Isomerism in coordination compounds

What is isomerism in coordination compounds?

In plain words: keep the same atoms but change either which atom is bonded to the metal, or only where the bonded groups sit in space, and you get a different compound. The first kind is structural isomerism, the second stereoisomerism.

Stereoisomerism and structural isomerism

NCERT divides the isomerism of coordination compounds into two principal types, each with subtypes (NCERT page 125; NCERT Exercise 5.8 asks for this list):

• Stereoisomers have the same chemical formula and the same chemical bonds, but a different spatial arrangement (NCERT page 125).
• Structural isomers have different bonds (NCERT page 125).

Geometric isomerism

For the idealised ligand arrangements considered here, geometrical isomerism arises in heteroleptic complexes because the ligands can take up different geometric arrangements; important examples come with coordination numbers 4 and 6 (NCERT Section 5.4.1, page 125).

• Square planar [MX₂L₂] (X and L unidentate): the two X are side by side in the cis isomer and opposite each other in the trans isomer, as in [Pt(NH₃)₂Cl₂] (NCERT page 125, Fig. 5.2).
• Square planar [MABXL] (four different unidentate ligands) has three geometrical isomers (NCERT page 125), each fixed by which ligand sits opposite A: B, X or L (reasoned). NCERT calls them “two cis and one trans”: take any one pair, say A and B, and it is trans in one isomer and cis in the other two (reasoned). None is chiral in this idealised model (Q7).
• Simple tetrahedral complexes with unidentate ligands show no geometrical isomerism, because all four positions are adjacent to one another: the relative positions of the ligands are the same in every arrangement (NCERT Example 5.4, page 126). A tetrahedral [MABCD] has no geometrical isomers but can be chiral, like a carbon atom carrying four different groups (reasoned from NCERT Class 12 Chemistry Part II, Unit 6, page 177).
• Octahedral [MX₂L₄]: the two X are cis or trans, as in [Co(NH₃)₄Cl₂]⁺ (NCERT page 125, Fig. 5.3). The same holds for [MX₂(L–L)₂] with a didentate ligand such as en, for example [CoCl₂(en)₂]⁺ (NCERT page 126, Fig. 5.4).
• Octahedral [Ma₃b₃], such as [Co(NH₃)₃(NO₂)₃]: if the three identical donor atoms occupy the corners of one face of the octahedron, it is the facial (fac) isomer; if they lie round a meridian, it is the meridional (mer) isomer (NCERT page 126, Fig. 5.5).

Optical isomerism

• Enantiomers. Optical isomers are mirror images that cannot be superimposed on one another; they are called enantiomers, and molecules or ions that cannot be superimposed on their mirror images are chiral (NCERT Section 5.4.2, page 126).
• d and l. An enantiomerically enriched sample may be dextro (d, +) or laevo (l, −), according to the direction in which it rotates the plane of polarised light in a polarimeter: d to the right, l to the left (NCERT page 126). An equal mixture of enantiomers is a racemate and has no net optical rotation. The measured sign is not inferred simply from the Δ or Λ configurational label.
• Where it occurs. Optical isomerism is common in octahedral complexes with didentate ligands, such as [Co(en)₃]³⁺ (NCERT page 126, Fig. 5.6).
• Only cis is chiral. In [PtCl₂(en)₂]²⁺ only the cis isomer shows optical activity (NCERT page 126, Fig. 5.7). The trans isomer has a mirror plane through the metal and both en ligands, so it is its own mirror image (reasoned).
• The idealised square planar examples here, with achiral ligands and ligand conformations ignored, show no optical isomerism: the plane of the four ligands is itself a mirror plane (reasoned).

Linkage isomerism

• Linkage isomerism arises in a compound that contains an ambidentate ligand, which can bind through either of two atoms (NCERT Section 5.4.3, page 127).
• Thiocyanate may bind through nitrogen, M–NCS, or through sulphur, M–SCN (NCERT page 127).
• Jørgensen’s nitrite complexes. [Co(NH₃)₅(NO₂)]Cl₂ is obtained as a red form, with the nitrite bound through oxygen (–ONO), and a yellow form, bound through nitrogen (–NO₂) (NCERT page 127). In names they are nitrito-O and nitrito-N.

