Which of the following is a bidentate negatively charged ligand?

Identify the total number of complexes having bidentate ligands in them from the following list of complexes:
$a$) Tetracyanonickelate$(II)$ ion
$b$) Trioxalatocobaltate$(III)$ ion
$c$) Sodium hexafluoroaluminate$(III)$
$d$) bis(ethylenediamine)dithiocyanatoplatinum$(IV)$

The bonds present in $K_4[Fe(CN)_6]$ are ........

The oxidation number of iron in the compound formed during the brown ring test for $NO_3^{-}$ ion is . . . . . .

$A$ $200 \ mL$ solution $X$ contains $0.02 \ mol$ of $[Co(NH_3)_5Br]Cl_2$ and $0.02 \ mol$ of $[Co(NH_3)_5Cl]SO_4$. If solution $X$ is treated with excess $AgNO_3$ and excess $BaCl_2$,what will be the number of moles of precipitates $Y$ and $Z$ formed,respectively?

In the complex ion $[Fe(C_2O_4)_3]^{3-}$,the coordination number of $Fe$ is