In the resonance hybrid structures, dashed-line bonds are used to indicate double or triple bond character between the bonded atoms. These partial multiple bonds represent the expanded `pi-bond` overlap. In each resonance contributing structures, the negative charge resides on a different `O` atom But in the hybrid structures, each `O` atom has a fractional negative charge.
(ii) `NO_2^(Ө)` : In `NO_2^(Ө)`, `N` has three `sp^2` hybrid orbitals. Two of them from `sigma-bonds` with the two `O` atoms and third `sp^2` `HO` has `LP overline e^, s`. Its pure `p-AO` overlaps laterally with a pure `p-AO` of each `O`atom, resulting in extended `pi-bond` overlap, comprising both the `O`atoms and the `N` atom, spreading charge over both the `O` atoms.
(FIg) Overlapping atomic orbitals `NO_3^(Ө)` is similar to `NO_2^(Ө)` except that a third `O` atom is involved in the extended `pi-bond` with the delocalisation of the negative charge over three `O` atoms, resulting a `(-2//3)` charge on each `O` atom [Fig. 4.16(b)]
(iii) `NO_3^(Ө)` has more delocalisation (or resonance) energy and is more stable than `NO_2^(Ө). NO_3^(Ө)` has three resonating structures in which negative charge is delocalised over all the three `O` atoms, whereas `NO_2^(Ө)` has two resonating structures in which negative charge is delocalised over two `O` atoms.
The energy of the hybrid is always lower than that of any cannonocal structures. The greater the number of resonance contributing structures with similar energies, the more stable and lower is the energy of the hybrid. More similar energy resonance contributing structures result in more extanded `pi-bonding`. Which allows the `overline e^, s` to move in a larger space and decreases `overline e^, s` repulsion.
(b) (i) and (ii)
Resonance structures of `(N_2O) (I)` :
`underset((I))underset(sp)underset(uarr)( :N)-=underset(sp)underset(uarr)overset(o+)(N)-underset(sp^(3))underset(uarr)(ddotunderset(..)O:^(Theta))harr underset((II))underset(sp^(2))underset(uarr)( :overset(Theta)(N))=underset(sp)underset(uarr)overset(o+)(N)=underset((III)("Hybride structures"))(underset(sp^(2))underset(uarr)ddotO:-=underset(sp)underset(uarr)overset(-delta)(N)overset(...)(=)underset(sp)underset(uarr)overset(o+)Noverset(...)(-) underset(sp^(2))underset(uarr)overset(-delta)underset(..)O: )`
Resonance structure of `(H_2 C-N_2)(II)` :
`underset((IV))underset(sp)underset(uarr)( H_(2)C)-=underset(sp)underset(uarr)overset(o+)(N)-underset(sp^(2))underset(uarr)(ddotN:^(Theta))harr underset((V))underset(sp^(3))underset(uarr)(H_(2)overset(ddot(Theta))C)-underset(sp)underset(uarr)overset(o+)N-=underset((VI)("Hybrid structures"))(underset(sp)underset(uarr)ddotN-=underset(sp^(2))underset(uarr)overset(-delta)(H_(2)C)=underset(sp)underset(uarr)overset(o+)Noverset(...)(=)underset(sp)underset(uarr)overset(delta-)N: )`
(iii) Bond lengths in hybrid structures are intermediate in value between those all the contributing structures (i.e., standard single, double, or driple bond (value). In `N_2O`, the `(O - N)` bond length in the hybrid (III) is shorter than the single bond in `(I)` but longer then the double bond in `(II)`. The `(N - N)` bond length in the hybrid `(III)` is longer than in `(I)` but shorter than in `(II)`. In `(H_2 C - N_2)`, the `(C - N)` bond length in the hybrid `(VI)` is longer than double bond in `(IV)` but shorter than the single bond in `(V)`
The `(N - N)` bond length in the hybrid `(VI)` is shorter than double bond in `(IV)` but longer than the triple bond in `(V)`.
The `(H - C - H)` bond angle expected form `(IV)` is `120^@` and from `(V)` is `105.5^@`. The actual bond angle has some intermediate value because the hybrid is a mixture of each resonating structure.
For maximum extended `pi-`bonding, the atoms in the hybrid structure should have `p-AOs`. In other words, the hybrid orbitals used to form `sigma-`bonds should have less`p` character.
Therefore, `C` should be `sp^2` - like rather than `sp^3`, and the terminal `N` should be `sp`- like rather than `sp^2`
Since the resonating structure of both `N_2 O` (I and II) and `CH_2N_2` (IV and V) are equally stable, so the hybrid (III) (for `N_2 O`) and hybrid `(VI)` (for `H_2 CN_2`) is a good mixture (blend) of their corresponding resonating structures.
