Question

According to the molecular orbital theory which of the following statement is incorrect?
[LUMO = lowest unoccupied molecular orbital]

• A. LUMO level for $C_2$ molecule is ${\sigma }_{2pz}$ orbital.
• B. In $C_2$ molecules both the bonds $\pi$ are bonds
• C. In $C_2^{2-}$ ion there is one $\sigma$ and two $\pi$ bonds
• D. $C_2$ is paramagnetic but $C_2^{2-}$ is diamagnetic.

Answer 1

Answer: (D)

$C_2\Rightarrow \sigma 1s^2<\stackrel{\ast }{\sigma }1s^2<\sigma 2s^2<\stackrel{\ast }{\sigma }2s^2<\frac{\pi 2p_s^2=\pi 2p_y^2}{\text{ HOMO }}<\frac{\sigma 2p_z^0}{\text{ LUMO }}$
All electrons are paired. So, the compound is diamagnetic. $C_2$ as well as $C_2^{2-}$ is diamagnetic. More over, both bonds in $C_2$ will be pi. In $C_2^{2-}$, there will be one sigma bond in addition to two pi bonds.