Which one of the following ions is colorless? (A) Cu+ (B) Co2+ (C) Ni2+ (D) Fe2+

How is Spectra Important for Colorless Cu+

Color in complexes:

• The hue of transition metal complexes often results from an electron transition.
• The following options exist for complexes’ color and electronic transition:
• ligand field spectra or d-d spectra:
• These result from electrons switching between orbitals that mostly have a metal orbital nature.

Charge transfer spectra:

• These kinds of spectra or colors result from electrons moving from ligand orbitals to molecular orbitals of metals or vice versa.
• The presence of open molecular orbitals is a prerequisite for charge transfer spectra.
• Charge transfer usually happens from the ligands when metals are depleted of their d electrons.
• In transition complexes, the charge transfer results in vibrant colors.

Ligand spectra:

• This type of spectra results from the ligands’ direct light absorption. Transitions from HOMO to LUMO or from non-bonding to bonding orbitals may take place.
• These spectra typically fall inside the UV range.

Counter-ion spectra:

• This particular sort of spectrum results from the typical counterions seen in ionic complex species absorbing light in the UV or visible region.
• The common ions are Cl-, SO42-, NO2-, etc.

Summary:

Which one of the following ions is colorless? (A) Cu+ (B) Co2+ (C) Ni2+ (D) Fe2+

Colorless ions are Cu+. The science behind this is really simple. When the outer orbit of Cu+ becomes complete, then no spectra of Ion remain visible.