Both normal oxidation states of iron, Fe, will form low spin complex ions with cyanide (CN-); i.e., [Fe(CN)6]4 minus (aq) and [Fe(CN)6] 3 minus(aq). However the color of these ions is not the same. One ion is green-yellow (more yellow than green), which means it absorbs violet to blue light and the other is blood red in color, which means that it absorbs blue-green light. Determine which ions is which color and explain your answer.
A Fe3+ ion has 5 electrons in the 5 3d orbitals. In going from Fe to Fe3+ it lost the 2 4s2 electrons and 1 3d electron.
A Fe2+ ion has 6 electrons in the 5 3d orbitals. In going from Fe to Fe2+ it lost the 2 4s2 electrons.
When a metal ion lies in an octahedron formed by six ligands, the energies of the d orbitals are no longer the same, and two opposing forces are set up. This splitting of the energies in the d orbitals is called the ligand field splitting and is symbolized by 10Dq. Two orbitals (eg orbitals) have higher energy, while three orbitals (t2g orbitals) have lower energy.
One force tends to keep the electrons of the metal ion distributed with unpaired spins within all of the d orbitals. The other force tends to reduce the average energy of the d electrons by placing as many of them as ...
This in-depth solution describes in detail the different oxidation states for Fe with full electron configurations and also clear explanations on the orbital level present. It also relates to the color shown on the electromagnetic spectrum.