What is the chemical formula of Tartar Emetic?

Grignard reagent is a

$CH_3MgI$ is an organometallic compound due to

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

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

The number of bridging $CO$ ligand$(s)$ and $Co-Co$ bond$(s)$ in $Co_2(CO)_8$ respectively are