Class 12th

Ligands are the neutral or negatively charged entities surrounding the central metal atom of the coordination complex which possesses at least one unshared pair of electrons.

Based on the number of donor sites of these ligands, Ligands are classified as:

Unidentate ligands: These Ligands which have only one donor site are called unidentate ligands.

Example: F^-, Cl ^– etc.

Didentate ligands: These Ligands which have only two donor site are called didentate ligands.

Example: Ethane-1,2-diamine, Oxalate ion etc.

Ambidentate ligands: These ligands which can attach them with the central metal atom by two different atoms are called as ambidentate ligands.

14.33

The different types of RNA found in the cell are listed below:-

(i) Messenger RNA (m-RNA)

It carries the genetic message code from the DNA to ribosomes. It is produced by the DNA; m-RNA is also single stranded and constitutes about 15% of total RNA.

(ii) Ribosomal RNA (r-RNA)

It is found in the ribosomes and it is usually associated with protein to form the ribosomes. It is synthesised in the nucleus by DNA. It is single stranded, comprising about 80% of total RNA. It is metabolically stable.

(iii) Transfer RNA (t-RNA)

It is synthesised in nucleus by DNA. It is also called soluble RNA. It is single stranded. There are 20 different kinds of t-RNA and each type has specificity for a particular amino acid. It constitutes about 5% of total RNA. It has very short life.

(1) Coordination Entity: Coordination entity is a charged entity having positive or negative charge in which the central atom is surrounded by molecules which may be neutral/negatively charged called Ligands

Examples:

i. Cationic Complexes: [Cu (H₂O)₆]²⁺, [Al (H₂O)₆]³⁺

ii. Anionic Complexes: [CuCl₄]^2-, [Al (H₂O)₂ (OH)₄]^-

iii. Neutral Complexes: [Co (NH₃)₄ Cl₂], [Ni (CO)₄]

(2) Ligands: Ligands are the neutral or negatively charged entities surrounding the central metal atom of the coordination complex which possesses at least one unshared pair of electrons

Example: F^-, Cl^-, Br^-, I^-, H₂0, and NH₃

(3) Coordination Number: Coordination Number which is also called as Ligancy is the total number of ligands that are attached to the central metal atom of the coordination

Example: [Cr (NH₃)₂Cl₂Br₂]^? has Cr³⁺ as its central cation, and has a coordination number of 6

Al³⁺ has coordination number 4 in [AlCl₄]^- but 6 in [AlF₆]^3-.

(4) Coordination polyhedron: Coordination polyhedron is defined as the spatial arrangement of the ligands around the central atom of a coordination

Example:

In the figure: The pink Sphere depicts the central atom of the coordination entity.

(5) Homoleptic complexes: Complexes in which the central metal atom is surrounded only by the same kind of donor groups which are the ligands.

Example: [Ag (CN)₂]^-, [Fe (CN₆)]⁴⁺ etc.

(6) Heteroleptic complexes: Complexes in which the central metal atom is surrounded by more than one kind of donor groups which are the ligands.

Example: [Co (NH₃)₄ Cl₂] ⁺, [Co (NH₃)₅ Cl]²⁺

Bonding in coordination compounds in terms of Werner's postulates is explained as:

(a) Metals can show two types of valencies which are Primary valency and Secondary

1. Primary Valency: Primary Valency shows Oxidation Primary valencies are ionizable.

2. Secondary Valency: Secondary Valency shows coordination These are non-ionizable.

(b) Both Primary and secondary valency of the metal are to be satisfied which is done by negative ions in case of primary valency and negative or neutral species in case of secondary

(c) Metals have a fixed number of secondary valencies/ Coordination number around the central atom, these secondary valencies are placed in such a way which leads to a specific geometry of the coordination

14.31

In the helical structure of DNA, the two strands are held together by hydrogen bonds between specific pairs of bases.

Cytosine from hydrogen bond with guanine, while adenine forms hydrogen bond with thymine. As a result, the two strands are complementary to each other.

DNA consists of two strands of nucleic acid chains coiled around each other in the form of a double helix. The base of one strand of DNA is paired with bases on other strand by means of hydrogen bonding.

This hydrogen bonding is very specific as the bases can only base pair in a complementary manner.

Adenine pairs with only thymine via 2 hydrogen bonds and guanine pairs with cytosine through 3 hydrogen bonds. 

Thus, the two strands of DNA are complementary to each other in the sense that the sequence of bases in one strand automatically determines that of the other. So the DNA stands cannot be identical, but they are complementary to each other.