Which of the following metals does not form a polynuclear carbonyl?

The number of bridging $CO$ ligands in $[Mn_{2}(CO)_{10}]$ is $....$ .

The Wilkinson catalyst is:

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:

The compound that is not an olefinic organometallic compound is:

Which of the following is not considered as an organometallic compound?