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

CONCEPT:

Isomerism in Coordination Complexes

• Tetrahedral complexes have symmetrical geometry, and due to their symmetry, they cannot exhibit geometric isomerism. This means tetrahedral complexes can exist only in one form.
• Square planar complexes, on the other hand, can exhibit geometric isomerism. For example, in a complex of the type [MA₂B₂], there can be two isomers: cis (where the same ligands are adjacent) and trans (where the same ligands are opposite).

EXPLANATION:

• "Tetrahedral complex can exist only in one form" - This is correct because tetrahedral complexes are symmetrical and cannot form isomers.
• "Square planar complex can exist in two isomeric forms" - This is also correct because square planar complexes can exhibit cis and trans isomerism.
•  "Tetrahedral complex can exist only in one form and Square planar complex can exist in two isomeric forms" - This combines the correct statements from Options 1 and 2, making this the correct choice.
•  "None of these" - This is incorrect because Options 1 and 2 are valid statements.

Therefore, the correct answer is Option 3: "Tetrahedral complex can exist only in one form and Square planar complex can exist in two isomeric forms."

CONCEPT:

Isoelectronic Species

• Two species are said to be isoelectronic if they have the same number of electrons.
• This includes atoms or ions, regardless of their chemical nature.
• To find the total number of electrons:

  Total Electrons = Atomic Number ± Charge

EXPLANATION:

• Sn₉⁴⁻:
  • Atomic number of Sn (in carbon Family)
  • Charge = 4− → Add 4 electrons
  • Total electrons = 9x4 + 4 = 40
• Bi₉⁵⁺:
  • Atomic number of Bi (Nitrogen family)
  • Charge = 95+ → Remove 5 electrons
  • Total electrons = 9x5 - 5 = 40

Therefore, the isoelectronic species are  Sn94-, Bi95+

CONCEPT:

Metal Clusters

• Metal clusters are chemical species in which two or more metal atoms are directly bonded together.
• These are commonly formed by transition metals that:
  • Have partially filled d orbitals
  • Exhibit multiple oxidation states
  • Show a high tendency for metal–metal bonding
• Clusters can be stabilized by ligands like CO, halides, etc.

EXPLANATION:

• Niobium (Nb), Molybdenum (Mo), and Technetium (Tc) belong to the middle of the d-block (groups 5–7).
• These elements have a high tendency to form stable metal–metal bonds, especially in their carbonyl and halide complexes.
• Examples include:
  • [Nb₆Cl₁₂]²⁻: a known niobium cluster
  • [Mo₆Cl₁₄]²⁻: molybdenum cluster with octahedral Mo–Mo bonding
  • Tc carbonyl clusters: like [Tc₂(CO)₁₀], [Tc₆(CO)₁₅]

Therefore, the metals with the greatest tendency to form metal clusters are:
Nb, Mo, Tc

CONCEPT:

Group IIB Elements and Their Coordination Behavior

• Group IIB includes Zinc (Zn), Cadmium (Cd), and Mercury (Hg).
• While all form +2 oxidation state, their tendency to form coordination compounds varies due to:
  • Atomic size and polarizability
  • Softness or hardness of the metal ion
  • Type of ligand (hard vs soft)
• Hg²⁺ is a soft Lewis acid and forms very stable complexes with soft ligands like CN⁻, S²⁻, and phosphines.
• This high affinity for soft ligands makes mercury form more stable and varied coordination compounds than Zn or Cd.

EXPLANATION:

• Zinc and cadmium, on the other hand, have smaller atomic radii and do not exhibit such a pronounced tendency to form coordination compounds as mercury does.
• Zn²⁺ and Cd²⁺ are hard acids → form weaker complexes with soft ligands.
• Hg²⁺ being larger and more polarizable (soft acid) → forms strong coordination complexes especially with soft ligands.
• Examples: [Hg(CN)₄]²⁻, [Hg(SCN)₄]²⁻

Therefore, among group IIB elements, mercury (Hg) has the greatest tendency to form coordination compounds, especially with soft ligands.

CONCEPT:

High Spin vs Low Spin Complexes

• The spin state of a complex depends on:
  • The metal's oxidation state
  • The ligand's field strength (spectrochemical series)
• High spin complexes occur with weak field ligands (like Cl⁻, F⁻, H₂O).
• Low spin complexes occur with strong field ligands (like CN⁻, bpy, CO).
• In high spin complexes, electrons occupy higher energy orbitals before pairing, resulting in more unpaired electrons.

EXPLANATION:

• [Mn(CN)₆]⁴⁻:
  • Mn²⁺ (3d⁵) with strong field ligand CN⁻ → Low spin 
    
• [Fe(CN)₆]³⁻:
  • Fe³⁺ (3d⁵) with strong field ligand CN⁻ → Low spin 
    
• [Fe(bpy)₃]³⁺:
  • Fe³⁺ (3d⁵) with strong field ligand bpy → Low spin 
• [FeCl₆]³⁻:
  • Fe³⁺ (3d⁵) with weak field ligand Cl⁻ → High spin 
    

Therefore, the high spin complex is [FeCl₆]³⁻

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 

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-.

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-

​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^–)