[NYM]

Why is Co(H₂O)₆²⁺ (aq)  light red, while CoCl₄^2-(aq) is blue?

Thanks.

[Dan]

This is to do with ligand field splitting.

Search Wikipedia for 'ligand field theory', 'crystal field theory' and 'spectrochemical series'

[NYM]

Thank you for the keywords, but it's a bit too complicated for me. I'll just go with "that's how it is"

[xiankai]

that is sooo an un-chemist way!

a very brief summary is that when light falls on the molecule, the electrons in it are excited to a higher energy state than its normal configuration. after the initial bombardment with light, the new high energy state is unstable and hence the electrons fall back to lower energy states (not necessarily the same as earlier), in the process releasing the excess energy in the form of 'light' with a wavelength that corresponds to the colours that they display.

the more exact mechanism is more complicated, and if u ever have free time or are feeling abit moee curious, you can look up the keywords mentioned previously