In which of the following coordination entites the magnitude of `Delta_(0)` (CFSE in octehedral field) will be maximum.
`(At. No .Co = 27)`

Options

A. `[Co(C_(2)O_(4))_(3)]^(3-)`
B. `[Co(H_(2)O)_(6)]^(3+)`
C. `[Co(NH_(3))_(6)]^(2+)`
D. `[Co(CN)_(6)]^(3-)`

Answer

D

Explanation

In all the given complex ions, the central metal is Co with the oxidation state `+3` . `Co(3d^(7)4s^(2))implies Co^(3+) (3d^(6))`
`d^(6)` configuration will be written as `t_(2g)^(4)e_(g)^(2)` in case of weak field ligands `C_(2)O_(4)^(2-)` , `H_(2)O` and `NH_(3)` but will be written as `t_(2g)^(6)` in case of strong field ligand, `CN^(-)` :
CFSE for `t_(2g)^(4)e_(g)^(2)` is (4) `(-2//5Delta_(o)) + (2) (3//5 Delta_(o))`
`= - 1.6 Delta_(o) + 1.2 Delta_(o) = -0.4 Delta_(o)`
CFSE for `t_(2g)^(6)e_(g)^(0)` is (6) `(-2//5Delta_(o)) = -2.4 Delta_(o)`
Thus the magnitude of `Delta_(o)` is maximum for `[Co(CN)_(6)]^(3-)` ion.