Cr3+ ions are coloured green and Cr2+ ions are coloured blue
Cr2+O72- is orange
The colour of the complex ion depends on the ligand used, not the metal centre.
Increasing the number of donor atoms in a ligand will increase its ability to form stable complexes with transition metals.
Transition metals have partially filled d orbitals which can be involved in bonding with other atoms or molecules to form transition metal compounds (complexes).
