Coordination Compounds MCQ Quiz - Objective Question with Answer for Coordination Compounds - Download Free PDF

CONCEPT:

Crystal Field Stabilization Energy (CFSE) and Wavelength of Light Absorbed

E = hν = hC / λ

• In coordination complexes, the wavelength of light absorbed is inversely related to the energy of the d-d transition. This relationship is expressed as:
• Where:
  • E is the energy of the absorbed light,
  • h is Planck’s constant,
  • ν is the frequency of light,
  • C is the speed of light,
  • λ is the wavelength of the absorbed light.
• The Crystal Field Stabilization Energy (CFSE) is related to the ligand field strength. The stronger the ligand field, the higher the CFSE, and the lower the wavelength of light absorbed (since energy is higher).
• The order of ligand field strength for various ligands is:
  • CN^- > NH₃ > H₂O > Cl^-

EXPLANATION:

• The order of CFSE is therefore: C > B > A > D. Since higher CFSE corresponds to shorter wavelengths of absorbed light, the order of wavelength is the reverse: D > A > B > C.

• In these complexes, all Co are in the +3 oxidation state, and the ligand field strength determines the CFSE, which in turn affects the wavelength of light absorbed.
• The order of ligand field strength is: CN^- > NH₃ > H₂O > Cl^-
• Based on this, the CFSE order is: C > B > A > D, and the wavelength order (since E ∝ 1/λ) is: D > A > B > C.

Therefore, the correct answer is: D > A > B > C.

CONCEPT:

Hybridisation and Geometry of Coordination Compounds

• In coordination chemistry, the hybridisation of the central metal ion determines the geometry and magnetic properties of the complex.
• Common hybridisations and corresponding geometries:
  • sp³: Tetrahedral
  • dsp²: Square planar
  • d²sp³: Inner orbital octahedral
  • sp³d²: Outer orbital octahedral
• Paramagnetic behavior arises due to the presence of unpaired electrons in d-orbitals.

EXPLANATION:

• A - I: [MnCl₄]²⁻
  • Manganese has a +2 oxidation state (d⁵ configuration).
  • Cl⁻ is a weak ligand → no pairing occurs.
  • Hybridisation: sp³ → Geometry: Tetrahedral
  • 
• B - II: [Cu(NH₃)₄]²⁺
  • Copper is in +2 state (d⁹ configuration).
  • Strong field NH₃ ligands → pairing occurs → dsp²
  • Geometry: Square planar
  • 
• C - III: [CoF₆]³⁻
  • Co³⁺ is d⁶.
  • F⁻ is a weak field ligand → no pairing.
  • Uses 4s, 4p, and 4d → sp³d² hybridisation (outer orbital)
  • Geometry: Octahedral
  • 
• D - IV: [Co(NH₃)₆]³⁺
  • Co³⁺ is d⁶.
  • NH₃ is a strong field ligand → pairing occurs.
  • Uses inner 3d orbitals → d²sp³ hybridisation (inner orbital)
  • Geometry: Octahedral
  • 

Therefore, the correct answer is: Option 4) A - I, B - II, C - III, D - IV

CONCEPT:

Conductance of Ionic Compounds in Solution

• The conductance of a compound in solution depends on the number of ions produced upon dissociation.
• More the number of ions, higher the conductance.
• Complex compounds can dissociate into different numbers of ions depending on their structure and coordination sphere.

EXPLANATION:

• [Co(NH₃)₃Cl₃]: This is a neutral complex where all three chlorides are coordinated to cobalt. It does not dissociate into ions. Hence, conductance is minimum.
• [Co(NH₃)₄Cl₂]Cl: This is a 1:2 electrolyte. It dissociates as:

  [Co(NH₃)₄Cl₂]Cl → [Co(NH₃)₄Cl₂]⁺ + Cl^-

  Producing 2 ions in solution.
• [Co(NH₃)₆]Cl₃: This is a 1:3 electrolyte. It dissociates as:

  [Co(NH₃)₆]Cl₃ → [Co(NH₃)₆]³⁺ + 3Cl^-

  Producing 4 ions in solution.
• [Co(NH₃)₅Cl]Cl: This is a 1:2 electrolyte. It dissociates as:

  [Co(NH₃)₅Cl]Cl → [Co(NH₃)₅Cl]⁺ + Cl^-

  Producing 2 ions in solution.

So, [Co(NH₃)₃Cl₃] produces no ions in solution and will have the minimum conductance.

Therefore, the compound with minimum conductance in solution is [Co(NH₃)₃Cl₃].

CONCEPT:

Wavelength of Light Absorbed by Complexes

• The wavelength of light absorbed by a complex depends on the energy gap (Δ_o, crystal field splitting) between the d-orbitals in the central metal ion.
• This energy gap is influenced by the nature of the ligands and their position in the spectrochemical series.
• Strong field ligands (e.g., CN⁻) cause a larger splitting of d-orbitals (higher Δ_o), leading to the absorption of light with shorter wavelengths (higher energy).
• Weak field ligands (e.g., H₂O) cause smaller splitting of d-orbitals (lower Δ_o), leading to the absorption of light with longer wavelengths (lower energy).

