Test: Color & Electronic Spectra - Chemistry MCQ

"Among the species given, the one possessing charge-transfer transition in the visible region is:"

And the options are:

• Option A: Th(C_2O_4)_3
• Option D: [Cr(NCS)_6]^3-
• Option C: ZnS
• Option B: HgI_2

Charge-transfer transitions occur when an electron moves from a donor to an acceptor within the molecule. In the visible region, we typically look for species that have a metal center capable of undergoing such transitions due to their electronic configuration.

Among the options given:

• Option A: Th(C_2O_4)_3 is a complex of thorium with oxalate ligands. Thorium is a f-block element, but the oxalate ligands typically do not participate in charge transfer with the visible region.

• Option D: [Cr(NCS)_6]^3- is a complex of chromium with thiocyanate ligands. Chromium, being a d-block element, can have d-d transitions as well as charge-transfer transitions, especially with ligands like thiocyanate that can participate in such transitions. This makes it a likely candidate for visible region charge-transfer transitions.

• Option C: ZnS is a semiconductor and generally does not show charge-transfer transitions in the visible region. It does have a band gap that can absorb UV light, but not typically visible light.

• Option B: HgI_2 is mercury(II) iodide, and while heavy metal halides can have interesting electronic properties, they do not usually exhibit charge-transfer transitions in the visible region.

Based on this information, the species most likely to show charge-transfer transition in the visible region is Option D: [Cr(NCS)_6]^3-. This complex can exhibit charge-transfer transitions where the electron transfer occurs from the thiocyanate ligands to the chromium metal center.