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NEET levelSingle correct optionDifficulty 1 of 3cc104

Which aqueous complex ion is expected to be colourless?

A $[Ni (H X_{2} O) X_{6}] X^{2 +}$
B $[Zn (H X_{2} O) X_{6}] X^{2 +}$
C $[Cu (H X_{2} O) X_{6}] X^{2 +}$
D $[Co (H X_{2} O) X_{6}] X^{2 +}$

Answer: B

Colour of these complexes comes from d-d transitions, which need a partly filled d subshell.

Zn X^{2 +}

is

3 d^{10}

, so no d-d transition is possible and the ion is colourless. Ni(II), Cu(II) and Co(II) are

d^{8}

,

d^{9}

,

d^{7}

.

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