EXPLANATION:

• In the given complexes:
  • [Co(NH₃)₆]³⁺: NH₃ is a moderate field ligand and causes moderate splitting of d-orbitals.
  • [Co(CN)₆]³⁻: CN⁻ is a strong field ligand and causes large splitting of d-orbitals, resulting in absorption of shorter wavelengths.
  • [Cu(H₂O)₄]²⁺: H₂O is a weak field ligand and causes small splitting of d-orbitals, resulting in absorption of longer wavelengths.
  • [Ti(H₂O)₆]³⁺: H₂O is also a weak field ligand, but the metal ion Ti³⁺ has a smaller splitting than Cu²⁺.
• Comparing the strength of the ligands and the splitting:
  • [Co(CN)₆]³⁻ absorbs the shortest wavelength due to CN⁻ (strong field ligand).
  • [Co(NH₃)₆]³⁺ absorbs a shorter wavelength than [Ti(H₂O)₆]³⁺ due to NH₃ (moderate field ligand).
  • [Cu(H₂O)₄]²⁺ absorbs the longest wavelength due to H₂O (weak field ligand) and Cu²⁺ having a higher splitting than Ti³⁺.

So, the correct order is B < A < D < C.

CONCEPT:

Paramagnetism

• A substance is considered paramagnetic if it has unpaired electrons in its electronic configuration. The presence of unpaired electrons leads to a net magnetic moment.
• If all the electrons are paired, the substance is diamagnetic and does not exhibit paramagnetism.
• The electronic configuration and ligand field strength play a major role in determining whether a compound is paramagnetic or diamagnetic.
• In the case of transition metal complexes, the ligand's field strength determines whether the d-electrons will pair up (low-spin) or remain unpaired (high-spin).

EXPLANATION:

​So, the correct answer is [NiCl₄]²⁻ and [Ni(H₂O)₆]²⁺ are paramagnetic.

Concept:

• IUPAC stands for International Union of Pure and Applied Chemistry.
• In this, the naming of the compound is followed by a certain rule to avoid confusion all over the world.
• As per IUPAC rules, additive nomenclature was founded in order to describe the structures of coordination entities or complexes, but this method is readily extended to other molecular entities as well. 
• Mononuclear complexes are considered to consist of a central atom, often a metal ion, which is bonded to surrounding small molecules or ions, which are referred to as ligands.

Explanation:

• From the above explanation we know that in mononuclear complexes we have a central atom which is metal in our case it is Cobalt which is surrounded by ammonia and ONO molecules then its IUPAC name will be Pentaamminenitritocobalt (III) ion

Confusing point:

• We can see that Pentaamminenitrocobalt (III) ion and  Pentaamminenitritocobalt (III) ion are both isomers and we can distinguish both as shown below 

Structure to be named
Centre atom	Cobalt III	Cobalt III
Name of ligands	(NH3)5⇒ Ammine  O-N-O (i.e., NO2) ⇒ nitro	(NH3)5⇒ Ammine  O-N=O (i.e.,ONO)⇒ nitrito
Assemble name	Pentaamminenitrocobalt (III) ion	Pentaamminenitritocobalt (III) ion

This both are 

Explanation:

Given,

[CoCl(en)₂]Cl

⇒ Cl ^- → monodentate ligand 

⇒ en → bidentate ligand

∴ Co-ordination Number of Co = (2 × 2) + 1 = 5

Now,

K₃ [Al(C₂O₄)₃]

⇒ C₂O₄ ^2-→ bidendate ligand

Explanation

Ligand Field Strength

• The field strength of ligands is determined by their ability to split the d-orbitals of a central metal ion in a coordination complex.
• This is often described using the spectrochemical series, which arranges ligands in order of their field strength.
• Stronger field ligands cause greater splitting of the d-orbitals, leading to higher energy differences between the d-orbital sets.
• Common order of field strength for some ligands is:
  • ( \(CO > CN^- > NO_2^- > en > NH_3 > NCS^- > H_2O > EDTA^{4-} > OH^- > F^- > S^{2-} \))

Conclusion

Based on the given options and the general spectrochemical series, the correct sequence in decreasing order of field strength is CO > H2O > F- > S2-

Concept:

According to Werner’s theory,

• one of the five molecules of H₂O molecules will attach to the SO₄ molecules in the form of hydrogen bonding.
• It will act as both a primary and secondary molecules so the coordination number will be 4.

Explanation:

• Copper sulphate pentahydrate contains copper (II) in a geometry best described as distorted octahedral.
• The copper (II) is bound to four water molecules in a square-planar geometry and two oxygen atoms from two sulphate ions (one H₂O is H-Bonded here).

This salt dissolves in water to produce the pale-blue [Cu(H₂O)₆]²⁺ ion, in which two of the water molecules are less tightly held and have longer bond distances.

• In CuSO_4.5H₂O, four H₂O molecules are directly coordinated to the central metal ion (Cu⁺²) while one H₂O molecule is hydrogen bonded with SO₄^2-.

