parenthesis. For example, py is used for pyridine, and en is used for ethane-1, 2-diamine, or ethylene diamine.

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

(ix) 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 –).

What is the IUPAC name of \([Pt(NH_3)_4(NO_2)Cl]\)?

Tetrammine chlorido nitro platinum (IV) sulphate

The correct IUPAC name of the complex \([Pt(NH_3)_4(NO_2)Cl]\) is (2) tetramminechloridonitroplatinum(IV) sulphate.

The name is derived from the chemical formula of the complex, which is \([Pt(NH_3)_4(NO_2)Cl]\). The name of the metal is written first, followed by the names of the ligands in alphabetical order. The name of the platinum metal is written as platinum, with the roman numeral IV indicating the oxidation state of platinum. The ammine ligand is written as NH₃, the chloride ligand is written as Cl, and the nitro ligand is written as NO₂. The sulphate ligand is written as SO₄.

The other options are incorrect. Option (1), chlorido nitro platinum (IV) sulphate, is incorrect because the name of the ligand nitro should be chloronitro. Option (3), chlorido tetrammine platinum (IV) sulphate, is incorrect because the name of the ligand chloride should be chlorido. Option (4), platinum (IV) tetrammine nitro chloride sulphate, is incorrect because the name of the complex should not include the oxidation state of the metal.