Question

The ${\left[ {Ti{{\left( {{H_2}O} \right)}_6}} \right]^{3 + }}$ complex ion shows a strong absorption at 490 mm. But ${\left[ {Ti{{\left( {N{H_3}} \right)}_6}} \right]^{3 + }}$ complex ion absorbs light of lower wavelength than 490mm. This is because:
A.$N{H_3}$ is a stronger ligand than water
B.${H_2}O$ is a stronger ligand than $N{H_3}$
C.Electron transition of higher energy is involved.
D.Both A and C

Answer 1

To answer this question, you must recall the electronic arrangement in the titanium ion and the colour of coordination compounds. Colour change in a coordination compound happens due to either ${\text{d - d}}$ transition or charge transfer.

Complete step by step solution:
${\left[ {Ti{{\left( {N{H_3}} \right)}_6}} \right]^{3 + }}$ complex absorbs light of lower wavelength than ${\left[ {Ti{{\left( {{H_2}O} \right)}_6}} \right]^{3 + }}$ because $N{H_3}$ is a stronger ligand than ${H_2}O$. As a result, a higher energy electron transition is involved. Since for stronger ligands, the crystal field splitting is higher, thus, the energy for ${\text{d - d}}$ transfer is higher in case of ${\left[ {Ti{{\left( {N{H_3}} \right)}_6}} \right]^{3 + }}$than that in ${\left[ {Ti{{\left( {{H_2}O} \right)}_6}} \right]^{3 + }}$.The higher is the energy of a radiation, lower is its wavelength.

Thus, the correct answer is D.

Additional Information:
Coordination compounds are a type of Addition compounds.
Coordination compounds are the compounds in which a central metal atom is linked by coordination bond to a number of ligands, which may either be ions or neutral molecules, i.e. by donation of lone pairs by the ligands to the central metal atom. If this compound carries a positive or negative charge, it is called a complex ion. These complex ions are relatively stable and they do not lose their identity in aqueous solution like double salts.
Apart from coordination compounds, the other types of addition compounds are double salts. Unlike coordination compounds, double salts are stable only in crystalline state and lose their identity in solution form.

Note:
The color shown by a coordination complex is predicted using the crystal field theory. When ligands attach to a metal and form a coordination complex, the d orbitals split into higher energy and lower energy orbitals. The electron may accept energy and excite to a higher energy level. Since radiations of certain wavelengths are absorbed, the complex becomes coloured due to subtractive colour mixing.