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.
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–, SO42– and OH–, etc. are cited at H, N, N, S, and O. The ligands containing carbon and hydrogen are cited only under C.
Consider the following statements about the rules for writing the formulas of mononuclear coordination entities.
(A) When ligands are polyatomic, their formulas are written without parentheses inside the square brackets.
(B) Polydentate ligands are also listed alphabetically; for an abbreviated ligand, the first letter of the abbreviation decides its position in the alphabetical order.
(C) When the formula of a charged coordination entity is written without that of the counter ion, the charge is indicated outside the square brackets as a right superscript with the sign before the number.
(D) There should be no space between the ligands and the metal within a coordination sphere.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (B) and (D) only
The formula of a coordination entity is written as one compact unit: the central atom first, then the ligands in alphabetical order, all inside square brackets.
(A) When ligands are polyatomic, their formulas are written without parentheses inside the square brackets. — False. Polyatomic ligands are enclosed in parentheses, as in \( \ce{[Co(NH3)6]^3+} \); ligand abbreviations are also put in parentheses.
(B) Polydentate ligands are also listed alphabetically; for an abbreviated ligand, the first letter of the abbreviation decides its position in the alphabetical order. — True. In \( \ce{[CoCl2(en)2]^+} \), en is placed by its first letter ‘e’, so it comes after Cl.
(C) When the formula of a charged coordination entity is written without that of the counter ion, the charge is indicated outside the square brackets as a right superscript with the sign before the number. — False. The charge is written outside the brackets with the number before the sign, as in \( \ce{[Co(CN)6]^3-} \) (3–, not –3).
(D) There should be no space between the ligands and the metal within a coordination sphere. — True. The whole entity is written without spaces, as in \( \ce{[Cr(H2O)6]^3+} \).
NCERT (2026-27 edition):
5.3.1 Formulas of Mononuclear Coordination Entities – “(iii) 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 the alphabetical order.”
“(iv) The formula for the entire coordination entity, whether charged or not, is enclosed in square brackets. When ligands are polyatomic, their formulas are enclosed in parentheses. Ligand abbreviations are also enclosed in parentheses.”
“(v) There should be no space between the ligands and the metal within a coordination sphere.”
“(vi) When the formula of a charged coordination entity is to be written without that of the counter ion, the charge is indicated outside the square brackets as a right superscript with the number before the sign. For example, [Co(CN)6]3–, [Cr(H2O)6]3+, etc.”