5 Marks Questions

Question 15 Marks

Explain valence bond theory for coordination compounds.

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Question 25 Marks

Mention the IUPAC rule used to write the formulas of mononuclear coordination compounds.

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Question 35 Marks

How does colour appear in coordination compounds? Its explanation is based on crystal field theory.

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Question 45 Marks

On the basis of crystal theory, octahedral in the presence of strong field ligand and weak field ligand. Explain the difference in crystal field splitting.

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Question 55 Marks

What is crystal field theory? On the basis of this, explain the crystal field splitting in octahedral coordinaton compounds.

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Question 65 Marks

Explain the limitations of valence bound theory.

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Question 75 Marks

Magnetic properties of coordination compounds. Explain in detail.

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Question 85 Marks

Explain the IUPAC method of nomenclature coordination compounds with examples.

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Question 95 Marks

Explain with two examples each of the following coordination entity, ligand coordination number, coordination polyhedron, homoleptic and heteroleptic.

Answer

(i) Coordination Entity : The species in which a certain number of ions or molecules are attached to the central metal atom or ion by forming covalent bonds is called coordination entity. For example : $\left[ CoCl _3\left( NH _3\right)_3\right]$, the cobalt ion is surrounded by three NH₃ molecules and three chloride ions. Other examples : $[Ni(CO)_4]$ and $\left[ Co \left( NH _3\right)_6\right]^{3+}$.
(ii) Ligand : The molecules or ions attached to the central metal atom or ion in the coordination entity (complex species) are called ligands. They form covalent bonds by donating an electron pair to the metal.
Example : $Cl , H _2 O$ and $NH _3$.
(iii) Coordination Number (CN) : In a complex species, the number of donor atoms of the ligands bound to the metal which are directly attached to it is called the coordination number of the metal.
Example : The coordination number of platinum Pt in the complex ion $\left[ PtCl _6\right]^{2-}$ is 6 and that of Ni in $\left[ Ni \left( NH _3\right)_4\right]^{2+}$ is 4.
(iv) Coordination polyhedra or Coordination polyhedron : The specific arrangement in space of the ligands directly attached to the central atom in a complex species is called coordination polyhedra.
Example : $\left[ Co \left( NH _3\right)_6\right]^{3+}$ is octahedral and $\left[ Ni ( CO )_4\right]$ is tetrahedral.
(v) Homoleptic and heteroleptic complexes : Complexes in which the metal atom is attached only to one type of donor groups is called homoleptic complexes, and those complexes in which the metal atom is attached to more than one type of donor groups are called heteroleptic complexes.
Example : $\left[ Co \left( NH _3\right)_4 Cl _2\right]^{+}$.

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Question 105 Marks

Write down the IUPAC name for each of the following complexes and indicate the oxidation state, electronic configuration and coordination number. Also give stereochemistry and magnetic moment of the complex :
(i) $K \left[ Cr \left( H _2 O \right)_2\left( C _2 O _4\right)_2\right] \cdot 3 H _2 O$
(ii) $\left[ Co \left( NH _3\right)_5 Cl \right] Cl _2$
(iii) $\left[ CrCl _3( py )_3\right]$
(iv) $Cs \left[ FeCl _4\right]$
(v) $K _4\left[ Mn ( CN )_6\right]$

Answer

(i) $K \left[ Cr \left( H _2 O \right)_2\left( C _2 O _4\right)_2\right] \cdot 3 H _2 O$ : IUPAC name of the complex is - Potassium diaqua di-oxalate chromate (III) trihydrate.
Oxidation state of metal = +3(3d³)
Electronic configuration of Cr³⁺ = $t_{2 g}^3 e_g^0$
Coordination number of metal = 6
Geometry of the complex = Octahedral

This complex has geometric and optical isomerism.
Magnetic moment $(\mu)=\sqrt{n(n+2)}$ B.M.
Cr³⁺ = 3d³ , no. of unpaired electrons, n = 3
$\mu=\sqrt{3(3+2)}=\sqrt{15}$ B.M. $=3.87$ B.M.
(ii)$\left[ CrCl _3( py )_3\right]:$ IUPAC name of the complex is-Trichloridotrispyridine chromium (III)
Oxidation state of metal = +3(3d³)
Electronic configuration of Cr³⁺= $t_{2 g}^3 e_g^0$
Coordination number of metal = 6
Geometry of the complex = Octahedral
It has special type of geometric isomerism (fac and mer).
$\begin{array}{r}\text {Magnetic moment} (\mu)=\sqrt{3(3+2)}=\sqrt{15}=3.87 B . M . \\ ( n =3)\end{array}$
(iii) $\left[ Co \left( NH _3\right)_5 Cl \right] Cl _2$ : IUPAC name of the complex is-Pentamine chlorido cobalt (III) chloride
Oxidation state of metal = +3(3d⁶)
Electronic configuration of Co³⁺$=t_{2 g}^6 e_g^0$
Coordination number of metal = 6
Geometry of the complex = Octahedral
Magnetic moment $\mu$ = 0, because it has all paired electrons.
(iv)$Cs \left[ FeCl _4\right]:$ IUPAC name of the complex is-Cesium tetrachlorido ferrate (III)
Oxidation state of metal =

