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 $\sigma _{2 pz}$ 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)

$C_{2} \Rightarrow \sigma 1 s^{2}< \overset{*}{\sigma} 1 s^{2}<\sigma 2 s^{2}< \overset{*}{\sigma} 2 s^{2}<\frac{\pi 2 p_{s}^{2}=\pi 2 p_{y}^{2}}{\text { HOMO }}<\frac{\sigma 2 p_{z}^{0}}{\text { LUMO }}$
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.