Farvet anioner

• Why are CrO₄^2– and Cr₂O₇^2– strongly coloured?

Formally, CrO₄^2– and Cr₂O₇^2– contain Cr in their highest possible oxidation states. The 3d orbitals on the metal centre are empty, so that d–d transitions cannot be the source of the colour. Instead, electronic transitions take place from filled orbitals on the O atoms to empty 3d orbitals on the metal. This is referred to as ligand–to–metal charge transfer (LMCT). LMCT transitions are completely "allowed" according to the rules of quantum mechanics (in contrast to d–d transitions, which are formally "forbidden"), so the probability of the electronic transition is high and the colours are strong.

Er der nogen der kan foklare mig hvorfor disse to forbindelser er farvet? Cr har 4 elektroner på sit 3d orbital, over halvdelen af orbitalerne vil stå tomme. Man vil forvente at elektronerne bevæger sig mellem energi tilstande, der øger energi mængden pga. frastødningkræfter mellem liganden, oxygen molekyler og 3d kationen Cr. Men forstår det ikke helt med tomme 3d orbitaler osv., er der nogen kan forklare det på andre måder?