Which of the following are organometallic compounds? $(I)$ Grignard reagent $(II)$ Trimethylboron $(III)$ Sodium methoxide $(IV)$ $TEL$

Which of the following is an organometallic compound?

The correct structural formula of Zeise's salt 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

The oxidation state of cobalt in the following molecule is

The metal which does not form a polynuclear carbonyl is