Which out of the following ions would form coloured complexes : `Ni^(2+), Cu^+` ?

Explanation

The electronic configuration of `Ni^(2+)` and `Cu^+` is given below:
`Ni^(2+) : [Ar] 3d^8 4s^0, Cu^+ : [Ar] 3d^10 4s^0`
Out of `Ni^(2+)` and `Cu^+ , Ni^(2+)` will form coloured complexes because it has two unpaired electrons.