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Coordination Compounds

Question
CBSEENCH12010492

Why a solution of [Ni(H2O)6]2+ is green while a solution of [Ni(CN)4]2– is colourless? (At. no. of Ni = 28)

Solution

In [Ni(H2O)6]+2  shows d-d transition t2g have 6 electrons and eg has 2 with octahedral structure and here d-d transition is allowed so it is coloured. While in [Ni(CN)4]2- the structure of the compound is square and hence has a centre of symmetry and hence it is Laporte forbidden and hence d-d transition is not allowed here with dsp2 hybridization.