Which of the following is an organometallic compound?

Assertion : $C-O$ bond in metal carbonyls is long.
Reason : There is delocalisation of electrons from filled $d$ orbitals into the empty $\pi^*$ orbitals on the $CO$ ligands.

Given below are two statements:
Statement $I$: The $M - C \sigma$ bond is formed by the donation of a lone pair of electrons on the carbonyl carbon into a vacant orbital of the metal.
Statement $II$: The $M - C \pi$ bond is formed by the donation of a pair of electrons from a filled $d$-orbital of the metal into the vacant antibonding $\pi^*$ orbital of carbon monoxide.
In the light of the above statements, choose the correct answer from the options given below:

The chemical formula of ferrocene is:

Among the following,which is not the $\pi$-bonded organometallic compound?

$CH_3-Mg-Br$ is an organometallic compound because...