+3(3d⁵)
Electronic configuration of $Fe ^{3+}= e _{ g }^2 t _{2 g}^3$
Coordination number of metal = 4
Geometry of the complex = Tetrahedral
Magnetic moment $(\mu)=\sqrt{5(5+2)}$
$=\sqrt{35}=5.91$ B.M. $( n =5)$
(v)$K _4\left[ Mn ( CN )_6\right]:$ IUPAC name of the complex is-Potassium hexacyano manganate (II)
Oxidation state of metal = +2(3d⁵)
Electronic configuration of $Mn ^{2+}= t _{2 g}^5 e _{ g }^0$
Coordination number of metal = 6
Geometry of the complex = Octahedral
Magnetic Magnetic $(\mu)=\sqrt{1(1+2)}=\sqrt{3}=1.73 BM$

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Question 115 Marks

Discuss the nature of bonding in the following coordination entities on the basis of valence bond theory :
(i) $\left[ Fe ( CN )_6\right]^{4-}$
(ii) $\left[ FeF _6\right]^{3-}$
(iii) $\left[ Co \left( C _2 O _4\right)_3\right]^{3-}$
(iv) $\left[ CoF _6\right]^{3-}$

Answer

(i) [Fe(CN)₆]^4-: In (i) [Fc(CN)₆]^4- the oxidation state of Fe is 2 and coordination number is 6.
Electronic configurtion of Fe²⁺ = 3d⁶ .

CN^- is a strong ligand due to which electrons get paired in 3d orbitals in its presence.

In this way, due to vacancy of two d-orbitals, $d ^2 sp ^3$ hybridization takes place. In these $d ^2 sp ^3$ hybridized orbitals, 6CN^- donates electron pairs and forms 6 covalent bonds.
The geometry of this complex is octahedral and due to the electrons being paired, it is diamagnetic. This is called inner orbital complex or low spin complex or spin paired complex.

(ii) $\left[ FeF _6\right]^{3-}: \operatorname{In}\left[ FeF _6\right]^{3-}$, the oxidation state of Fe is +3 and the coordination number is 6.
Electronic configuration of Fe³⁺ = 3d⁵

F^- is a weak ligand and hence, in its presence electrons do not get paired.

Therefore, sp³d² hybridization takes place in this complex. These sp³d² hybridized orbitals forms 6 covalent bonds with 6F^- by donating electrons pairs. It is an octahedral complex and due to unpaired electrons present in it, it is paramagnetic. This is called outer orbitals complex or high spin complex or spin complex.

(iii) $\left[ Co \left( C _2 O _4\right)_3\right]^{3-}$ : In this complex, the oxidation state of Co is + 3 and coordination number is 6.
Electronic configuration of Co³⁺ = 3d⁶

$C _2 O _4{ }^{2-}$ is a strong ligand, hence in its presence, the electrons get paired in 3d orbitals.

Due to vacancy of two d-orbitals, d²sp³ hybridization takes place. In these hybridized orbitals 3C₂O_4^2- denotes electron pairs and form 6 covalent bonds. It is an octahedral complex and having all the electrons paired it is diamagnetic. This is called inner orbital complex.

(iv) $\left[ CoF _6\right]^{3-}$ : In $\left[ CoF _6\right]^{3-}$, the oxidation state of Co is + 3 and coordination number is 6.
Electronic configuration of Co³⁺ = 3d⁶

F^- a weak ligand hence electrons do not get paired in its presence.

Therefore, sp³d² hybridization occurs in it. These hybridized orbitals denote 6F^- electron pairs and form 6 covalent bonds. The octahedral complex is paramagnetic due to unpaired electrons and is called outer orbital complex.

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Question 125 Marks

Draw all the isomers (geometrical and optical) of :
(i) $\left[ CoCl _2( en )_2\right]^{+}$
(ii) $\left[ Co \left( NH _3\right) Cl ( en )_2\right]^{2+}$
(iii) $\left[ Co \left( NH _3\right)_2 Cl _2( en )\right]^{+}$

Answer

(i) The complex ion [CoCl₂(en)₂]⁺ shows geo-metric isomerism, hence it has cis and trans form, of these, the counterpart form or cis form is assymetric and hence shows optical isomerism.

(ii) The complex ion [Co(NH₃)Cl(en)₂]²⁺ has geometric isomerism whose cis and trans form are as follows :
Its cis form also shows optical isomerism.

(iii) The complex ion [Co(NH₃)₂Cl₂(en)]²⁺ has geometric isomerism whose cis and trans isomers are as follows :

Both its cis and trans form shows optical isomerism whose structures can be made like (i) and (ii).

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Question 135 Marks

Draw the structures of optical isomers of :
(i) $\left[ Cr \left( C _2 O _4\right)_3\right]^{3-}$
(ii) $\left[ PtCl _2( en )_2\right]^{2+}$
(iii) $\left[ Cr \left( NH _3\right)_2 Cl _2( en )\right]^{+}$

Answer

(i) [Cr(C₂O₄)₃]^3- has $C _2 O _4^{2-}$ (oxalate) ion whose symbol is used as ox. The structure of the two optical isomers of this complex is as follows :

(ii) Following are the optical isomers of $\left[ PtCl _2( en )_2\right]^{2+}$ complex ion :

(iii) Both the cis and trans isomers of [Cr(NH₃)₂Cl₂(en)]⁺ shows optical isomerism.
(a) Optical isomers of cis form are shown as follows :

(b) Optical isomers of trans form are shows as follows :

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