How will you account for the following ? (a) `K_4[Fe(CN)_6]` is not toxic whereas KCN is highly toxic. (b) Oxalic acid is commonly used to remove rust

(a) Both the compounds in water ionise are :
`K_4[Fe(CN)_6] ltimplies 4K^(+)` + `[Fe(CN)_6]^(4-)`
KCN `rArr` `K^(+)` + `CN^(-)`
The `CN^(-)` ions are highly toxic in nature. These are released by KCN and not by the complex. Therefore, `K_4[Fe(CN)_6]` is not toxic ion furnished by oxalic acid.
(b) Rust is chemically hydrated iron (III) oxide with the formula `Fe_(2)O_(3).xH_(2)O`. The compound forms a soluble complex with oxilate ion furnished by oxalic acid.
`Fe_(2)O_(3)+ 6C_(2)O_(4)^(2-) + 6H_(3)O^(+)rarr2[Fe(C_(2)O_(4))_(3)]^(3-)("Soluble") + 9H_(2)O`.
Therefore, rust stains are removed by oxalic acid.
(c ) In a tetrahedral complex such as `Ni(CO)_(4)` , all the four positions occupied by the ligands are adjacent to each other. . Therefore , cis and trans isomers are not possible , in this case or the complex fails to show any geometrical isomerism.