The chemical formula of ferrocene is:

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

Select the true statement from the following for metal carbonyls?

Carbon monoxide reacts with iron to form

The metal which does not form a polynuclear carbonyl is

Which of the following is not an organometallic compound?