Ncert Solutions Chemistry Class 12th

14.29

Nucleic acids are Biomolecules which are found in the nuclei of all living cells, inform of nucleoproteins or chromosomes (proteins containing nucleic acids as the prosthetic group). Nucleic acids are of two types: – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleic acids are also known as Polynucleotide as they are long- chain polymers of nucleotides.

The two important functions of nucleic acids are listed below:-

(i) DNA which is responsible for the transference of hereditary effects from one generation to another, which is due to their property of replication during cell division as a result of which two identical DNA strands are transferred to the daughter

(ii) Nucleic acids (both DNA and RNA) are responsible for synthesis of all proteins needed for the growth and maintenance of our body. Actually, the proteins are synthesised by various RNA molecules in the cell but the message for the synthesis of a particular protein is given by DNA molecules

In [Fe (H₂O)₆]³⁺

Electronic configuration of Fe is: [Ar]3d⁶4s²

[Ar] = 1s²2s²2p⁶3s²3p⁶

Electronic configuration of Fe⁺³ = [Ar]3d⁵

Outer electronic configuration of Fe⁺³ = 3d⁵

Overall charge balance:

X + 6 (0) = 3

X = + 3

In [Fe (CN)₆]^3-

Overall charge balance:

X + 6 (-1) = -3

X = + 3

In both the compounds Fe is in + 3 oxidation state.

In case of [Fe (H₂O)₆]³⁺

H₂O is weak field ligand so it does not pair the unpaired electron. Total no. of the unpaired electron, n =5, Spin only magnetic moment is given by:

μ = [n (n + 2)]^1/2

μ = [5*7]^1/2

μ = 5.916BM

In case of [Fe (CN)₆]^3-

CN^- is a strong field ligand so it pairs up the electron.

Total no. of unpaired electrons = 1

Spin only magnetic moment is given by:

μ = [n (n + 2)]^1/2

μ = [1*3]^1/2

μ = 1.732BM

as we can see spin only magnetic moment of [Fe (H₂O)₆]³⁺ is more than [Fe (CN)₆]^3- .

so, [Fe (H₂O)₆]³⁺ is strongly paramagnetic whereas [Fe (CN)₆]³ weakly paramagnetic.

Fe in ground state

Fe in + 3 oxidation state

Electrons from 5 CN^- ligands

In [Ni (Cl)₄]^2- ion, Cl^- is a weak field ligand so it will not pair the unpaired electrons of Ni⁺² ion. Electronic configuration of Ni is: [Ar]3d⁸4s² where [Ar] = 1s²2s²2p⁶3s²3p⁶

Electronic configuration of Ni⁺² = [Ar]3d⁸ Outer electronic configuration of Ni⁺² = 3d⁸ Overall charge balance:

X + 4 (-1) = -2 X = + 2.

Therefore it undergoes sp³ hybridization. So it will have tetrahedral geometry.

Since there are 2 unpaired electrons in the d orbital so it is a paramagnetic compound. In [Ni (Co)₄]:

Overall charge is neutral and oxidation state of Ni can be calculated as:

X + 4 (0) = 0

x = 0

Ni is in zero oxidation state.

Co is a strong field ligand it causes pairing of the 4 unpaired electrons in d orbital. Also, it causes the 4s electrons to shift to the 3d orbital, thereby undergoes sp³ hybridisation forming tetrahedral geometry. Since it has no unpaired electron, therefore it is diamagnetic compound.

According to the valence band theory, the central metal atom or ion under the influence of ligands can use its (n-1)d, ns, np (inner orbital complex) or ns, np, and (outer orbital complex)orbitals for hybridisation to form equivalent set of orbitals of definite geometry.

In [Ni (CN)₄]^2-, oxidation state of Ni can be calculated as :

Using overall charge balance as the whole ion has overall -2 charge:

x + 4 (-1) = -2 (? CN^- has -1 negative charge) x = + 2

Ni is in + 2 oxidation state.

Electronic configuration of Ni is: [Ar]3d⁸4s² Where, [Ar] = 1s²2s²2p⁶3s²3p⁶

Electronic configuration of Ni⁺² = [Ar]3d⁸ Outer electronic configuration of Ni⁺² = 3d⁸

Since there are 4 CN ions so they can either form tetrahedral or square planar geometry. And CN^- is a strong field ligand (according to experimental data of spectro-chemical series) it causes pairing of the 2 unpaired electrons.

It undergoes dsp² (one d orbital, one s and two p orbitals used by the ligands) hybridization and forms square planar structure.

Since all the electrons are paired so it is diamagnetic.

Paramagnetic compounds- those compounds which have one or more no. of unpaired electrons in their atomic orbitals.

Diamagnetic compounds-those compounds in which all the electrons in their atomic orbitals are paired.

In case of [Ni (Cl)₄]^2- ion, Cl^- is a weak field ligand so it will not pair the unpaired electrons of Ni⁺² ion. Therefore it undergoes sp³ hybridization.

Overall charge balance: X + 4 (-1) = -2

X = + 2.

Since there are 2 unpaired electrons in the d orbital so it is a paramagnetic compound.