Which of the following species contains equal number of $\sigma$ and $\pi$ bonds?

Let us calculate the number of $\sigma$ and $\pi$ bonds in each species:

• $\text{HCO}_3^-$: The bicarbonate ion has the structure $\text{HO-C(=O)O}^-$. It contains $4$ $\sigma$ bonds (one $\text{C-OH}$, one $\text{C-O}^-$, one $\text{C=O}$, and one $\text{O-H}$) and $1$ $\pi$ bond (in the $\text{C=O}$ double bond).
• $\text{XeO}_4$: In Xenon tetroxide, the central Xenon atom is $sp^3$ hybridised and forms four double bonds with four oxygen atoms. Each double bond consists of $1$ $\sigma$ and $1$ $\pi$ bond. Thus, there are $4$ $\sigma$ bonds and $4$ $\pi$ bonds in total.
• $(\text{CN})_2$: Cyanogen ($\text{N}\equiv\text{C-C}\equiv\text{N}$) contains $3$ $\sigma$ bonds (one $\text{C-C}$ and two $\text{C-N}$) and $4$ $\pi$ bonds (two in each $\text{C}\equiv\text{N}$ triple bond).
• $\text{CH}_2(\text{CN})_2$: Malononitrile ($\text{NC-CH}_2\text{-CN}$) contains $6$ $\sigma$ bonds (two $\text{C-C}$, two $\text{C-H}$, and two $\text{C-N}$) and $4$ $\pi$ bonds. Therefore, $\text{XeO}_4$ is the only species that has an equal number of $\sigma$ and $\pi$ bonds.