Which of the following complex ion is not expected to absorb visible light?

Options

A. `[Ni(CN)_(4)]^(2-)`
B. `[Cr(NH_(3))_(6)]^(3+)`
C. `[Fe(H_(2)O)_(6)]^(2+)`
D. `[Ni(H_(2)O)_(6)]^(2+)`

Answer

A

Explanation

(a) Key Idea For the absorption of visible light presence of unpaired d-electrons is the necessity.
(a) In `[Ni(CN)_(4)]^(2-)`, Ni is peresent as `Ni^(2+).`
`Ni^(2+)=[Ar] 3d^(8) 4s^(0)`
`therefore" "[Ni(CN)_(4)]^(2-)=`

(Pairing occurs because `CN^(-)` is a strong field ligand). Since in `[Ni(CN)_(4)]^(2-)`, no unpaired electron is present in d-orbitals so it does not absorb visilble light.
(b) In `[Cr(NH_(3))_(6)]^(3+)`, Cr is present as `Cr^(3+).`
`Cr^(3+)=[Ar] 3d^(3) 4s^(0)` (Three unpaired electrons)
(c) In `[Fe(H_(2)O)_(6)]^(3+)`, Fe is present as `Fe^(2+)`.
`[Ar] 3d^(6) 4s^(0)` (Four unpaired electrons)
(d) In `[Ni (H_(2)O)_(6)]^(2+),` Ni is present as `Ni^(2+).`
`Ni^(2+)=[Ar] 3d^(8) 4s^(0)` (Two paired electrons)
The complex given in option (b), (c), (d) have unpaired electrons, thus absorb visible light.
In presence of `NH_(3)" and H_(2)O` pairing does not occur as they are strong field ligand.