Ligands which cause a large splitting Δ of the d -orbitals are referred to as strong-field ligands, such as CN and CO from the spectrochemical series. In complexes with these ligands, it is unfavourable to put electrons into the high energy orbitals. Therefore, the lower energy orbitals are completely filled before population of the upper sets starts according to the Aufbau principle. Complexes such as this are called "low spin". For example, NO 2 is a strong-field ligand and produces a large Δ. The octahedral ion [Fe(NO 2) 6 ], which has 5 d -electrons, would have the octahedral splitting diagram shown at right with all five electrons in the t 2 g level. This low spin state therefore does not follow Hund's rule.