Identify the complexes which are expected to be coloured.
a. `[Ti(NO_(3))_(4)]` , b. `[Cu(NC CH_(3))_(4)]^(o+) BF_(4)^(ɵ)`
c. `[Cr(NH_(3))_(6)]^(3+)3Cl^(ɵ)` , d. `K_(3)[VF_(6)]`

Explanation

a. `[overset(+4)(Ti)overset(-1 xx 4)((NO_(3))_(4))], Ti = 3d^(2)4s^(2), Ti^(4+) = 3d^(10) 4s^(0)`.
There is no unpaired electron in `Ti^(4+)` ion, hence no `d-d` transition occurs, so colourless.
b. `[overset(+1)(Cu)(NC-CH_(3))_(4)]^(o+) BF_(4)^(ɵ)`,
`Cu = 3d^(10) 4s^(1), Cu^(o+) = 3d^(10) 4s^(0)`
`d^(10)` configuration of `Cu^(o+)` ion have no unpaired electron, hence colourless.
c. `[Cr^(3+)(NH_(3)^(0))_(6)]^(3+), Cr = 3d^(5) 4s^(1), Cr^(3+) = 3d^(3) 4s^(0)`.
`cr^(+3)` has `d^(3)` configuration, thus have three unpaired electrons, d-d transition occurs, hence coloured.
d. `overset(+1 xx 3)(K_(3))[overset(+3-6)(VF_(6))]^(-3) , V = 3d^(3) 4s^(2), V^(3+) = 3d^(2) 4s^(0)`.
`V^(3+)` has `d^(2)` configuration, thus have two unpaired electrons, d-d transition occurs, hence coloured