Which of the following organometallic compound is a $\sigma$ and $\pi$-bonded?

Which of the following organometallic compound is a $\sigma$ and $\pi$-bonded?
  1. $\left[\mathrm{Fe}\left(\eta^{5}-\mathrm{C}_{5} \mathrm{H}_{5}ight)_{2}ight]$
  2. $\mathrm{K}\left[\mathrm{PtCl}_{3}\left(\eta^{2}-\mathrm{C}_{2} \mathrm{H}_{4}ight)ight]$
  3. $\left[\mathrm{Co}(\mathrm{CO})_{5} \mathrm{NH}_{3}ight]^{2+}$
  4. $\left[\mathrm{Fe}\left(\mathrm{CH}_{3}ight)_{3}ight]$

Solution

$\left[\mathrm{Co}(\mathrm{CO})_{5} \mathrm{NH}_{3}ight]^{+2}$ is $\sigma$ and $\pi$ bonded. In this organometallic compound,bond between cobalt and CO ligands have both $\sigma$ and $\pi$ character. $\sigma$-bond formation between metal and carbon of CO takes place by overlap of filled bonding $\pi_{2 \mathrm{p}}$ of CO with an empty metal $\mathrm{d}$ - orbital. The 2nd overlap takes place between metal d- orbital with an empty antibonding pi orbital of the $\mathrm{CO}$ resulting in additional $\pi-$ bond between the metal and same CO molecule. .

Asked in: JEE-TOPICTESTS-CHEMISTRY

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