In which of the following compounds is the oxidation state of the transition element zero?

The correct statements about metal carbonyls are:
$A$. The metal-carbon bonds in metal carbonyls possess both $\sigma$ and $\pi$-character.
$B$. Due to synergic bonding interactions between metal and $CO$ ligand, the metal-carbon bond becomes weak.
$C$. The metal-carbon $\sigma$ bond is formed by the donation of a lone pair of electrons on the carbonyl carbon into a vacant orbital of metal.
$D$. The metal-carbon $\pi$ bond is formed by the donation of electrons from a filled $d$-orbital of metal into a vacant $\pi^*$ orbital of $CO$.
Choose the correct answer from the options given below:

Which one of the following is not an organometallic compound?

The correct sequence of $CO$ bond order in the given compounds is:
$(P) \ Fe(CO)_5, (Q) \ CO, (R) \ H_3B \leftarrow CO, (S) \ [Mn(CO)_5]^-$

Which of the following is an organometallic compound?

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