Nature of a Chemical Bonds B.Sc. 2nd Year Notes

Nature of a Chemical Bonds B.Sc. 2nd Year Notes

Nature of a Chemical Bonds

Sigma Bond (σ Bond)

When the overlapping of orbitals of two atoms takes place in face to face or head on manner along the molecular axis at an angle of 180o, the covalent bond formed is called σ Bond.
sigma bond is formed by overlapping between s-s, s-pz, pz-pz, spn-spn, spn-s, pz-dz2, dx2-y2-dx2-y2 and dz2-dz2 orbitals,(where z is defined as the axis of the bond or the internuclear axis). Sigma bond formed by overlapping between s-s orbital is weakest as s-orbital is direction less.
Sigma bonds are the strongest type of covalent bonds due to the direct overlap of orbitals, and the electrons in these bonds are sometimes referred to as sigma electrons.
sigma bond

Pi Bond (π Bond)

When the overlapping of orbitals of two atoms takes place in sidewise or in lateral manner along the molecular axis at an angle of 45o, the covalent bond formed is called π Bond.
π Bonds are generally weaker than σ bonds, due to the lower degree of overlapping. Generally, double bonds consist of one σ and one π bond, whereas a typical triple bond is made up of two π bonds and one σ bond. It is important to note that a combination of s&igma; and π bonds is always stronger than a single sigma bond.
Pi bonds are more diffuse bonds than the σ bonds and electrons in π bonds are sometimes referred to as pi electrons. Hence, π bond is more reactive than σ bond.
A pi bond can exist between two atoms that do not have a net sigma-bonding effect between them.
pi bond

Tau Bond (𝜏 Bond)

Tau bond is a type of covalent chemical bond with a geometry somewhat reminiscent of a banana. So It is also called Banana bond or Bent bond. It is a type of chemical bonding where the ordinary hybridization state of two atoms making up a chemical bond are modified with increased or decreased s-orbital character in order to accommodate a particular molecular geometry.
The formation of tau bond is observed in the dimerization process of BH3 to B2H6.
In B2H6 there are 12 valence electrons. Out of 6H, 4H (i.e. terminal H) are bonded covalently with two Boron atoms hence 8 electrons are used. The remaining 4 electrons are shared between the two bridging H atoms and the two B atoms consequently two bridging B-H-B bonds are formed each of which consists of 2 electrons( 2 electrons are shared with 3 atoms i.e. 3C-2e Bond) which is sometimes called 'banana bond', as they are not linear but curved.
banana bond

Delta Bond (δ Bond)

When four lobes of one d- orbital overlap with four lobes of the other d- orbital in sidewise manner, the covalent bond formed is called delta(δ) bond and is also called quadruple bond.
This overlap leads to the formation of a bonding molecular orbital with two nodal planes which contain the internuclear axis and go through both atoms.Delta bonds are weaker than sigma bonds but stronger than pi bonds.
Delta bonds are usually observed in organometallic species. Some ruthenium and molybdenum compounds contain a quadruple bond, which can only be explained by invoking the delta bond. Common example of delta bond is [Re2Cl8]2-.
If we consider z- axis as internuclear axis between two Re- metals then (dx2-y2, dxy) will form delta bond because four lobes interaction will be there in that case and dz2 will form sigma bond and dxz and dyz will form pie bond.
delta bond

Hydrogen Bond

Hydrogen bond is an attractive force between the hydrogen atom of one molecule and more electronegative atoms(i.e. N, O & F) of the same molecule or other molecules.It is weaker than a ionic and covalent bond while stronger than vander waal force of attraction. Hydrogen bond is denoted by a dotted line (.....).
Hydrogen bond is of two types. One is intermolecular hydrogen bond and other is intramolecular hydrogen bond. Intermolecular hydrogen bond is formed between at least two same or different molecules (e.g. H2O, NH3, HF etc.) while intramolecular hydrogen bond is formed within a single molecule (e.g. o-nitrophenol, salicylic acid or aldehyde etc.).
hydrogen bond
Hydrogen bond affect the physical properties of the molecules. Some of important properties are given below-
1. Boiling and Melting Point
Boiling and Melting points increases due to hydrogen bond as H-bond increases the intermolecular force of attraction among the molecules as well as size of the molecules.
2. Density
The compounds have greater extent of H-bond between molecules, increases the density of the compounds.
3. Surface Tension and Viscosity
Surface tension and Viscosity also increases as the molecules on the surface are bound to one another via H-bonds.
4. Molecular association
Compounds having hydrogen bonds form aggregates as a result of a weak association between molecules. The formation of dimers and trimers in different compounds occurs due to hydrogen bonding.
5. Solubility
Hydrogen bonding is responsible for the solubility of different compounds as it is involved in the hydration of anions in aqueous solutions.
6. Trouton's rule
Statement: Entropy of vaporization is almost same (about 85 - 88J/mol/K1 ) for different type of liquids at their boiling points.
Molecules having hydrogen bond does not follow Trouton's rule because due to hydrogen bonding the boiling point of the compound changes and hence entropy of vaporization changes.

