Arrange the following in order of decreasing number of unpaired electrons `:`
i. `[Fe(H_(2)O_(6))]^(2+)`
ii. `[Fe(CN)_(6) ]^(3-)`
iii. `[Fe(CN)_(6)]^(4-)`
iv. `[Fe(H_(2)O)_(6)]^(3+)`

Answer

A

Explanation

`[Fe^(2+)(H_(2)O)_(6)]^(2+),Fe^(2+) =3d^(6)`
`H_(2)O` is a weak field ligand so, no pairing of elctrons occurs
(II) `[Fe^(3+)(CN)_(6)]^(3-),Fe^(3+) =3d^(5)`
Since cyanide ion is a strong field ligand, hence pairing occurs
(III) `[Fe^(2+) (CN)_(6)]^(4-),Fe^(2+) =3d^(6)`
Pairing occurs
(IV) `[Fe^(3+)(H_(2O))_(6)]^(3+),fe^(3+) =3d^(5)`
pairing does not occur
The decreasing number of unpaired electrons is
`IV gtIgtIIgtIII(5 gt4gt1gt0)`