Coordination isomerism

• When both ions of a salt are complex, the ligands can be shared out differently between the two metal ions (NCERT Section 5.4.4, page 127).
• In [Co(NH₃)₆][Cr(CN)₆] the NH₃ ligands are bound to Co³⁺ and the CN⁻ ligands to Cr³⁺; in its coordination isomer [Cr(NH₃)₆][Co(CN)₆] it is the other way round (NCERT page 127).

Ionisation isomerism

• Ionisation isomerism arises when the counter ion of a complex salt is itself a potential ligand: it can take the place of a ligand, which then becomes the counter ion (NCERT Section 5.4.5, page 127).
• [Co(NH₃)₅(SO₄)]Br and [Co(NH₃)₅Br]SO₄ are ionisation isomers (NCERT page 127). In cold, prompt tests they supply different counter ions, so they react differently: only the second gives a precipitate of BaSO₄ with Ba²⁺, and only the first gives AgBr with Ag⁺ (NCERT answer to Intext Question 5.4, page 140).
• NCERT slip (page 140): Intext Question 5.4 asks about [Co(NH₃)₅Cl]SO₄ and [Co(NH₃)₅(SO₄)]Cl, but the printed answer works the bromide pair above. The same two tests decide the chloride pair, with AgCl in place of AgBr (Q2; reasoned). In an exam, write the tests for the pair the question names.

Solvate isomerism

• Solvate isomers differ by whether or not a solvent molecule is directly bonded to the metal ion or merely present as free solvent molecules in the crystal lattice; with water it is called hydrate isomerism, and it is similar to ionisation isomerism (NCERT Section 5.4.6, page 128). A lattice water molecule, though not bonded to the metal, may still hydrogen-bond (reasoned).
• Example: the aqua complex [Cr(H₂O)₆]Cl₃ (violet) and its solvate isomer [Cr(H₂O)₅Cl]Cl₂·H₂O (grey-green) (NCERT page 128).
• Telling them apart. In water, [Cr(H₂O)₆]Cl₃ releases three Cl⁻ per formula and [Cr(H₂O)₅Cl]Cl₂·H₂O only two. In a prompt cold test with excess Ag⁺, one mole of each gives 3 mol and 2 mol of AgCl; the lattice water is not a ligand (reasoned).

Solved examples

• NCERT slip (page 140): the answer to Intext Question 5.3(iii) says “There are 10 possible isomers”, and its hint lists geometrical isomers. An octahedral complex with five identical ligands and one different one has no geometrical isomers, and the linkage and ionisation isomers of Example 4 give three compounds in all (reasoned). In an exam, name linkage and ionisation isomerism and say that no geometrical isomers are possible. The printed 10 is inconsistent with this restricted structural model; do not treat it as a correct count for the stated assumptions.

Common mistakes to avoid

Quick revision summary

• Stereoisomers: same bonds, different arrangement in space (geometrical, optical). Structural isomers: different bonds (linkage, coordination, ionisation, solvate).
• Geometrical: cis/trans for square planar [MX_₂L_₂] and octahedral [MX_₂L_₄], [MX_₂(L–L)_₂]; fac/mer for [Ma_₃b_₃]; none for tetrahedral.
• Optical: non-superimposable mirror images (d and l), common with didentate ligands: [Co(en)_₃]^³⁺, cis-[PtCl_₂(en)_₂]^²⁺.
• Linkage: ambidentate ligand (–NO_₂/–ONO, –SCN/–NCS); coordination: [Co(NH_₃)_₆][Cr(CN)_₆] and [Cr(NH_₃)_₆][Co(CN)_₆].
• Ionisation: [Co(NH_₃)_₅(SO_₄)]Br and [Co(NH_₃)_₅Br]SO_₄; solvate: [Cr(H_₂O)_₆]Cl_₃ (violet) and [Cr(H_₂O)_₅Cl]Cl_₂·H_₂O (grey-green).

