Chapter 3: – Chemical Bonding
B.sc 1st year Book
(Page 7)

Odd electron bond

Lewis (1916) proposed that there are a number of molecules or ions that are formed by sharing one, three, or five electrons between two or more than two atoms or ions, such bands are called “odd electron bonds” and the molecules or ions containing such bond’s as “odd electron molecule or odd molecule”. Thus, odd electron bonds are a type of covalent bonds which are formed by sharing an odd number of electrons between two atoms or ions since they have an odd number of electrons hence they show paramagnetic behavior. The two atoms or ions forming odd electron bonds may be similar or dissimilar e.g. H2+, L2+, He2+, N2+, O2, O2+, O2, NO, NO−, ClO2, etc.

One electron bond :

It is formed by the sharing of one electron between two combined atoms or ions of nearly equal electronegativity value. e.g. ln⁡H2+ion H-atom shares one electron but H+ion does not. Thus H2+ ion is an example of a one-electron bond. Since this ion has only one electron hence it shows paramagnetic behavior. According to valency bond theory H2+, ion shows a resonance structure a and b as depicted below.
H2+ ion:
[H∗⋅H]⟹[H⋅H+]⟹[H⋅H+ (Bond order of H2+inn is 0.5)]
(a) <—–>  (b) =>Reserince hybrid
The bond energy of the H2+ ion is 61Kcal/ mole whereas the H2 molecule has bond energy equal to 109Kcal/mole and their bond distances are 1.06 A∘ and 0.74 AC respectively. Thus, the odd bond distance is longer than a normal covalent bond. Other examples are U2+Na2+, K2+, B2H6, etc in which one electron bonds are present. Due to one electron bond, such molecules or ions are not stable but are very reactive and show paramagnetic behavior.

Three electron bonds:

The molecules like H2∼He2+, O2, O2+, O2−N2+, NO, NO−, NO2, ClO2, BrO2, etc. are examples of odd electron molecules or ions because they are formed by three electron bonds. These molecules also possess resonance structures. e.g.

  1. He2+ ion :

    This ion is represented as a resonance hybrid of (a) and (b) respectively.
    [He*He+]<------> [He+ x He]The bond energy of one electron bond and three electron bonds ars nearly identical. The bond length of the He2+ ion is equal to 58Kcal/mole.

  2. O2 molecule :

    Generally, it is considered that the O2 molecule is formed by sharing of two electrons each: between two oxygen atoms, Thus, the O2 molecule should be diamagnetic, while this molecule Is paramagnetic. It is, therefore, assumed that in this molecule two electrons are involved in a single covalent bond formation between the two O-atoms, and the remaining 6 electrons (three of each O-atom) form two 3 electrons bonds. as shown below:

    (a) O2 Molecule with 2 - cloctron (b) O2 Molecule vith two 3. elactron two centre bond. two cantre band.
    Thus, this molecule is called a double odd molecule. The paramagnetic behavior of this molecule is also confirmed by M.O.theory, The molecular orbital electronic configuration of the O2 molecule shows there are two unpaired electrons present in antibonding p-molecular orbitals as shown below:
    O2=(σ1s)2(σ∗1s)2(σ2s)2(σ∗2s)2(σ2px)2(π2py)2=(π2pz)2(π∗2py)1=(π∗2pz)1

    1. NO molecule :

      Nitric oxide is the most stable odd molecule. Its valence bond representation is given as :NO2 Molecules electron distribution
      Structure(b) is expected to be less stable as compared to structure(a) because of the distribution of unfavorable charges, i.e., we are placing a positive charge on a more electronegative oxygen atom. However, structures(a) and (b) both have nearly equal stability and thus there is complete resonance(structure c) between the two structures. This resonating hybrid structure shows NO molecule consists one double bond and one 3-electron bond. Thus, due to the presence of one odd electron bond NO molecule is paramagnetic:

    2. ClO2 molecule:

      This molecule exists in two resonating forms, both forms have equal stability. These structures shows of the ClO2 molecule consists of one 3 – electrons bond between either one of the Cl−O bond: In this case, the central Cl atom contributes one electron and one of the neighboring oxygen atoms contributes two electrons in odd bond formation,ClO2 molecule formation
      Thus, this molecule is paramagnetic.

    3. NO2 molecule :

      This molecule also exists in two resonating forms, both forms have equal stability, In this case, the central N atom contributes two electrons, and one of the neighboring oxygen atoms contributes only one electron in odd bond formation.

NO2 molecule formation

Related Topic | Chemical Bonding
Chemical Bonding Inert Pair Effect
Lattice energy Fajans Rule
Covalent Bond Coordinate Bond
Odd electron bond Metallic bond
Hydrogen Bonding M.O. Theory
Sidgwick-Powell theory VSEPR theory
Hybridization of atomic orbitals

Characteristics of three electron bonds:

(i) It has been found that three electrons bond can never be formed if the electronegativity difference exceeds 0.5. If there is a large difference in electronegative value the molecule will be unstable.
(ii) Three_electron bond must have a greater bond length as compared to two electrons bond and possess lesser bond energy.
(iii) This type of bond forms a hybrid between two extreme structures.

[A⋅0×B1→[A⋅x×B∥1]

(iv) Three electrons bond molecules have a tendency to dimerize. e.g. NO2 dimerizes into N2O4 and ClO2 dimerizes into Cl2O4.
(v) The three electrons borid is one and a half as an electron pair bond.
(vi) An odd bond is mostly equivalent to one-half of the normal covalent bond.
(vii) Most of the compounds which have three electrons bond are colored and show paramagnetic behavior.
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