Question

In solutions and complexes, all the 5 d orbitals within the same d-subshell are degenerate – True or False?

• A. True
• B. False

Answer 1

The correct option is B False

Unlike s and p block metal compounds, most of the d-block metal compounds are coloured in the solid or in solution states. The colour of transition metal ions is due to the presence of unpaired or incomplete d-orbitals.
In a free isolated gaseous ion, the five d-orbitals are degenerate. However, the surrounding groups which can be solvent molecules (in solution), ligands (in a complex) or other ions (in a crystal lattice) affect the energy of some
d-orbitals more than the others. Thus, all the five d-orbitals no longer have identical energy. In the simplest of such scenarios, the orbitals within the same d-subshell form two groups of orbitals of different energy.

Thus, in a transition element with a partly filled d-orbital, electron can be
promoted from one d level to another d* level of higher energy (d -d * transition). In general, this promotion requires small amount of energy, which corresponds to the visible light.
The colour of a transition metal complex is dependent on the amount of energy required for d - d * transition which in turn depends on the nature of the ligand, and the type of the complex. The colour which we see is the colour of the transmitted wavelength, which is complementary to the colour absorbed by the compound.

Suppose, if the electrons in an octahedral metal complex are able to absorb red light and get promoted from the lower
$d_{xy},d_{yz}{d}_{xz}$ orbital to the $d_{z2}{d}_{x2-y2}$ orbital, the compound reflects all wavelengths except red. We can use use the colour wheel shown above to find the complementary colour of red – cyan, which will be the colour of the compound.