Questions students ask

Is there a formula for the number of geometrical isomers of a coordination compound?

There is no general formula for counting the geometrical isomers of a complex: place the ligands on the corners of its polyhedron and discard any arrangement that a rotation or reflection turns into one already drawn (reasoned). For octahedral complexes of unidentate ligands this gives 2 for [Ma₄bc], 3 for [Ma₃b₂c], 5 for [Ma₂b₂c₂], 9 for [Ma₂bcde] and 15 for [Mabcdef]; counting mirror images separately, the last three have 6, 15 and 30 stereoisomers (beyond the NCERT text). The organic 2ⁿ rules are on the organic isomerism page.

How can you tell whether a coordination compound is optically active?

A coordination entity is chiral when its mirror image cannot be superimposed on it (NCERT page 126). A sample shows net optical rotation only if the enantiomer contributions do not cancel. A mirror plane or inversion centre rules out chirality; more generally any improper rotation symmetry does so (reasoned). Didentate ligands make chirality common in octahedral complexes (NCERT page 126), but they are not needed: the octahedral [Ma₂b₂c₂] isomer in which every pair of identical ligands is cis has neither and exists as two enantiomers, while each of its other four geometrical isomers has a mirror plane (reasoned).

Has isomerism in coordination compounds been deleted from the JEE Main 2027 or NEET (UG) 2026 syllabus?

No, isomerism in coordination compounds has not been deleted: Unit 12 of both the JEE Main 2027 and the NEET (UG) 2026 syllabus lists “isomerism” right after the “IUPAC nomenclature of mononuclear co-ordination compounds”, and the 2026–27 NCERT reprint keeps it as Section 5.4 (pages 125–128). Neither syllabus names the subtypes, so prepare all six that NCERT treats, geometrical, optical, linkage, coordination, ionisation and solvate isomerism.

What is polymerisation isomerism in coordination compounds?

Polymerisation isomerism is a term some textbooks use, though NCERT and the IUPAC Red Book 2005 do not, for compounds that share an empirical formula but have formula units of different sizes, such as [Pt(NH₃)₂Cl₂] and Magnus’s green salt, [Pt(NH₃)₄][PtCl₄] (beyond the NCERT text; Wikipedia, “Magnus’s green salt”). They are not isomers in NCERT’s sense, which asks for the same chemical formula (NCERT page 125): the salt has two platinum atoms per formula unit, so its molar mass is twice as large (reasoned). The salt is named in Example 4 of the nomenclature page.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Which of these can show geometrical isomerism: (i) a tetrahedral [MA₂B₂] complex, (ii) square planar [Pt(NH₃)₂Cl₂], (iii) octahedral [Co(NH₃)₅Cl]²⁺? (Based on NCERT Example 5.4.)

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   Only (ii), as cis and trans (NCERT Fig. 5.2, page 125). In (i) every position of a tetrahedron is adjacent to every other (NCERT Example 5.4, page 126). In (iii) every position of the single Cl is equivalent (reasoned, as in Example 4).

2. Q2. Give evidence that [Co(NH₃)₅Cl]SO₄ and [Co(NH₃)₅(SO₄)]Cl are ionisation isomers. (NCERT Intext Question 5.4.)

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   They give different ions in water. [Co(NH₃)₅Cl]SO₄ has free SO₄²⁻, so it gives a precipitate of BaSO₄ with Ba²⁺ but no immediate AgCl from coordinated chloride in the cold test. [Co(NH₃)₅(SO₄)]Cl has free Cl⁻, so it gives immediate AgCl with Ag⁺ but no immediate BaSO₄ from coordinated sulphate (reasoned, as in NCERT’s answer for the bromide pair, page 140).

3. Q3. Write the coordination isomer of [Cu(NH₃)₄][PtCl₄].

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   [Pt(NH₃)₄][CuCl₄]: the NH₃ and Cl⁻ ligands change metals, and both metals stay +2 (reasoned from NCERT page 127).