Concept-

​Crystal field theory:

• A metal ion in a complex is surrounded by coordinating ligands anions or negative ends.
• An electric field is produced at the metal ion by the surrounding ligands.
• This electrostatic approach towards the interaction between metals and ligands is known as crystal field theory.
• CFT theory considers the ligands as point charges.
• There is no overlap between the ligand orbitals and metal ion orbitals.
• Ligands donate lone pair of electrons to the metal atoms and form coordinate complexes.

Explanation:

• The metal ion has two valencies:
  • The oxidation state or the primary valency.
  • The co-ordination number or the secondary valency.
• The secondary valencies are satisfied by co-ordinating with primary ligands.
• The number of ligands satisfying the secondary valency is called the coordination number of the metal.
• The bond formed by the ligands and the metal ion forms a coordination sphere.
• The coordination sphere is non-ionizable in solution because of the co-ordinate bond between them.
• The oxidation number or primary valency of the metal ion is satisfied by the side ions which form the ionizable sphere.
• In the complex [Cu(NH3)4]SO4, [Cu(NH3)4]²⁺ is the coordination sphere. The number of ligands attached to the metal Cu is four.

​

Hence, the coordination number of the complex [Cu(NH3)4]SO4 is four.

​Additional Information

The geometry of the complex formed.

• Placement of the donor atoms at different stereochemical positions will perturb the metal ions to a different extent.
• This will result in the formation of different stereochemistry or structure.
• The stereochemistry depends on the co-ordination number.

Concept:

The structure of Co₂(CO)₈ (a polynuclear metal carbonyl) can be written as:

Concept:

Cis-Trans Isomers are isomers that differ in the arrangement of two ligands in square planar and octahedral geometry.

Cis isomers are isomers where the two ligands are 90 degrees apart from one another in relation to the central molecule. This is because Cis isomers have a bond angle of 90^o, between two same atoms.

Trans isomers are isomers where the two ligands are on opposite sides in a molecule because trans isomers have a bond angle of 180^o, between the two same atoms.

When naming cis or trans isomers, the name begins either with cis or trans, whichever applies, followed by a hyphen and then the name of a molecule.

Cis-trans Isomerism is possible with [Pt(en)₂Cl₂]²⁺

Only square planar and octahedral geometries can have cis or trans isomers.

(Dichloro(ethelyenediamine)platinum (II)) shows only optical isomerism. The other complexes do not show stereoisomerism.

Stereoisomerism, is also called as spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space. This contrasts with structural isomers which shares the same molecular formula, but the bond connections or their order differs.

Concept:

Wilkinson's catalyst is an σ –bonded organometallic compound [(Ph₃P)₃ RhCl]. It is commercially used for hydrogenation of alkenes and vegetable oils (unsaturated).

Concept:

Co-ordination complexes and CFT:

• A metal ion in a complex is surrounded by coordinating ligands anions or negative ends.
• An electric field is produced at the metal ion by the surrounding ligands.
• This electrostatic approach towards the interaction between metals and ligands is known as crystal field theory.
• CFT theory considers the ligands as point charges.
• There is no overlap between the ligand orbitals and metal ion orbitals.
• Ligands donate lone pair of electrons to the metal atoms and form coordinate complexes.

Explanation:

• The metal ion has two valencies:
  • The oxidation state or the primary valency.
  • The coordination number or the secondary valency.
• The secondary valencies are satisfied by coordinating with primary ligands.
• The bond formed by the ligands and the metal ion forms a coordination sphere.
• The coordination sphere is non-ionizable in solution because of the co-ordinate bond between them.
• The oxidation number or primary valency of the metal ion is satisfied by the side ions which form the ionizable sphere.

• The compound will thus dissociate as:

\(\left[ {Co{{\left( {N{H_3}} \right)}_6}} \right]C{l_2} \to {\left[ {Co{{\left( {N{H_3}} \right)}_6}} \right]^{ + 2}} + 2Cl^-\)

Thus, the total number of ions is = 3.

Hence, the number of ions that are produced from the complex [Co(NH3)6] Cl2 in solution is 3.

[V(H₂O)₆]³⁺ → d²sp³

₂₃V :- [Ar]3d³4s²

V⁺³ :- [Ar]3d² , n = 2 (even number of unpaired e^–)

[Cr(H₂O)₆]²⁺ → sp³d²

₂₄Cr :- [Ar]3d⁵4s¹

Cr⁺² :- [Ar]3d⁴ , n = 4 (even number of unpaired e^–)

[Fe(H₂O)₆]³⁺ → sp³d²

Fe³⁺ :- [Ar]3d⁵4s⁰

n = 5 (odd number of unpaired e^–)

[Ni(H₂O)₆]³⁺ → sp³d²

Ni :- [Ar]3d⁸4s²

Ni⁺³ :- [Ar]3d⁷ , n = 3 (odd number of unpaired e^–)

[Cu(H₂O)₆]²⁺ → sp³d²

Cu :- [Ar]3d⁹4s⁰

n = 1 (odd number of unpaired e^–)