Coordination Compounds — NEET UG Questions

34 NEET UG practice questions on Coordination Compounds, part of Chemistry. Below are 12 of them in full, each with the answer and a written explanation.

Questions & explanations

1. Which of the following is an example of the biological importance of a coordination compound?

  1. Chlorophyll contains a magnesium-porphyrin complex
  2. Sodium chloride is essential for nerve transmission
  3. Carbon monoxide binds to haemoglobin but is not a coordination compound
  4. Ammonia is used as a ligand in laboratory tests

Answer: Chlorophyll contains a magnesium-porphyrin complex

Chlorophyll is a coordination compound where magnesium is coordinated to a porphyrin ring; it is vital for photosynthesis. Sodium chloride is an ionic compound, not a coordination compound. Carbon monoxide binding to haemoglobin involves coordination chemistry (haemoglobin is a coordination compound). Ammonia as a ligand is an analytical application, not a biological one.

2. Which of the following square planar complexes exhibits geometrical (cis-trans) isomerism?

  1. [Pt(NH3)2Cl2]
  2. [Pt(NH3)4]Cl2
  3. [PtCl4]2-
  4. [Pt(NH3)Cl3]-

Answer: [Pt(NH3)2Cl2]

Square planar complexes with formula [MA2B2] (two identical ligands A and two identical ligands B) show cis-trans isomerism. [Pt(NH3)2Cl2] has two NH3 and two Cl ligands, allowing the two Cl to be adjacent (cis) or opposite (trans). The other complexes have either all ligands identical or only one type of ligand substitution, so no geometrical isomerism is possible.

3. Which of the following pairs of compounds exhibit ionization isomerism?

  1. [Co(NH3)5SO4]Br and [Co(NH3)5Br]SO4
  2. [Co(NH3)5NO2]Cl2 and [Co(NH3)5ONO]Cl2
  3. [Cr(H2O)6]Cl3 and [Cr(H2O)5Cl]Cl2·H2O
  4. [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6]

Answer: [Co(NH3)5SO4]Br and [Co(NH3)5Br]SO4

Ionisation isomerism occurs when the counter ion exchanges with a ligand in the coordination sphere, yielding different ions in solution. In the given pair, one compound has SO4 coordinated and Br as counter ion, while the other has Br coordinated and SO4 as counter ion. The other options are linkage, hydrate, and coordination isomerism, respectively.

4. Which of the following complexes is expected to exhibit optical isomerism?

  1. [Co(NH3)6]3+
  2. trans-[Co(NH3)4Cl2]+
  3. [Co(en)3]3+
  4. [Co(NH3)5Cl]2+

Answer: [Co(en)3]3+

Optical isomerism arises from chirality, i.e., the absence of a plane of symmetry. The complex [Co(en)3]3+ (en = ethylenediamine, a bidentate ligand) has a non-superimposable mirror image due to its three chelate rings, making it optically active. The other complexes possess planes of symmetry and are achiral, hence they do not show optical isomerism.

5. In the complex [Co(en)2Cl2]+, what are the denticity of ethylenediamine (en) and the coordination number of cobalt, respectively?

  1. 2, 4
  2. 2, 6
  3. 1, 4
  4. 1, 6

Answer: 2, 6

Ethylenediamine (en) is a bidentate ligand, meaning it donates two pairs of electrons through two nitrogen atoms, so its denticity is 2. The complex has two en ligands (each forming two coordinate bonds, total 4 bonds) and two chloride ligands (each forming one coordinate bond, total 2 bonds). Hence the coordination number of cobalt is 4 + 2 = 6.

6. The effective atomic number (EAN) of cobalt in the complex [Co(NH3)6]3+ is: (Atomic number of Co = 27)

  1. 27
  2. 36
  3. 24
  4. 33

Answer: 36

Cobalt in [Co(NH3)6]3+ has an oxidation state of +3. Co (Z=27) loses three electrons to form Co3+, leaving 24 electrons. Each NH3 ligand donates 2 electrons, so six NH3 donate 12 electrons. Total electrons around cobalt = 24 + 12 = 36, which is the effective atomic number (EAN). This corresponds to the electron count of krypton (noble gas).

