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

In $Fe(CO)_5$,the $Fe-C$ bond possesses

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

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

Carbon monoxide reacts with iron to form

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