What is the oxidation state of the metal in metal carbonyls?

Which of the following statements is incorrect?
$(1)$ $Al_4C_3$ is an organometallic compound.
$(2)$ Metal carbonyls are organometallic compounds.
$(3)$ $TEL$ (Tetraethyl lead) is a $\pi$-bonded organometallic compound.
$(4)$ Frankland reagent is a $\sigma$-bonded organometallic compound.

Tetraethyl lead is used as

What are organometallic compounds? Explain in detail.

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

Carbon monoxide reacts with iron to form