According to the valence bond theory, the metal atom or ion under the influence of ligands can use its (n-1)d, ns, np, nd orbitals for hybridization to yield a set of equivalent orbitals of definite geometry. These hybridized orbitals are allowed to overlap with ligand orbitals that can donate electron pairs for bonding. It is usually possible to predict the geometry of a complex from the knowledge of its magnetic behaviour on the basis of the valence bond theory. Consider the formation of \([Co(NH_3)_5Cl]Cl_2\) and answer the following question:
The IUPAC name of the above coordination entity is
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Pentaamminechloridocobalt (III) chloride
Name the cation first, then the anion. In the complex ion \( \ce{[Co(NH3)5Cl]^{2+}} \):
The ligands are named in alphabetical order: ammine (a) before chlorido (c). Five \( \ce{NH3} \) give “pentaammine” (the prefix is not counted in the alphabetical order).
Oxidation state of Co: x + 5(0) + (−1) = +2, so x = +3, written (III).
The complex ion is a cation, so the metal keeps its own name, cobalt; the ending “-ate” is used only for anionic complexes.
The counter ion is named chloride: Pentaamminechloridocobalt(III) chloride.
NCERT (2026-27 edition):
Naming of Mononuclear Coordination Compounds – “… (i) The cation is named first in both positively and negatively charged coordination entities.
(ii) The ligands are named in an alphabetical order before the name of the central atom/ion. (This procedure is reversed from writing formula).
(iii) Names of the anionic ligands end in –o, those of neutral and cationic ligands are the same except aqua for H2O, ammine for NH3, carbonyl for CO and nitrosyl for NO. While writing the formula of coordination entity, these are enclosed in brackets ( ).
… (v) Oxidation state of the metal in cation, anion or neutral coordination entity is indicated by Roman numeral in parenthesis.
(vi) If the complex ion is a cation, the metal is named same as the element. For example, Co in a complex cation is called cobalt and Pt is called platinum.”