Question

The wavelength of light absorbed is highest in:
A) $[Co(N{H}_3{)}_{5}Cl{]}^{2+}$
B) $[Co(N{H}_3{)}_5{H}_{2}O{]}^{3+}$
C) $[Co(N{H}_3{)}_6{]}^{3+}$
D) $[Co(en{)}_3{]}^{3+}$

Answer 1

Hint: As following the crystal field theory and electric spectra series, increasing strength of ligand wavelength of absorbed radiation decreases the light absorbed.

Complete answer:
Light absorption is a process by which light is absorbed and converted into energy. Absorption of electromagnetic radiation depends on the electromagnetic field. If a material or matter absorbs light of certain wavelengths (or colors) of the spectrum it colour cannot be observed whereas if the colors is reflected back the colour can be seen.
This electromagnetic spectrum ranges from very short wavelengths to very long wavelengths regions of this spectrum, depicting the inverse relationship between wavelength and frequency
In the coordination complex as stated in crystal field theory that have high spin are considered "weak fields which induce the absorption of longer wavelength whereas ligands that have low spin are considered "strong field inducing reflection of the wavelength.
Absorption of radiation in organic molecules is limited to certain functional groups (chromophores) that contain valence electrons of low excitation energy. When an atom or molecule absorbs energy, electrons are promoted from their ground state to an excited state.
Among all the coordination complex aforesaid in complex $[Co(N{H}_3{)}_{5}Cl{]}^{2+}$ unpaired electrons are present, with the central element cobalt so it required less energy for excitation. Thus, the wavelength of light absorbed will be highest.

Therefore the correct option is option (A).

Note: The colour of the coordination complex is also determined based on the ligands attachment to a transition metal to form a coordination complex, allowing electrons in the orbital to split into high energy and low energy orbitals.