Which of the following organometallic compounds is both $\sigma$ and $\pi$ bonded?

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

Which one of the following is not an organometallic compound?

In the cobalt-carbonyl complex: $[Co_{2}(CO)_{8}]$,the number of $Co-Co$ bonds is "$X$" and the number of terminal $CO$ ligands is "$Y$". Find the value of $X + Y$.

Which element does not form a mononuclear carbonyl?

The correct statement$(s)$ regarding the binary transition metal carbonyl compounds is (are) (Atomic numbers: $Fe = 26, Ni = 28$)
$A$. Total number of valence shell electrons at metal centre in $Fe(CO)_5$ or $Ni(CO)_4$ is $16$
$B$. These are predominantly low spin in nature
$C$. Metal-carbon bond strengthens when the oxidation state of the metal is lowered
$D$. The carbonyl $C-O$ bond weakens when the oxidation state of the metal is increased