7. Which of the following is an example of hydrate (solvate) isomerism?

  1. [Cr(H2O)6]Cl3 and [Cr(H2O)5Cl]Cl2·H2O
  2. [Co(NH3)5NO2]Cl2 and [Co(NH3)5ONO]Cl2
  3. [Co(NH3)5SO4]Br and [Co(NH3)5Br]SO4
  4. [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6]

Answer: [Cr(H2O)6]Cl3 and [Cr(H2O)5Cl]Cl2·H2O

Hydrate (solvate) isomerism involves water molecules either inside the coordination sphere or outside as water of crystallisation. The given pair has different number of water molecules in the coordination sphere and as lattice water. The other options are examples of linkage, ionization, and coordination isomerism, respectively.

8. Isomerism in coordination compounds is broadly divided into which of the following two categories?

  1. Structural and stereo
  2. Optical and geometrical
  3. Ionisation and hydrate
  4. Linkage and coordination

Answer: Structural and stereo

Isomerism in coordination compounds is classified into structural isomerism (where the bonding or connectivity differs) and stereoisomerism (where the spatial arrangement differs). Optical and geometrical are types of stereoisomerism; ionisation, hydrate, linkage, and coordination are types of structural isomerism.

9. The colour of a transition metal complex is attributed to:

  1. d-d transitions involving absorption of light in the visible region
  2. charge transfer from metal to ligand only
  3. absorption of infrared radiation causing vibrational transitions
  4. excitation of inner shell electrons

Answer: d-d transitions involving absorption of light in the visible region

According to Crystal Field Theory, the colour of a complex arises from d-d transitions: an electron absorbs a photon of visible light to jump from a lower d-orbital to a higher d-orbital. The energy difference (Δ) corresponds to a specific wavelength, and the complementary colour is observed.

10. Which of the following pairs represents coordination isomerism?

  1. [Co(NH3)5NO2]Cl2 and [Co(NH3)5ONO]Cl2
  2. [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6]
  3. [Co(NH3)5SO4]Br and [Co(NH3)5Br]SO4
  4. [Cr(H2O)6]Cl3 and [Cr(H2O)5Cl]Cl2·H2O

Answer: [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6]

Coordination isomerism occurs when the cation and anion in a coordination compound exchange ligands. In this pair, the ligands NH3 and CN are swapped between Co and Cr centers in the complex cation and anion. The other options are linkage, ionization, and hydrate isomerism, respectively.

11. Which of the following pairs of compounds are linkage isomers?

  1. [Co(NH3)5NO2]Cl2 and [Co(NH3)5ONO]Cl2
  2. [Co(NH3)5SO4]Br and [Co(NH3)5Br]SO4
  3. [Cr(H2O)6]Cl3 and [Cr(H2O)5Cl]Cl2·H2O
  4. [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6]

Answer: [Co(NH3)5NO2]Cl2 and [Co(NH3)5ONO]Cl2

Linkage isomerism arises when an ambidentate ligand coordinates through different donor atoms. Here, NO2 can bind through nitrogen (nitro) or oxygen (nitrito), giving two isomers. The other pairs are examples of ionization, hydrate, and coordination isomerism, respectively.

12. According to Valence Bond Theory (VBT), the formation of a coordination compound involves:

  1. Use of d-orbitals of the ligand to form bonds
  2. Hybridisation of the metal ion's valence orbitals to accommodate lone pairs from ligands
  3. Complete transfer of electrons from the metal to the ligand
  4. Electrostatic attraction between metal and ligand without any orbital overlap

Answer: Hybridisation of the metal ion's valence orbitals to accommodate lone pairs from ligands

VBT explains coordination by stating that the metal ion uses suitable hybrid orbitals (e.g., d2sp3, sp3d2) to accept lone pairs from ligands. It does not involve ligand d-orbitals, nor is it purely electrostatic (that is CFT), and electron transfer is not a feature of VBT.

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