Q. The liquid that deviates from the Trouton’s rule is

a. Hydrochloric acid
b. Sulphuric acid
c. Phosphoric acid
d. Acetic acid

Structure of Copper(II) Acetate

Copper(II) Acetate
Copper(II) acetate, also called cupric acetate and is chemical compound having molecular formula Cu2(CH3COO)4. The hydrated derivative, which contains one molecule of water for each Cu atom, is available commercially. Cu2(CH3COO)4 is a dark green crystalline solid, whereas Cu2(CH3COO)4(H2O)2 is more bluish-green. Cu2(CH3COO)4 is used as a source of copper(II) in inorganic synthesis and as a catalyst or an oxidizing agent in organic synthesis. Copper acetate, like all copper compounds, emits a blue-green glow in a flame.
Cu2(CH3COO)4(H2O)2 adopts the "Chinese lantern" structure. One oxygen atom on each acetate is bound to one copper at 1.97 Å. Completing the coordination sphere are two water ligands, with Cu-O distances of 2.20 Å. The two five-coordinate copper atoms are separated by only 2.65 Å, which is close to the Cu--Cu separation in metallic copper. The two copper centers interact resulting in a diminishing of the magnetic moment such that near 90 K, Cu2(CH3COO)4(H2O)2 is essentially diamagnetic due to cancellation of the two opposing spins. Cu2(CH3COO)4(H2O)2 was a critical step in the development of modern theories for antiferromagnetic coupling.

Structure of Diborane

The molecular formula of diborane is B2H6. Out of six hydrogen, four hydorgen atoms are terminal, while two hydrogens are in bridge between the boron centers. The lengths of the B–H bridge bonds and the B–H terminal bonds are 1.31 and 1.19 Å respectively. The B–H bridge bond is weaker than B–H terminal bonds.
The bonds between boron and the terminal hydrogen atoms as conventional 2-center 2-electron (2c-2e) covalent bonds. The bonding between the boron atoms and the bridging hydrogen atoms is, however, different from that in molecules such as hydrocarbons. Each boron uses two electrons in bonding to the terminal hydrogen atoms and has one valence electron remaining for additional bonding. The bridging hydrogen atoms provide one electron each. The B2H2 ring is held together by four electrons forming two 3-center 2-electron bonds. This type of bond is sometimes called a "banana bond".
Bonding and Structure of diborane
The orbital theory explains its formation, in which boron atoms show sp3 hybridisation. Because three electrons are available hence out of four sp3 hybrid orbitals, one sp3 hybrid orbital is empty. There is one s-orbital of hydrogen atom. Now two sp3 hybrid orbitals of one boron atom overlap with two s-orbitals of two hydrogen atoms and form sp3 - s bonds with hydrogen atoms which are known as terminal hydrogen atoms in the diborane. The third hydrogen atom form bond with one sp3 hybrid orbital of the first boron atom and one sp3 hybrid orbital of the second boron atom which is empty hybrid orbital. This concept of three center bond formation in which the three nuclei are bonded by two electrons hence also known as 3c-2e bond. Such type of overlapping cause of banana type bond.
Bonding and Structure of diborane

Q. The bonding in diborane (B2H6) can be described by

a. two centre - two electron bonds 2 three - centre - two electron bonds
b. two centre - two electron bonds 3 three centre - two electron bonds
c. two centre - two electron bonds and 4 three centre - two electron bonds
d. two centre - two electron bonds and 4 two centre - two electron bonds