Which of the following is a $\pi$-complex?

Explain the Grignard reaction in detail.

The $C-O$ bond distance in $[V(CO)_6]$ and $[V(CO)_6]^-$ are:

Which of the following compounds is used as an antiknock agent?

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 $\pi$-bonded organometallic compound which has ethene as one of its components is