1.

A solution of [Ni(H_(2)O)_(6)]^(2+) is green but a solution of [Ni(CN)_(4)]^(2-) is colourless. Explain.

Answer»

Solution :In `[NI(H_(2)O)_(6)]^(2+)`, Ni is in +2 state with the configuration `3d^(8)`, i.e., it has TWO unpaired electrons which do not pair up in the presence of the weak `H_(2)O` LIGAND. Hence, it is coloured. The d-d transition absorbs red LIGHT and the complementary light emitted is green
In CASE of `[Ni(CN)_(4)]^(2-)`, Ni is again in +2 state with the configuration `3d^(8)` but in presence of the strong `CN^(-)` ligand, the two unpaired electrons in the 3d orbitals pair up. Thus, there is no unpaired electron present. Hence, it is colourless.


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