Crystal field theory views the bonding in complexes as arising from electrostatic interaction and considers the effect of the ligand charges on the energies of the metal ion d-orbitals
In this theory, a ligand lone pair is modelled as a point negative charge that repels electrons in the d-orbitals of the central metal ion.The theory concentrated on the resulting splitting of the d-orbitals in two groups with different energies and used that splitting to rationalize and correlate the optical spectra,thermodynamic stability, and magnetic properties of complexes.This energy splitting between the two sets of d-orbitals is called the crystal field splitting `Delta`.
In general, the crystal field splitting energy `Delta` corresponds to wavelenght of light in visible region of the spectrum, and colours of the complexes can therefore be attributed to electronic transition between the lower and higher energy sets of d-orbitals.
In general, the colour that we see is complementry to the colour absorbed.

Different metal ions have different value of `Delta`, which explains why their complexes with the same ligand have different colour.
Similarly the crystal field splitting also depends on the nature of ligands and as the ligand for the same metal varies from `H_2O` to `NH_3` to ethylenediamine, `Delta` for complexes increase.Accordingly, the electronic transition shifts to higher energy (shorter wavelength) as the ligand varies from `H_2O` from `NH_3` to en, thus accounting for the variation in colour.
Crystal field theory accounts for the magnetic properties of complexes in terms of the relative values of `Delta` and the spin pairing energy P. Small `Delta` values favour high spin complexes, and large `Delta` values favour low spin complexes.
Which of the following statements is incorrect ?

Options

A. The `Ni^(2+)` (aq) cation is coloured because `Ni^(2+)` ion can absorb light, which promotes electrons from the filled d-orbitals to the higher energy half filled d-orbitals
B. The `Zn^(2+)` (aq) cation is colourless because the d-orbitals are completely filled and no electrons can be promoted, so no light is absorbed
C. A complex which has just one absorption band at 455 nm, must be red coloured
D. None

Answer

C

Explanation

As it absorbs blue colour light (`lambda`=455 nm), the colour of the complex must be orange.