Which of the following species has a bond order of $2.5$ and is paramagnetic?

Solution

1. The molecular orbital configuration for ${\text{O}}_2^{+}$ (15 electrons) is $(\sigma 1s{)}^2({\sigma }^{\ast }1s{)}^2(\sigma 2s{)}^2({\sigma }^{\ast }2s{)}^2(\sigma 2p_z{)}^2(\pi 2p_x{)}^2(\pi 2p_y{)}^2({\pi }^{\ast }2p_x{)}^1$.
2. The bond order is calculated as $$\begin{array}{rl}\frac{1}{2}(N_b-N_a)& =\frac{1}{2}(10-5)\\ & =2.5\end{array}$$.
3. The presence of one unpaired electron in the ${\pi }^{\ast }2p_x$ orbital makes it paramagnetic.
4. Hence the answer is (A).