A-Level Properties and complexes of first-row transition metals: worked solution

Question

In a free Fe2+ ion the five 3d orbitals are degenerate. Sketch an energy-level diagram contrasting the relative 3d orbital energies in the isolated Fe2+ ion with those when the ion is placed in an octahedral complex.

In the isolated Fe2+ ion the five 3d orbitals are all at the same energy - draw a single degenerate level containing five orbitals. In an octahedral complex the six ligands approach along the x, y and z axes: this raises the energy of the two orbitals pointing directly at the ligands (eg set: dx²-y² and dz²) and lowers the three orbitals pointing between the axes (t2g set: dxy, dxz, dyz). Diagram: the single degenerate free-ion level splits into a lower t2g set (3 orbitals) and an upper eg set (2 orbitals), separated by the crystal-field splitting energy (delta oct).